|
THE ANCIENT LIFE-HISTORY OF THE EARTH
A COMPREHENSIVE OUTLINE OF THE PRINCIPLES AND LEADING FACTS OF
PALAEONTOLOGICAL SCIENCE
BY H. ALLEYNE NICHOLSON
M.D., D.SC., M.A., PH. D. (GOeTT), F.R.S.E, F.L.S.
PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ST ANDREWS
PREFACE.
The study of Palaeontology, or the science which is concerned
with the living beings which flourished upon the globe during
past periods of its history, may be pursued by two parallel but
essentially distinct paths. By the one method of inquiry, we may
study the anatomical characters and structure of the innumerable
extinct forms of life which lie buried in the rocks simply as
so many organisms, with but a slight and secondary reference
to the _time_ at which they lived. By the other method, fossil
animals are regarded principally as so many landmarks in the
ancient records of the world, and are studied _historically_
and as regards their relations to the chronological succession
of the strata in which they are entombed. In so doing, it is of
course impossible to wholly ignore their structural characters,
and their relationships with animals now living upon the earth;
but these points are held to occupy a subordinate place, and to
require nothing more than a comparatively general attention.
In a former work, the Author has endeavoured to furnish a summary
of the more important facts of Palaeontology regarded in its strictly
scientific aspect, as a mere department of the great science of
Biology. The present work, on the other hand, is an attempt to
treat Palaeontology more especially from its historical side, and
in its more intimate relations with Geology. In accordance with
this object, the introductory portion of the work is devoted to a
consideration of the general principles of Palaeontology, and the
bearings of this science upon various geological problems--such
as the mode of formation of the sedimentary rocks, the reactions
of living beings upon the crust of the earth, and the sequence
in time of the fossiliferous formations. The second portion of
the work deals exclusively with Historical Palaeontology, each
formation being considered separately, as regards its lithological
nature and subdivisions, its relations to other formations, its
geographical distribution, its mode of origin, and its characteristic
life-forms.
In the consideration of the characteristic fossils of each successive
period, a general account is given of their more important zoological
characters and their relations to living forms; but the technical
language of Zoology has been avoided, and the aid of illustrations
has been freely called into use. It may therefore be hoped that
the work may be found to be available for the purposes of both the
Geological and the Zoological student; since it is essentially an
outline of Historical Palaeontology, and the student of either of
the above-mentioned sciences must perforce possess some knowledge
of the last. Whilst primarily intended for students, it may be
added that the method of treatment adopted has been so far
untechnical as not to render the work useless to the general
reader who may desire to acquire some knowledge of a subject of
such vast and universal interest.
In carrying out the object which he has held before him, the
Author can hardly expect, from the nature of the materials with
which he has had to deal, that he has kept himself absolutely
clear of errors, both of omission and commission. The subject,
however, is one to which he has devoted the labour of many years,
both in studying the researches of others and in personal
investigations of his own; and he can only trust that such errors
as may exist will be found to belong chiefly to the former class,
and to be neither serious nor numerous. It need only be added
that the work is necessarily very limited in its scope, and that
the necessity of not assuming a thorough previous acquaintance
with Natural History in the reader has inexorably restricted its
range still further. The Author does not, therefore, profess to
have given more than a merely general outline of the subject; and
those who desire to obtain a more minute and detailed knowledge
of Palaeontology, must have recourse to other and more elaborate
treatises.
UNITED COLLEGE, ST ANDREWS.
October 2, 1876.
CONTENTS.
PART I.
PRINCIPLES OF PALAEONTOLOGY.
INTRODUCTION.
The general objects or geological science--The older theories of
catastrophistic and intermittent action--The more modern doctrines
of continuous and uniform action--Bearing of these doctrines
respectively on the origin or the existing terrestrial
order--Elements or truth in Catastrophism--General truth of the
doctrine of Continuity--Geological time.
CHAPTER I.
Definition of Palaeontology--Nature of Fossils--Different processes
of fossilisation.
CHAPTER II.
Aqueous and igneous rocks--General characters of the sedimentary
rocks--Mode or formation of the sedimentary rocks--Definition
of the term "formation"--Chief divisions of the aqueous
rocks--Mechanically-formed rocks, their characters and mode of
origin--Chemically and organically formed rocks--Calcareous
rocks--Chalk, its microscopic structure and mode of
formation--Limestone, varieties, structure, and origin--Phosphate
of lime--Concretions--Sulphate of lime--Silica and siliceous
deposits of various kinds--Greensands--Red clays--Carbon and
carbonaceous deposits.
CHAPTER III.
Chronological succession of the fossiliferous rocks--Tests or age
of strata--Value of Palaeontological evidence in stratigraphical
Geology--General sequence of the great formations.
CHAPTER IV.
The breaks in the palaeontological and geological record--Use of
the term "contemporaneous" as applied to groups of strata--General
sequence of strata and of life-forms interfered with by more or
less extensive gaps--Unconformability--Phenomena implied by
this--Causes of the imperfection of the palaeontological record.
CHAPTER V.
Conclusions to be drawn from fossils--Age of rocks--Mode of origin
of any fossiliferous bed--Fluviatile, lacustrine, and marine
deposits--Conclusions as to climate--Proofs of elevation and
subsidence of portions of the earth's crust derived from fossils.
CHAPTER VI.
The biological relations of fossils--Extinction of
life-forms--Geological range of different species--Persistent types
of life--Modern origin of existing animals and plants--Reference
of fossil forms to the existing primary divisions of the animal
kingdom--Departure of the older types of life from those now in
existence--Resemblance of the fossils of a given formation to
those of the formation next above and next below--Introduction
of new life-forms.
PART II.
HISTORICAL PALAEONTOLOGY.
CHAPTER VII.
The Laurentian and Huronian periods--General nature, divisions,
and geographical distribution of the Laurentian deposits--Lower
and Upper Laurentian--Reasons for believing that the Laurentian
rocks are not azoic based upon their containing limestones, beds of
oxide of iron, and graphite--The characters, chemical composition,
and minute structure of _Eozooen Canadense_--Comparison of _Eozooen_
with existing Foraminifera--_Archoeosphoerinoe_--Huronian
formation--Nature and distribution of Huronian deposits--Organic
remains of the Huronian--Literature.
CHAPTER VIII.
The Cambrian period--General succession of Cambrian deposits in
Wales--Lower Cambrian and Upper Cambrian--Cambrian deposits of
the continent of Europe and North American--Life of the Cambrian
period--Fucoids--Eophyton--Oldhamia--Sponges--Echinoderms--Annelides
--Crustaceans--Structure of Trilobites--Brachiopods--Pteropods,
Gasteropods, and Bivalves--Cephalopods--Literature.
CHAPTER IX.
The Lower Silurian period--The Silurian rocks generally--Limits
of Lower and Upper Silurian--General succession, subdivisions,
and characters of the Lower Silurian rocks of Wales--General
succession, subdivisions, and characters of the Lower Silurian
rocks of the North American continent--Life of the
period--Fucoids--Protozoa--Graptolites--Structure of
Graptolites--Corals--General structure of Corals--Crinoids--
Cystideans--General characters of Cystideans--Annelides--
Crustaceans--Polyzoa--Brachiopods--Bivalve and Univalve
Molluscs--Chambered Cephalopods--General characters of the
Cephalopoda--Conodonts.
CHAPTER X.
The Upper Silurian period--General succession of the Upper Silurian
deposits of Wales--Upper Silurian deposits of North America--Life
of the Upper Silurian--Plants--Protozoa--Graptolites--Corals--
Crinoids--General structure of Crinoids--Star-fishes--Annelides--
Crustaceans--Eurypterids--Polyzoa--Brachiopods--Structure of
Brachiopods--Bivalves and Univalves--Pteropods--Cephalopods--
Fishes--Silurian literature.
CHAPTER XI.
The Devonian period--Relations between the Old Red Sandstone
and the marine Devonian deposits--The Old Red Sandstone of
Scotland--The Devonian strata of Devonshire--Sequence and
subdivisions of the Devonian deposits of North America--Life
of the period--Plants--Protozoa--Corals-Crinoids--Pentremites--
Annelides--Crustaceans--Insects--Polyzoa--Brachiopods--Bivalves--
Univalves--Pteropods--Cephalopods--Fishes--General divisions of
the Fishes--Palaeontological evidence as to the independent
existence of the Devonian system as a distinct
formation--Literature.
CHAPTER XII.
The Carboniferous period--Relations of Carboniferous rocks to
Devonian--The Carboniferous Limestone or Sub-Carboniferous
series--The Millstone-grit and the Coal-measures--Life of the
period--Structure and mode of formation of Coal--Plants of the
Coal.
CHAPTER XIII.
Animal life of the Carboniferous period--Protozoa--Corals--
Crinoids--Pentremites--Structure of Pentremites--Echinoids--
Structure of Echinoidea--Annelides--Crustacea--Insects--
Arachnids--Myriapods--Polyzoa--Brachiopods--Bivalves and
Univalves--Cephalopods--Fishes--Labyrinthodont Amphibians--
Literature.
CHAPTER XIV.
The Permian period--General succession, characters, and mode
of formation of the Permian deposits--Life of the period--
Plants--Protozoa--Corals--Echinoderms--Annelides--Crustaceans--
Polyzoa--Brachiopods--Bivalves-Univalves--Pteropods--
Cephalopods--Fishes--Amphibians--Reptiles--Literature.
CHAPTER XV.
The Triassic period--General characters and subdivisions of the
Trias of the Continent of Europe and Britain--Trias of North
America--Life of the period--Plants--Echinoderms--Crustaceans--
Polyzoa--Brachiopods--Bivalves--Univalves--Cephalopods--
Intermixture of Palaeozoic with Mesozoic types of Molluscs--
Fishes--Amphibians--Reptiles--Supposed footprints of Birds--
Mammals--Literature.
CHAPTER XVI.
The Jurassic period--General sequence and subdivisions of the
Jurassic deposits in Britain--Jurassic rocks of North America--Life
of the period--Plants--Corals--Echinoderms--Crustaceans--Insects--
Brachiopods--Bivalves--Univalves-Pteropods--Tetrabranchiate
Cephalopods--Dibranchiate Cephalopods--Fishes--Reptiles--Birds--
Mammals--Literature.
CHAPTER XVII.
The Cretaceous period--General succession and subdivisions of
the Cretaceous rocks in Britain--Cretaceous rocks of North
America--Life of the period--Plants--Protozoa--Corals--Echinoderms--
Crustaceans--Polyzoa--Brachiopods--Bivalves--Univalves--
Tetrabranchiate and Dibranchiate Cephalopods--Fishes--Reptiles--
Birds--Literature.
CHAPTER XVIII.
The Eocene period--Relations between the Kainozoic and Mesozoic
rocks in Europe and in North America--Classification of the Tertiary
deposits--The sequence and subdivisions of the Eocene rocks of
Britain and France--Eocene strata of the United States--Life of the
period--Plants--Foraminifera--Corals--Echinoderms--Mollusca--Fishes--
Reptiles--Birds--Mammals.
CHAPTER XIX.
The Miocene period--Miocene strata of Britain--Of France--Of
Belgium--Of Austria--Of Switzerland--Of Germany--Of Greece--Of
India--Of North America--Of the Arctic regions--Life of the
period--Vegetation of the Miocene period--Foraminifera--Corals--
Echinoderms--Articulates--Mollusca--Fishes-Amphibians--Reptiles--
Mammals.
CHAPTER XX.
The Pliocene period--Pliocene deposits of Britain--Of Europe--Of
North America--Life of the period--Climate of the period as indicated
by the Invertebrate animals--The Pliocene Mammalia--Literature
relating to the Tertiary deposits and their fossils.
CHAPTER XXI.
The Post-Pliocene period--Division of the Quaternary deposits into
Post-Pliocene and Recent--Relations of the Post-Pliocene deposits
of the northern hemisphere to the "Glacial period"--Pre-Glacial
deposits--Glacial deposits--Arctic Mollusca in Glacial
beds--Post-Glacial deposits--Nature and mode of formation of
high-level and low-level gravels--Nature and mode of formation
of cavern-deposits--Kent's Cavern-Post--Pliocene deposits of
the southern hemisphere.
CHAPTER XXII.
Life of the Post-Pliocene period--Effect of the coming on and
departure of the Glacial period upon the animals inhabiting the
northern hemisphere--Birds of the Post-Pliocene--Mammalia of the
Post-Pliocene--Climate of the Post-Glacial period as deduced from
the Post-Glacial Mammals--Occurrence of the bones and implements
of Man in Post-Pliocene deposits in association with the remains
of extinct Mammalia--Literature relating to the Post-Pliocene
period.
CHAPTER XXIII.
The succession of life upon the globe--Gradual and successive
introduction of life-forms--What is meant by "lower" and "higher"
groups of animals and plants--Succession in time of the great
groups of animals in the main corresponding with their zoological
order--Identical phenomena in the vegetable kingdom--Persistent
types of life--High organisation of many early forms--Bearings
of Palaeontology on the general doctrine of Evolution.
APPENDIX.--Tabular view of the chief Divisions of the Animal Kingdom.
GLOSSARY.
INDEX.
LIST OF ILLUSTRATIONS
FIG.
1. Cast of _Trigonia longa_.
2. Microscopic section of the wood of a fossil Conifer.
3. Microscopic section of the wood of the Larch.
4. Section of Carboniferous strata, Kinghorn, Fife.
5. Diagram illustrating the formation of stratified deposits.
6. Microscopic section of a calcareous breccia.
7. Microscopic section of White Chalk.
8. Organisms in Atlantic Ooze.
9. Crinoidal marble.
10. Piece of Nummulitic limestone, Pyramids.
11. Microscopic section of Foraminiferal
limestone--Carboniferous, America.
12. Microscopic section of Lower Silurian limestone.
13. Microscopic section of oolitic limestone, Jurassic.
14. Microscopic section of oolitic limestone, Carboniferous.
15. Organisms in Barbadoes earth.
16. Organisms in Richmond earth.
17. Ideal section of the crust of the earth.
18. Unconformable junction of Chalk and Eocene rocks.
19. Erect trunk of a _Sigillaria_.
20. Diagrammatic section of the Laurentian rocks.
21. Microscopic section of Laurentian limestone.
22. Fragment of a mass of _Eozooen Canadense_.
23. Diagram illustrating the structure of _Eozooen_.
24. Microscopic section of _Eozooen Canadense_.
25. _Nonionina_ and _Gromia_.
26. Group of shells of living _Foraminifera_.
27. Diagrammatic section of Cambrian strata.
28. _Eophyton Linneanum_.
29. _Oldhamia antiqua_.
30. _Scolithus Canadensis_.
31. Group of Cambrian Trilobites.
32. Group of characteristic Cambrian fossils.
33. Fragment of _Dictyonema sociale_.
34. Generalised section of the Lower Silurian rocks
of Wales.
35. Generalised section of the Lower Silurian rocks
of North America.
36. _Licrophycus Ottawaensis_.
37. _Astylospongia proemorsa_.
38. _Stromatopora rugosa_.
39. _Dichograptus octobrachiatus_.
40. _Didymograptus divaricatus_.
41. _Diplograptus pristis_.
42. _Phyllograptus typus_.
43. _Zaphrentis Stokesi_.
44. _Strombodes pentagonus_.
45. _Columnaria alveolata_.
46. Group of Cystideans.
47. Group of Lower Silurian Crustaceans.
48. _Ptilodictya falciformis_.
49. _Ptilodictya Schafferi_.
50. Group of Lower Silurian Brachiopods.
51. Group of Lower Silurian Brachiopods.
52. _Murchisonia gracilis_.
53. _Bellerophon argo_.
54. _Maclurea crenulata_.
55. _Orthoceras crebriseptum_.
56. Restoration of _Orthoceras_.
57. Generalised section of the Upper Silurian rocks.
58. _Monograptus priodon_.
59. _Halysites catenularia_ and _H. agglomerata_.
60. Group of Upper Silurian Star-fishes.
61. _Protaster Sedgwickii_.
62. Group of Upper Silurian Crinoids.
63. _Planolites vulgaris_.
64. Group of Upper Silurian Trilobites.
65. _Pterygotus Anglicus_.
66. Group of Upper Silurian _Polyzoa_.
67. _Spirifera hysterica_.
68. Group of Upper Silurian Brachiopods.
69. Group of Upper Silurian Brachiopods.
70. _Pentamerus Knightii_.
71. _Cardiola interrupta, C. fibrosa_, and _Pterinoea
subfalcata_.
72. Group of Upper Silurian Univalves.
73. _Tentaculites ornatus_.
74. _Pteraspis Banksii_.
75. _Onchus tenuistriatus_ and _Thelodus_.
76. Generalised section of the Devonian rocks of North America.
77. _Psilophyton princeps_.
78. _Prototaxites Logani_.
79. _Stromatopora tuberculata_.
80. _Cystiphyllum vesiculosum_.
81. _Zaphrentis cornicula_.
82. _Heliophyllum exiguum_.
83. _Crepidophyllum Archiaci_.
84. _Favosites Gothlandica_.
85. _Favosites hemisphoerica_.
86. _Spirorbis omphalodes_ and _S. Arkonensis_.
87. _Spirorbis laxus_ and _S. Spinulifera_.
88. Group of Devonian Trilobites.
89. Wing of _Platephemera antiqua_.
90. _Clathropora intertexta_.
91. _Ceriopora Hamiltonensis_.
92. _Fenestella magnifica_.
93. _Retepora Phillipsi_.
94. _Fenestella cribrosa_.
95. _Spirifera sculptilis_.
96. _Spirifera mucronata_.
97. _Atrypa reticularis_.
98. _Strophomena rhomboidalis_.
99. _Platyceras dumosum_.
100. _Conularia ornata_.
101. _Clymenia Sedgwickii_.
102. Group of Fishes from the Devonian rocks of North America.
103. _Cephalaspis Lyellii_.
104. _Pterichthys cornutus_.
105. _Polypterus_ and _Osteolepis_.
106. _Holoptychius nobilissimus_.
107. Generalised section of the Carboniferous rocks of the
North of England.
108. _Odontopteris Schlotheimii_.
109. _Calamites cannoeformis_.
110. _Lepidodendron Sternbergii_.
111. _Sigillaria Groeseri_.
112. _Stigmaria ficoides_.
113. _Trigonocarpum ovatum_.
114. Microscopic section of Foraminiferal
limestone--Carboniferous, North America.
115. _Fusulina cylindrica_.
116. Group of Carboniferous Corals.
117. _Platycrinus tricontadactylus_.
118. _Pentremites pyriformis_ and _P. conoideus_.
119. _Archoeocidaris ellipticus_.
120. _Spirorbis Carbonarius_.
121. _Prestwichia rotundata_.
122. Group of Carboniferous Crustaceans.
123. _Cyclophthalmus senior_.
124. _Xylobius Sigillarioe_.
125. _Haplophlebium Barnesi_.
126. Group of Carboniferous _Polyzoa_.
127. Group of Carboniferous _Brachiopoda_.
128. _Pupa vetusta_.
129. _Goniatites Fossoe_.
130. _Amblypterus macropterus_.
131. _Cochliodus contortus_.
132. _Anthracosaurus Russelli_.
133. Generalised section of the Permian rocks.
134. _Walchia piniformis_.
135. Group of Permian _Brachiopods_.
136. _Arca antiqua_.
137. _Platysomus gibbosus_.
138. _Protorosaurus Speneri_.
139. Generalised section of the Triassic rocks.
140. _Zamia spiralis_.
141. Triassic Conifers and Cycads.
142. _Encrinus liliiformis_.
143. _Aspidura loricata_.
144. Group of Triassic Bivalves.
145. _Ceratites nodosus_.
146. Tooth of _Ceratodus serratus_ and _C. Altus_.
147. _Ceratodus Fosteri_.
148. Footprints of _Cheirotherium_.
149. Section of tooth of _Labyrinthodont_.
150. Skull of _Mastodonsaurus_.
151. Skull of _Rhynchosaurus_.
152. _Belodon_, _Nothosaurus_, _Paloeosaurus_, &c.
153. _Placodus gigas_.
154. Skulls of _Dicynodon_ and _Oudenodon_.
155. Supposed footprint of Bird, from the Trias of Connecticut.
156. Lower jaw of _Dromatherium sylvestre_.
157. Molar tooth of _Microlestes antiquus_.
158. _Myrmecobius fasciatus_.
159. Generalised section of the Jurassic rocks.
160. _Mantellia megalophylla_.
161. _Thecosmilia annularis_.
162. _Pentacrinus fasciculosus_.
163. _Hemicidaris crenularis_.
164. _Eryon arctiformis_.
165. Group of Jurassic Brachiopods.
166. _Ostrea Marshii_.
167. _Gryphoea incurva_
168. _Diceras arietina_.
169. _Nerinoea Goodhallii_.
170. _Ammonites Humphresianus_.
171. _Ammonites bifrons_.
172. _Beloteuthis subcostata_.
173. Belemnite restored; diagram of Belemnite; _Belemnites
canaliculata_.
174. _Tetragonolepis_.
175. _Acrodus nobilis_.
176. _Ichthyosaurus communis_.
177. _Plesiosaurus dolichodeirus_.
178. _Pterodactylus crassirostris_.
179. _Ramphorhynchus Bucklandi_, restored.
180. Skull of _Megalosaurus_.
181. _Archoeopteryx macrura_.
182. _Archoeopteryx, restored_.
183. Jaw of _Amphitherium Prevostii_.
184. Jaws of Oolitic Mammals.
185. Generalised section of the Cretaceous rocks.
186. Cretaceous Angiosperms.
187. _Rotalia Boueana_.
188. _Siphonia ficus_.
189. _Ventriculites simplex_.
190. _Synhelia Sharpeana_.
191. _Galerites albogalerus_.
192. _Discoidea cylindrica_.
193. _Escharina Oceani_.
194. _Terebratella Astieriana_.
195. _Crania Ignabergensis_.
196. _Ostrea Couloni_.
197. _Spondylus spinosus_.
198. _Inoceramus sulcatus_.
199. _Hippurites Toucasiana_.
200. _Voluta elongata_.
201. _Nautilus Danicus_.
202. _Ancyloceras Matheronianus_.
203. _Turrilites catenatus_
204. Forms of Cretaceous _Ammonitidoe_.
205. _Belemnitella mucronata_.
206. Tooth of _Hybodus_.
207. Fin-spine of _Hybodus_.
208. _Beryx Lewesiensis_ and _Osmeroides Mantelli_.
209. Teeth of _Iguanodon_.
210. Skull of _Mosasaurus Camperi_.
211. _Chelone Benstedi_.
212. Jaws and vertebrae of _Odontornithes_.
213. Fruit of _Nipadites_.
214. _Nummulina loevigata_.
215. _Turbinolia sulcata_.
216. _Cardita planicosta_.
217. _Typhis tubifer_.
218. _Cyproea elegans_.
219. _Cerithium hexagonum_.
220. _Limnoea pyramidalis_.
221. _Physa columnaris_.
222. _Cyclostoma Arnoudii_.
223. _Rhombus minimus_.
224. _Otodus obliquus_.
225. _Myliobatis Edwardsii_.
226. Upper jaw of Alligator.
227. Skull of _Odontopteryx toliapicus_.
228. _Zeuglodon cetoides_.
229. _Paloeotherium magnum_, restored.
230. Feet of _Equidoe_.
231. _Anoplothelium commune_.
232. Skull of _Dinoceras mirabilis_.
233. _Vespertilio Parisiensis_.
234. Miocene Palms.
235. _Platanus aceroides_.
236. _Cinnamomum polymorphum_.
237. _Textularia Meyeriana_.
238. _Scutella subrotunda_.
239. _Hyalea Orbignyana_.
240. Tooth of _Oxyrhina_.
241. Tooth of _Carcharodon_.
242. _Andrias Scheuchzeri_.
243. Skull of _Brontotherium ingens_.
244, _Hippopotamus Sivalensis_.
245. Skull of _Sivatherium_.
246. Skull of _Deinotherium_.
247. Tooth of _Elephas planfrons_ and of _Mastodon
Sivalensis_.
248. Jaw of _Pliopithecus_.
249. _Rhinoceros Etruscus_ and _R. megarhinus_.
250. Molar tooth of _Mastodon Arvernensis_.
251. Molar tooth of _Etephas meridionalis_.
252. Molar tooth of _Elephas antiquus_.
253. Skull and tooth of _Machairodus cultridens_.
254. _Pecten Islandicus_.
255. Diagram of high-level and low-level gravels.
256. Diagrammatic section of Cave.
257. _Dinornis elephantopus_.
258. Skull of _Diprotodon_.
259. Skull of _Thylacoleo_.
260. Skeleton of _Megatherium_.
261. Skeleton of _Mylodon_.
262. _Glyptodon clavipes_.
263. Skull of _Rhinoceros tichorhinus_.
264. Skeleton of _Cervus megaceros_.
265. Skull of _Bos primigenius_.
266. Skeleton of Mammoth.
267. Molar tooth of Mammoth.
268. Skull of _Ursus speloeus_.
269. Skull of _Hyoena speloea_.
270. Lower jaw of _Trogontherium Cuvieri_.
PART I.
PRINCIPLES OF PALAEONTOLOGY.
INTRODUCTION.
THE LAWS OF GEOLOGICAL ACTION.
Under the general title of "Geology" are usually included at
least two distinct branches of inquiry, allied to one another in
the closest manner, and yet so distinct as to be largely capable
of separate study. _Geology_,[1] in its strict sense, is the
science which is concerned with the investigation of the materials
which compose the earth, the methods in which those materials
have been arranged, and the causes and modes of origin of these
arrangements. In this limited aspect, Geology is nothing more than
the Physical Geography of the past, just as Physical Geography
is the Geology of to-day; and though it has to call in the aid
of Physics, Astronomy, Mineralogy, Chemistry, and other allies
more remote, it is in itself a perfectly distinct and individual
study. One has, however, only to cross the threshold of Geology
to discover that the field and scope of the science cannot be
thus rigidly limited to purely physical problems. The study of
the physical development of the earth throughout past ages brings
us at once in contact with the forms of animal and vegetable
life which peopled its surface in bygone epochs, and it is found
impossible adequately to comprehend the former, unless we possess
some knowledge of the latter. However great its physical advances
may be, Geology remains imperfect till it is wedded with
Palaeontology,[2] a study which essentially belongs to the vast
complex of the Biological Sciences, but at the same time has its
strictly geological side. Dealing, as it does, wholly with the
consideration of such living beings as do not belong exclusively
to the present order of things, Palaeontology is, in reality, a
branch of Natural History, and may be regarded as substantially
the Zoology and Botany of the past. It is the ancient life-history
of the earth, as revealed to us by the labours of palaeontologists,
with which we have mainly to do here; but before entering upon
this, there are some general questions, affecting Geology and
Palaeontology alike, which may be very briefly discussed.
[Footnote 1: Gr. _ge_, the earth; _logos_, a discourse.]
[Footnote 2: Gr. _palaios_, ancient; _onta_, beings; _logos_,
discourse.]
The working geologist, dealing in the main with purely physical
problems, has for his object to determine the material structure
of the earth, and to investigate, as far as may be, the long chain
of causes of which that structure is the ultimate result. No wider
or more extended field of inquiry could be found; but philosophical
geology is not content with this. At all the confines of his
science, the transcendental geologist finds himself confronted
with some of the most stupendous problems which have ever engaged
the restless intellect of humanity. The origin and primaeval
constitution of the terrestrial globe, the laws of geologic action
through long ages of vicissitude and development, the origin of
life, the nature and source of the myriad complexities of living
beings, the advent of man, possibly even the future history of
the earth, are amongst the questions with which the geologist
has to grapple in his higher capacity.
These are problems which have occupied the attention of philosophers
in every age of the world, and in periods long antecedent to
the existence of a science of geology. The mere existence of
cosmogonies in the religion of almost every nation, both ancient
and modern, is a sufficient proof of the eager desire of the
human mind to know something of the origin of the earth on which
we tread. Every human being who has gazed on the vast panorama
of the universe, though it may have been but with the eyes of
a child, has felt the longing to solve, however imperfectly,
"the riddle of the painful earth," and has, consciously or
unconsciously, elaborated some sort of a theory as to the why and
wherefore of what he sees. Apart from the profound and perhaps
inscrutable problems which lie at the bottom of human existence,
men have in all ages invented theories to explain the common
phenomena of the material universe; and most of these theories,
however varied in their details, turn out on examination to have a
common root, and to be based on the same elements. Modern geology
has its own theories on the same subject, and it will be well to
glance for a moment at the principles underlying the old and
the new views.
It has been maintained, as a metaphysical hypothesis, that there
exists in the mind of man an inherent principle, in virtue of
which he believes and expects that what has been, will be; and
that the course of nature will be a continuous and uninterrupted
one. So far, however, from any such belief existing as a necessary
consequence of the constitution of the human mind, the real fact
seems to be that the contrary belief has been almost universally
prevalent. In all old religions, and in the philosophical systems
of almost all ancient nations, the order of the universe has
been regarded as distinctly unstable, mutable, and temporary.
A beginning and an end have always been assumed, and the course
of terrestrial events between these two indefinite points has
been regarded as liable to constant interruption by revolutions
and catastrophes of different kinds, in many cases emanating from
supernatural sources. Few of the more ancient theological creeds,
and still fewer of the ancient philosophies, attained body and
shape without containing, in some form or another, the belief
in the existence of periodical convulsions, and of alternating
cycles of destruction and repair.
That geology, in its early infancy, should have become imbued
with the spirit of this belief, is no more than might have been
expected; and hence arose the at one time powerful and
generally-accepted doctrine of "Catastrophism." That the succession
of phenomena upon the globe, whereby the earth's crust had assumed
the configuration and composition which we find it to possess,
had been a discontinuous and broken succession, was the almost
inevitable conclusion of the older geologists. Everywhere in
their study of the rocks they met with apparently impassable
gaps, and breaches of continuity that could not be bridged over.
Everywhere they found themselves conducted abruptly from one system
of deposits to others totally different in mineral character or
in stratigraphical position. Everywhere they discovered that
well-marked and easily recognisable groups of animals and plants
were succeeded, without the intermediation of any obvious lapse
of time, by other assemblages of organic beings of a different
character. Everywhere they found evidence that the earth's crust
had undergone changes of such magnitude as to render it seemingly
irrational to suppose that they could have been produced by any
process now in existence. If we add to the above the prevalent
belief of the time as to the comparative brevity of the period
which had elapsed since the birth of the globe, we can readily
understand the general acceptance of some form of catastrophism
amongst the earlier geologists.
As regards its general sense and substance, the doctrine of
catastrophism held that the history of the earth, since first
it emerged from the primitive chaos, had been one of periods
of repose, alternating with catastrophes and cataclysms of a
more or less violent character. The periods of tranquillity were
supposed to have been long and protracted; and during each of them
it was thought that one of the great geological "formations" was
deposited. In each of these periods, therefore, the condition of
the earth was supposed to be much the same as it is now--sediment
was quietly accumulated at the bottom of the sea, and animals and
plants flourished uninterruptedly in successive generations.
Each period of tranquillity, however, was believed to have been,
sooner or later, put an end to by a sudden and awful convulsion
of nature, ushering in a brief and paroxysmal period, in which
the great physical forces were unchained and permitted to spring
into a portentous activity. The forces of subterranean fire,
with their concomitant phenomena of earthquake and volcano, were
chiefly relied upon as the efficient causes of these periods of
spasm and revolution. Enormous elevations of portions of the
earth's crust were thus believed to be produced, accompanied by
corresponding and equally gigantic depressions of other portions.
In this way new ranges of mountains were produced, and previously
existing ranges levelled with the ground, seas were converted into
dry land, and continents buried beneath the ocean--catastrophe
following catastrophe, till the earth was rendered uninhabitable,
and its races of animals and plants were extinguished, never to
reappear in the same form. Finally, it was believed that this
feverish activity ultimately died out, and that the ancient peace
once more came to reign upon the earth. As the abnormal throes
and convulsions began to be relieved, the dry land and sea once
more resumed their relations of stability, the conditions of
life were once more established, and new races of animals and
plants sprang into existence, to last until the supervention
of another fever-fit.
Such is the past history of the globe, as sketched for us, in
alternating scenes of fruitful peace and revolutionary destruction,
by the earlier geologists. As before said, we cannot wonder at the
former general acceptance of Catastrophistic doctrines. Even in
the light of our present widely-increased knowledge, the series
of geological monuments remains a broken and imperfect one; nor
can we ever hope to fill up completely the numerous gaps with
which the geological record is defaced. Catastrophism was the
natural method of accounting for these gaps, and, as we shall see,
it possesses a basis of truth. At present, however, catastrophism
may be said to be nearly extinct, and its place is taken by the
modern doctrine of "Continuity" or "Uniformity"--a doctrine with
which the name of Lyell must ever remain imperishably associated.
The fundamental thesis of the doctrine of Uniformity is, that,
in spite of all apparent violations of continuity, the sequence
of geological phenomena has in reality been a regular and
uninterrupted one; and that the vast changes which can be shown
to have passed over the earth in former periods have been the
result of the slow and ceaseless working of the ordinary physical
forces--acting with no greater intensity than they do now, but
acting through enormously prolonged periods. The essential element
in the theory of Continuity is to be found in the allotment of
indefinite time for the accomplishment of the known series of
geological changes. It is obviously the case, namely, that there
are two possible explanations of all phenomena which lie so far
concealed in "the dark backward and abysm of time," that we can
have no direct knowledge of the manner in which they were produced.
We may, on the one hand, suppose them to be the result of some
very powerful cause, acting through a short period of time. That
is Catastrophism. Or, we may suppose them to be caused by a much
weaker force operating through a proportionately prolonged period.
This is the view of the Uniformitarians. It is a question of
_energy_ versus _time_ and it is _time_ which is the true element
of the case. An earthquake may remove a mountain in the course
of a few seconds; but the dropping of the gentle rain will do
the same, if we extend its operations over a millennium. And
this is true of all agencies which are now at work, or ever have
been at work, upon our planet. The Catastrophists, believing
that the globe is but, as it were, the birth of yesterday, were
driven of necessity to the conclusion that its history had been
checkered by the intermittent action of paroxysmal and almost
inconceivably potent forces. The Uniformitarians, on the other
hand, maintaining the "adequacy of existing causes," and denying
that the known physical forces ever acted in past time with greater
intensity than they do at present, are, equally of necessity,
driven to the conclusion that the world is truly in its "hoary
eld," and that its present state is really the result of the
tranquil and regulated action of known forces through unnumbered
and innumerable centuries.
The most important point for us, in the present connection, is
the bearing of these opposing doctrines upon the question, as
to the origin of the existing terrestrial order. On any doctrine
of uniformity that order has been evolved slowly, and, according
to law, from a pre-existing order. Any doctrine of catastrophism,
on the other hand, carries with it, by implication, the belief
that the present order of things was brought about suddenly and
irrespective of any pre-existent order; and it is important to
hold clear ideas as to which of these beliefs is the true one. In
the first place, we may postulate that the world had a beginning,
and, equally, that the existing terrestrial order had a beginning.
However far back we may go, geology does not, and cannot, reach the
actual beginning of the world; and we are, therefore, left simply
to our own speculations on this point. With regard, however, to
the existing terrestrial order, a great deal can be discovered,
and to do so is one of the principal tasks of geological science.
The first steps in the production of that order lie buried in
the profound and unsearchable depths of a past so prolonged as
to present itself to our finite minds as almost in eternity.
The last steps are in the prophetic future, and can be but dimly
guessed at. Between the remote past and the distant future, we
have, however, a long period which is fairly open to inspection;
and in saying a "long" period, it is to be borne in mind that
this term is used in its _geological_ sense. Within this period,
enormously long as it is when measured by human standards, we
can trace with reasonable certainty the progressive march of
events, and can determine the laws of geological action, by which
the present order of things has been brought about.
The natural belief on this subject doubtless is, that the world,
such as we now see it, possessed its present form and configuration
from the beginning. Nothing can be more natural than the belief
that the present continents and oceans have always been where
they are now; that we have always had the same mountains and
plains; that our rivers have always had their present courses,
and our lakes their present positions; that our climate has always
been the same; and that our animals and plants have always been
identical with those now familiar to us. Nothing could be more
natural than such a belief, and nothing could be further removed
from the actual truth. On the contrary, a very slight acquaintance
with geology shows us, in the words of Sir John Herschel, that
"the actual configuration of our continents and islands, the
coast-lines of our maps, the direction and elevation of our
mountain-chains, the courses of our rivers, and the soundings
of our oceans, are not things primordially arranged in the
construction of our globe, but results of successive and complex
actions on a former state of things; _that_, again, of similar
actions on another still more remote; and so on, till the original
and really permanent state is pushed altogether out of sight
and beyond the reach even of imagination; while on the other
hand, a similar, and, as far as we can see, interminable vista
is opened out for the future, by which the habitability of our
planet is secured amid the total abolition on it of the present
theatres of terrestrial life."
Geology, then, teaches us that the physical features which now
distinguish the earth's surface have been produced as the ultimate
result of an almost endless succession of precedent changes.
Palaeontology teaches us, though not yet in such assured accents,
the same lesson. Our present animals and plants have not been
produced, in their innumerable forms, each as we now know it,
as the sudden, collective, and simultaneous birth of a renovated
world. On the contrary, we have the clearest evidence that some
of our existing animals and plants made their appearance upon the
earth at a much earlier period than others. In the confederation
of animated nature some races can boast of an immemorial antiquity,
whilst others are comparative _parvenus_. We have also the clearest
evidence that the animals and plants which now inhabit the globe
have been preceded, over and over again, by other different
assemblages of animals and plants, which have flourished in
successive periods of the earth's history, have reached their
culmination, and then have given way to a fresh series of living
beings. We have, finally, the clearest evidence that these successive
groups of animals and plants (faunae and florae) are to a greater
or less extent directly connected with one another. Each group
is, to a greater or less extent, the lineal descendant of the
group which immediately preceded it in point of time, and is
more or less fully concerned with giving origin to the group
which immediately follows it. That this law of "evolution" has
prevailed to a great extent is quite certain; but it does not
meet all the exigencies of the case, and it is probable that
its action has been supplemented by some still unknown law of
a different character.
We shall have to consider the question of geological "continuity"
again. In the meanwhile, it is sufficient to state that this
doctrine is now almost universally accepted as the basis of all
inquiries, both in the domain of geology and that of palaeontology.
The advocates of continuity possess one immense advantage over
those who believe in violent and revolutionary convulsions, that
they call into play only agencies of which we have actual knowledge.
We _know_ that certain forces are now at work, producing certain
modifications in the present condition of the globe; and we _know_
that these forces are capable of producing the vastest of the
changes which geology brings under our consideration, provided
we assign a time proportionately vast for their operation. On
the other hand, the advocates of catastrophism, to make good
their views, are compelled to invoke forces and actions, both
destructive and restorative, of which we have, and can have, no
direct knowledge. They endow the whirlwind and the earthquake,
the central fire and the rain from heaven, with powers as mighty
as ever imagined in fable, and they build up the fragments of a
repeatedly shattered world by the intervention of an intermittently
active creative power.
It should not be forgotten, however, that from one point of view
there is a truth in catastrophism which is sometimes overlooked
by the advocates of continuity and uniformity. Catastrophism
has, as its essential feature, the proposition that the known
and existing forces of the earth at one time acted with much
greater intensity and violence than they do at present, and they
carry down the period of this excessive action to the commencement
of the present terrestrial order. The Uniformitarians, in effect,
deny this proposition, at any rate as regards any period of the
earth's history of which we have actual cognisance. If, however,
the "nebular hypothesis" of the origin of the universe be well
founded--as is generally admitted--then, beyond question, the
earth is a gradually cooling body, which has at one time been
very much hotter than it is at present. There has been a time,
therefore, in which the igneous forces of the earth, to which we
owe the phenomena of earthquakes and volcanoes, must have been
far more intensely active than we can conceive of from anything
that we can see at the present day. By the same hypothesis, the
sun is a cooling body, and must at one time have possessed a
much higher temperature than it has at present. But increased
heat of the sun would seriously alter the existing conditions
affecting the evaporation and precipitation of moisture on our
earth; and hence the aqueous forces may also have acted at one
time more powerfully than they do now. The fundamental principle
of catastrophism is, therefore, not wholly vicious; and we have
reason to think that there must have been periods--very remote,
it is true, and perhaps unrecorded in the history of the earth--in
which the known physical forces may have acted with an intensity
much greater than direct observation would lead us to imagine.
And this may be believed, altogether irrespective of those great
secular changes by which hot or cold epochs are produced, and
which can hardly be called "catastrophistic," as they are produced
gradually, and are liable to recur at definite intervals.
Admitting, then, that there _is_ a truth at the bottom of the once
current doctrines of catastrophism, still it remains certain that
the history of the earth has been one of _law_ in all past time,
as it is now. Nor need we shrink back affrighted at the vastness
of the conception--the vaster for its very vagueness--that we
are thus compelled to form as to the duration of _geological
time_. As we grope our way backward through the dark labyrinth
of the ages, epoch succeeds to epoch, and period to period, each
looming more gigantic in its outlines and more shadowy in its
features, as it rises, dimly revealed, from the mist and vapour
of an older and ever-older past. It is useless to add century
to century or millennium to millennium. When we pass a certain
boundary-line, which, after all, is reached very soon, figures
cease to convey to our finite faculties any real notion of the
periods with which we have to deal. The astronomer can employ
material illustrations to give form and substance to our conceptions
of celestial space; but such a resource is unavailable to the
geologist. The few thousand years of which we have historical
evidence sink into absolute insignificance beside the unnumbered
aeons which unroll themselves one by one as we penetrate the dim
recesses of the past, and decipher with feeble vision the ponderous
volumes in which the record of the earth is written. Vainly does
the strained intellect seek to overtake an ever-receding
commencement, and toil to gain some adequate grasp of an apparently
endless succession. A beginning there must have been, though we
can never hope to fix its point. Even speculation droops her
wings in the attenuated atmosphere of a past so remote, and the
light of imagination is quenched in the darkness of a history so
ancient. In _time_, as in _space_, the confines of the universe
must ever remain concealed from us, and of the end we know no
more than of the beginning. Inconceivable as is to us the lapse
of "geological time," it is no more than "a mere moment of the
past, a mere infinitesimal portion of eternity." Well may "the
human heart, that weeps and trembles," say, with Richter's pilgrim
through celestial space, "I will go no farther; for the spirit of
man acheth with this infinity. Insufferable is the glory of God.
Let me lie down in the grave, and hide me from the persecution
of the Infinite, for end, I see, there is none."
CHAPTER I.
THE SCOPE AND MATERIALS OF PALAEONTOLOGY.
The study of the rock-masses which constitute the crust of the
earth, if carried out in the methodical and scientific manner of
the geologist, at once brings us, as has been before remarked, in
contact with the remains or traces of living beings which formerly
dwelt upon the globe. Such remains are found, in greater or less
abundance, in the great majority of rocks; and they are not only of
great interest in themselves, but they have proved of the greatest
importance as throwing light upon various difficult problems in
geology, in natural history, in botany, and in philosophy. Their
study constitutes the science of palaeontology; and though it is
possible to proceed to a certain length in geology and zoology
without much palaeontological knowledge, it is hardly possible to
attain to a satisfactory general acquaintance with either of
these subjects without having mastered the leading facts of the
first. Similarly, it is not possible to study palaeontology without
some acquaintance with both geology and natural history.
Palaeontology, then, is the science which treats of the living
beings, whether animal or vegetable, which have inhabited the earth
during past periods of its history. Its object is to elucidate,
as far as may be, the structure, mode of existence, and habits
of all such ancient forms of life; to determine their position
in the scale of organised beings; to lay down the geographical
limits within which they flourished; and to fix the period of
their advent and disappearance. It is the ancient life-history
of the earth; and were its record complete, it would furnish
us with a detailed knowledge of the form and relations of all
the animals and plants which have at any period flourished upon
the land-surfaces of the globe or inhabited its waters; it would
enable us to determine precisely their succession in time; and
it would place in our hands an unfailing key to the problems of
evolution. Unfortunately, from causes which will be subsequently
discussed, the palaeontological record is extremely imperfect,
and our knowledge is interrupted by gaps, which not only bear
a large proportion to our solid information, but which in many
cases are of such a nature that we can never hope to fill them
up.
Fossils.--The remains of animals or vegetables which we now find
entombed in the solid rock, and which constitute the working
material of the palaeontologist, are termed "fossils,"[3] or
"petrifactions." In most cases, as can be readily understood,
fossils are the actual hard parts of animals and plants which
were in existence when the rock in which they are now found was
being deposited. Most fossils, therefore, are of the nature of
the shells of shell-fish, the skeletons of coral-zoophytes, the
bones of vertebrate animals, or the wood, bark, or leaves of
plants. All such bodies are more or less of a hard consistence
to begin with, and are capable of resisting decay for a longer
or shorter time--hence the frequency with which they occur in
the fossil condition. Strictly speaking, however, by the term
"fossil" must be understood "any body, _or the traces of the
existence of any body_, whether animal or vegetable, which has
been buried in the earth by natural causes" (Lyell). We shall
find, in fact, that many of the objects which we have to study
as "fossils" have never themselves actually formed parts of any
animal or vegetable, though they are due to the former existence
of such organisms, and indicate what was the nature of these.
Thus the footprints left by birds, or reptiles, or quadrupeds
upon sand or mud, are just as much proofs of the former existence
of these animals as would be bones, feathers, or scales, though
in themselves they are inorganic. Under the head of fossils,
therefore, come the footprints of air-breathing vertebrate animals;
the tracks, trails, and burrows of sea-worms, crustaceans, or
molluscs; the impressions left on the sand by stranded jelly-fishes;
the burrows in stone or wood of certain shell-fish; the "moulds"
or "casts" of shells, corals, and other organic remains; and
various other bodies of a more or less similar nature.
[Footnote 3: Lat. _fossus_, dug up.]
Fossilisation.-- The term "fossilisation" is applied to all those
processes through which the remains of organised beings may pass
in being converted into fossils. These processes are numerous
and varied; but there are three principal modes of fossilisation
which alone need be considered here. In the first instance, the
fossil is to all intents and purposes an actual portion of the
original organised being--such as a bone, a shell, or a piece
of wood. In some rare instances, as in the case of the body of
the Mammoth discovered embedded in ice at the mouth of the Lena
in Siberia, the fossil may be preserved almost precisely in its
original condition, and even with its soft parts uninjured. More
commonly, certain changes have taken place in the fossil, the
principal being the more or less total removal of the organic
matter originally present. Thus bones become light and porous
by the removal of their gelatine, so as to cleave to the tongue
on being applied to that organ; whilst shells become fragile, and
lose their primitive colours. In other cases, though practically
the real body it represents, all the cavities of the fossil,
down to its minutest recesses, may have become infiltrated with
mineral matter. It need hardly be added, that it is in the more
modern rocks that we find the fossils, as a rule, least changed
from their former condition; but the original structure is often
more or less completely retained in some of the fossils from
even the most ancient formations.
In the second place, we very frequently meet with fossils in the
state of "casts" or moulds of the original organic body. What
occurs in this case will be readily understood if we imagine any
common bivalve shell, as an Oyster, or Mussel, or Cockle, embedded
in clay or mud. If the clay were sufficiently soft and fluid, the
first thing would be that it would gain access to the interior
of the shell, and would completely fill up the space between the
valves. The pressure, also, of the surrounding matter would insure
that the clay would everywhere adhere closely to the exterior of
the shell. If now we suppose the clay to be in any way hardened
so as to be converted into stone, and if we were to break up the
stone, we should obviously have the following state of parts.
The clay which filled the shell would form an accurate cast of
the _interior_ of the shell, and the clay outside would give us
an exact impression or cast of the _exterior_ of the shell (fig.
1). We should have, then, two casts, an interior and an exterior,
and the two would be very different to one another, since the
inside of a shell is very unlike the outside. In the case, in
fact, of many univalve shells, the interior cast or "mould" is
so unlike the exterior cast, or unlike the shell itself, that
it may be difficult to determine the true origin of the former.
[Illustration: Fig. 1.--_Trigonia longa_, showing casts to of
the exterior and interior of the shell.--Cretaceous (Neocomian).]
It only remains to add that there is sometimes a further
complication. If the rock be very porous and permeable by water,
it may happen that the original shell is entirely dissolved away,
leaving the interior cast loose, like the kernel of a nut, within
the case formed by the exterior cast. Or it may happen that
subsequent to the attainment of this state of things, the space
thus left vacant between the interior and exterior cast--the space,
that is, formerly occupied by the shell itself--may be filled up
by some foreign mineral deposited there by the infiltration of
water. In this last case the splitting open of the rock would
reveal an interior cast, an exterior cast, and finally a body
which would have the exact form of the original shell, but which
would be really a much later formation, and which would not exhibit
under the microscope the minute structure of shell.
[Illustration: Fig. 2.--Microscopic section of the silicified
wood of a Conifer (_Sequoia_) cut in the long direction of the
fibres. Post-tertiary? Colorado. (Original.)]
[Illustration: Footnote: Fig. 3.--Microscopic section of the wood
of the common Larch (_Abies larix_), cut in the long direction of
the fibres. In both the fresh and the fossil wood (fig. 2) are
seen the discs characteristic of coniferous wood. (Original.)]
In the third class of cases we have fossils which present with
the greatest accuracy the external form, and even sometimes the
internal minute structure, of the original organic body, but
which, nevertheless, are not themselves truly organic, but have
been formed by a "replacement" of the particles of the primitive
organism by some mineral substance. The most elegant example
of this is afforded by fossil wood which has been "silicified"
or converted into flint (_silex_). In such cases we have fossil
wood which presents the rings of growth and fibrous structure of
recent wood, and which under the microscope exhibits the minutest
vessels which characterise ligneous tissue, together with the even
more minute markings of the vessels (fig. 2). The whole, however,
instead of being composed of the original carbonaceous matter of
the wood, is now converted into flint. The only explanation that
can be given of this by no means rare phenomenon, is that the
wood must have undergone a slow process of decay in water charged
with silica or flint in solution. As each successive particle of
wood was removed by decay, its place was taken by a particle of
flint deposited from the surrounding water, till ultimately the
entire wood was silicified. The process, therefore, resembles
what would take place if we were to pull down a house built of
brick by successive bricks, replacing each brick as removed by
a piece of stone of precisely the same size and form. The result
of this would be that the house would retain its primitive size,
shape, and outline, but it would finally have been converted
from a house of brick into a house of stone. Many other fossils
besides wood--such as shells, corals, sponges, &c.--are often
found silicified; and this may be regarded as the commonest form
of fossilisation by replacement. In other cases, however, though
the principle of the process is the same, the replacing substance
may be iron pyrites, oxide of iron, sulphur, malachite, magnesite,
talc, &c.; but it is rarely that the replacement with these minerals
is so perfect as to preserve the more delicate details of internal
structure.
CHAPTER II.
THE FOSSILIFEROUS ROCKS.
Fossils are found in rocks, though not universally or promiscuously;
and it is therefore necessary that the palaeontologist should
possess some acquaintance with, at any rate, those rocks which
yield organic remains, and which are therefore said to be
"_fossiliferous_." In geological language, all the materials
which enter into the composition of the solid crust of the earth,
be their texture what it may--from the most impalpable mud to
the hardest granite--are termed "rocks;" and for our present
purpose we may divide these into two great groups. In the first
division are the _Igneous Rocks_--such as the lavas and ashes of
volcanoes--which are formed within the body of the earth itself,
and which owe their structure and origin to the action of heat.
The Igneous Rocks are formed primarily below the surface of the
earth, which they only reach as the result of volcanic action;
they are generally destitute of distinct "stratification," or
arrangement in successive layers; and they do not contain fossils,
except in the comparatively rare instances where volcanic ashes
have enveloped animals or plants which were living in the sea
or on the land in the immediate vicinity of the volcanic focus.
The second great division of rocks is that of the _Fossiliferous,
Aqueous_, or _Sedimentary_ Rocks. These are formed at the surface
of the earth, and, as implied by one of their names, are invariably
deposited in water. They are produced by vital or chemical action,
or are formed from the "sediment" produced by the disintegration
and reconstruction of previously existing rocks, without previous
solution; they mostly contain fossils; and they are arranged
in distinct layers or "strata." The so-called "aerial" rocks
which, like beds of blown sand, have been formed by the action
of the atmosphere, may also contain fossils; but they are not
of such importance as to require special notice here.
For all practical purposes, we may consider that the Aqueous
Rocks are the natural cemetery of the animals and plants of bygone
ages; and it is therefore essential that the palaeontological
student should be acquainted with some of the principal facts as
to their physical characters, their minute structure and mode of
origin, their chief varieties, and their historical succession.
The Sedimentary or Fossiliferous Rocks form the greater portion of
that part of the earth's crust which is open to our examination, and
are distinguished by the fact that they are regularly "stratified" or
arranged in distinct and definite layers or "strata." These layers
may consist of a single material, as in a block of sandstone, or
they may consist of different materials. When examined on a large
scale, they are always found to consist of alternations of layers
of different mineral composition. We may examine any given area,
and find in it nothing but one kind of rock--sandstone, perhaps,
or limestone. In all cases, however, if we extend our examination
sufficiently far, we shall ultimately come upon different rocks;
and, as a general rule, the thickness of any particular set of
beds is comparatively small, so that different kinds of rock
alternate with one another in comparatively small spaces.
[Illustration: Fig. 4.--Sketch of Carboniferous strata at Kinghorn,
in Fife, showing stratified beds (limestone and shales) surmounted
by an unstratified mass of trap. (Original.)]
As regards the origin of the Sedimentary Rocks, they are for
the most part "derivative" rocks, being derived from the wear
and tear of pre-existent rocks. Sometimes, however, they owe
their origin to chemical or vital action, when they would more
properly be spoken of simply as Aqueous Rocks. As to their mode
of deposition, we are enabled to infer that the materials which
compose them have formerly been spread out by the action of water,
from what we see going on every day at the mouths of our great
rivers, and on a smaller scale wherever there is running water.
Every stream, where it runs into a lake or into the sea, carries
with it a burden of mud, sand, and rounded pebbles, derived from
the waste of the rocks which form its bed and banks. When these
materials cease to be impelled by the force of the moving water,
they sink to the bottom, the heaviest pebbles, of course, sinking
first, the smaller pebbles and sand next, and the finest mud
last. Ultimately, therefore, as might have been inferred upon
theoretical grounds, and as is proved by practical experience,
every lake becomes a receptacle for a series of stratified rocks
produced by the streams flowing into it. These deposits may vary
in different parts of the lake, according as one stream brought
down one kind of material and another stream contributed another
material; but in all cases the materials will bear ample evidence
that they were produced, sorted, and deposited by running water.
The finer beds of clay or sand will all be arranged in thicker or
thinner layers or laminae; and if there are any beds of pebbles
these will all be rounded or smooth, just like the water-worn
pebbles of any brook-course. In all probability, also, we should
find in some of the beds the remains of fresh-water shells or
plants or other organisms which inhabited the lake at the time
these beds were being deposited.
In the same way large rivers--such as the Ganges or
Mississippi--deposit all the materials which they bring down
at their mouths, forming in this way their "deltas." Whenever
such a delta is cut through, either by man or by some channel of
the river altering its course, we find that it is composed of a
succession of horizontal layers or strata of sand or mud, varying
in mineral composition, in structure, or in grain, according to
the nature of the materials brought down by the river at different
periods. Such deltas, also, will contain the remains of animals
which inhabit the river, with fragments of the plants which grew
on its banks, or bones of the animals which lived in its basin.
Nor is this action confined, of course, to large rivers only,
though naturally most conspicuous in the greatest bodies of water.
On the contrary, all streams, of whatever size, are engaged in
the work of wearing down the dry land, and of transporting the
materials thus derived from higher to lower levels, never resting
in this work till they reach the sea.
[Illustration: Fig. 5.--Diagram to illustrate the formation of
sedimentary deposits at the point where a river debouches into
the sea.]
Lastly, the sea itself--irrespective of the materials delivered
into it by rivers--is constantly preparing fresh stratified deposits
by its own action. Upon every coast-line the sea is constantly
eating back into the land and reducing its component rocks to
form the shingle and sand which we see upon every shore. The
materials thus produced are not, however, lost, but are ultimately
deposited elsewhere in the form of new stratified accumulations,
in which are buried the remains of animals inhabiting the sea
at the time.
Whenever, then, we find anywhere in the interior of the land
any series of beds having these characters--composed, that is,
of distinct layers, the particles of which, both large and small,
show distinct traces of the wearing action of water--whenever and
wherever we find such rocks, we are justified in assuming that
they have been deposited by water in the manner above mentioned.
Either they were laid down in some former lake by the combined
action of the streams which flowed into it; or they were deposited
at the mouth of some ancient river, forming its delta; or they
were laid down at the bottom of the ocean. In the first two cases,
any fossils which the beds might contain would be the remains
of fresh-water or terrestrial organisms. In the last case, the
majority, at any rate, of the fossils would be the remains of
marine animals.
The term "formation" is employed by geologists to express "any
group of rocks which have some character in common, whether of
origin, age, or composition" (Lyell); so that we may speak of
stratified and unstratified formations, aqueous or igneous
formations, fresh-water or marine formations, and so on.
CHIEF DIVISIONS OF THE AQUEOUS ROCKS.
The Aqueous Rocks may be divided into two great sections, the
Mechanically-formed and the Chemically-formed, including under
the last head all rocks which owe their origin to vital action,
as well as those produced by ordinary chemical agencies.
[Illustration: Fig. 6.--Microscopic section of a calcareous breccia
in the Lower Silurian (Coniston Limestone) of Shap Wells,
Westmoreland. The fragments are all of small size, and consist of
angular pieces of transparent quartz, volcanic ashes, and limestone
embedded in a matrix of crystalline limestone. (Original.)]
A. MECHANICALLY-FORMED ROCKS.--These are all those Aqueous Rocks
of which we can obtain proofs that their particles have been
mechanically transported to their present situation. Thus, if
we examine a piece of _conglomerate_ or puddingstone, we find
it to be composed of a number of rounded pebbles embedded in an
enveloping matrix or paste, which is usually of a sandy nature,
but may be composed of carbonate of lime (when the rock is said to
be a "calcareous conglomerate"). The pebbles in all conglomerates
are worn and rounded by the action of water in motion, and thus
show that they have been subjected to much mechanical attrition,
whilst they have been mechanically transported for a greater
or less distance from the rock of which they originally formed
part. The analogue of the old conglomerates at the present day
is to be found in the great beds of shingle and gravel which
are formed by the action of the sea on every coast-line, and
which are composed of water-worn and well-rounded pebbles of
different sizes. A _breccia_ is a mechanically-formed rock, very
similar to a conglomerate, and consisting of larger or smaller
fragments of rock embedded in a common matrix. The fragments,
however, are in this case all more or less angular, and are not
worn or rounded. The fragments in breccias may be of large size,
or they may be comparatively small (fig. 6); and the matrix may
be composed of sand (arenaceous) or of carbonate of lime
(calcareous). In the case of an ordinary sandstone, again, we
have a rock which may be regarded as simply a very fine-grained
conglomerate or breccia, being composed of small grains of sand
(silica), sometimes rounded, sometimes more or less angular,
cemented together by some such substance as oxide of iron, silicate
of iron, or carbonate of lime. A sandstone, therefore, like a
conglomerate is a mechanically-formed rock, its component grams
being equally the result of mechanical attrition and having equally
been transported from a distance; and the same is true of the
ordinary sand of the sea-shore, which is nothing more than an
unconsolidated sandstone. Other so-called sands and sandstones,
though equally mechanical in their origin, are truly calcareous in
their nature, and are more or less entirely composed of carbonate
of lime. Of this kind are the shell-sand so common on our coasts,
and the coral-sand which is so largely formed in the neighbourhood
of coral-reefs. In these cases the rock is composed of fragments
of the skeletons of shellfish, and numerous other marine animals,
together, in many instances, with the remains of certain sea-weeds
(_Corallines_, _Nullipores_, &c,) which are endowed with the
power of secreting carbonate of lime from the sea-water. Lastly,
in certain rocks still finer in their texture than sandstones,
such as the various mud-rocks and shales, we can still recognise
a mechanical source and origin. If slices of any of these rocks
sufficiently thin to be transparent are examined under the
microscope, it will be found that they are composed of minute
grains of different sizes, which are all more or less worn and
rounded, and which clearly show, therefore, that they have been
subjected to mechanical attrition.
All the above-mentioned rocks, then, are _mechanically-formed_
rocks; and they are often spoken of as "Derivative Rocks," in
consequence of the fact that their particles can be shown to
have been mechanically _derived_ from other pre-existent rocks.
It follows from this that every bed of any mechanically-formed
rock is the measure and equivalent of a corresponding amount of
destruction of some older rock. It is not necessary to enter
here into a minute account of the subdivisions of these rocks, but
it may be mentioned that they may be divided into two principal
groups, according to their chemical composition. In the one group
we have the so-called _Arenaceous_ (Lat. _arena_, sand) or
_Siliceous_ Rocks, which are essentially composed of larger or
smaller grains of flint or silica. In this group are comprised
ordinary sand, the varieties of sandstone and grit, and most
conglomerates and breccias. We shall, however, afterwards see
that some siliceous rocks are of organic origin. In the second
group are the so-called _Argillaceous_ (Lat. _argilla_, clay)
Rocks, which contain a larger or smaller amount of clay or hydrated
silicate of alumina in their composition. Under this head come
clays, shales, marls, marl-slate, clay-slates, and most flags
and flagstones.
B. CHEMICALLY-FORMED ROCKS.--In this section are comprised all
those Aqueous or Sedimentary Rocks which have been formed by
chemical agencies. As many of these chemical agencies, however,
are exerted through the medium of living beings, whether animals
or plants, we get into this section a number of what may be called
"_organically-formed rocks_." These are of the greatest possible
importance to the palaeontologist, as being to a greater or less
extent composed of the actual remains of animals or vegetables,
and it will therefore be necessary to consider their character
and structure in some detail.
By far the most important of the chemically-formed rocks are
the so-called _Calcareous Rocks_ (Lat. _calx_, lime), comprising
all those which contain a large proportion of carbonate of lime,
or are wholly composed of this substance. Carbonate of lime is
soluble in water holding a certain amount of carbonic acid gas
in solution; and it is, therefore, found in larger or smaller
quantity dissolved in all natural waters, both fresh and salt,
since these waters are always to some extent charged with the
above-mentioned solvent gas. A great number of aquatic animals,
however, together with some aquatic plants, are endowed with
the power of separating the lime thus held in solution in the
water, and of reducing it again to its solid condition. In this
way shell-fish, crustaceans, sea-urchins, corals, and an immense
number of other animals, are enabled to construct their skeletons;
whilst some plants form hard structures within their tissues
in a precisely similar manner. We do meet with some calcareous
deposits, such as the "stalactites" and "stalagmites" of caves,
the "calcareous tufa" and "travertine" of some hot springs, and
the spongy calcareous deposits of so-called "petrifying springs,"
which are purely chemical in their origin, and owe nothing to the
operation of living beings. Such deposits are formed simply by
the precipitation of carbonate of lime from water, in consequence
of the evaporation from the water of the carbonic acid gas which
formerly held the lime in solution; but, though sometimes forming
masses of considerable thickness and of geological importance,
they do not concern us here. Almost all the limestones which
occur in the series of the stratified rocks are, primarily at any
rate, of _organic_ origin, and have been, directly or indirectly,
produced by the action of certain lime-making animals or plants,
or both combined. The presumption as to all the calcareous rocks,
which cannot be clearly shown to have been otherwise produced,
is that they are thus organically formed; and in many cases this
presumption can be readily reduced to a certainty. There are
many varieties of the calcareous rocks, but the following are
those which are of the greatest importance:--
_Chalk_ is a calcareous rock of a generally soft and pulverulent
texture, and with an earthy fracture. It varies in its purity,
being sometimes almost wholly composed of carbonate of lime,
and at other times more or less intermixed with foreign matter.
Though usually soft and readily reducible to powder, chalk is
occasionally, as in the north of Ireland, tolerably hard and
compact; but it never assumes the crystalline aspect and stony
density of limestone, except it be in immediate contact with
some mass of igneous rock. By means of the microscope, the true
nature and mode of formation of chalk can be determined with
the greatest ease. In the case of the harder varieties, the
examination can be conducted by means of slices ground down to
a thinness sufficient to render them transparent; but in the
softer kinds the rock must be disintegrated under water, and the
_debris_ examined microscopically. When investigated by either
of these methods, chalk is found to be a genuine organic rock,
being composed of the shells or hard parts of innumerable marine
animals of different kinds, some entire, some fragmentary, cemented
together by a matrix of very finely granular carbonate of lime.
Foremost amongst the animal remains which so largely compose
chalk are the shells of the minute creatures which will be
subsequently spoken of under the name of _Foraminifera_ (fig.
7), and which, in spite of their microscopic dimensions, play a
more important part in the process of lime-making than perhaps
any other of the larger inhabitants of the ocean.
[Illustration: Fig. 7.--Section of Gravesend Chalk, examined
by transmitted light and highly magnified. Besides the entire
shells of _Globigerina_, _Rotalia_, and _Textularia_, numerous
detached chambers of _Globigerina_ are seen. (Original.)]
As chalk is found in beds of hundreds of feet in thickness,
and of great purity, there was long felt much difficulty
in satisfactorily accounting for its mode of formation and origin.
By the researches of Carpenter, Wyville Thomson,
Huxley, Wallich, and others, it has, however, been shown
that there is now forming, in the profound depths of our
great oceans, a deposit which is in all essential respects
identical with chalk, and which is
generally known as the "Atlantic ooze," from its having been
first discovered in that sea. This ooze is found at great
depths (5000 to over 15,000 feet) in both the Atlantic and
Pacific, covering enormously large areas of the sea-bottom,
and it presents itself as a whitish-brown, sticky, impalpable mud,
very like greyish chalk when dried. Chemical examination
shows that the ooze is composed almost wholly of carbonate of
lime, and microscopical examination proves it to be of organic
origin, and to be made up of the remains of living beings.
The principal forms of these belong to the _Foraminifera_, and
the commonest of these are the irregularly-chambered shells of
_Globigerina_, absolutely indistinguishable from the
_Globigerinoe_ which are so largely present in the chalk (fig. 8).
Along with these occur fragments of the skeletons of other larger
creatures, and a certain proportion of the flinty cases of minute
animal and vegetable organisms (_Polycystina_ and _Diatoms_).
Though many of the minute animals, the hard parts of which form
the ooze, undoubtedly live at or near the surface of the sea,
others, probably, really live near the bottom; and the ooze
itself forms a congenial home for numerous sponges, sea-lilies,
and other marine animals which flourish at great
depths in the sea. There is thus established an intimate
and most interesting parallelism between the chalk and
the ooze of modern oceans. Both are formed essentially in
the same way, and the latter only requires consolidation to
become actually converted into chalk. Both are fundamentally
organic deposits, apparently requiring a great depth of water
for their accumulation, and mainly composed of the remains of
_Foraminifera_, together with the entire or broken skeletons
of other marine animals of greater dimensions. It is to be
remembered, however, that the ooze, though strictly
representative of the chalk, cannot be said in any proper sense
to be actually _identical_ with the formation so called by
geologists. A great lapse of time separates the two, and though
composed of the remains of representative classes or groups of
animals, it is only in the case of the lowly-organised
_Globigerinoe_, and of some other organisms of little higher
grade, that we find absolutely the same kinds or species of
animals in both.
[Illustration: Fig. 8.--Organisms in the Atlantic Ooze, chiefly
_Foraminifera_ (_Globigerina_ and _Textularia_), with _Polycystina_
and sponge-spicules; highly magnified. (Original.)]
[Illustration: Fig. 9.--Slab of Crinoidal marble, from the
Carboniferous limestone of Dent, in Yorkshire, of the natural
size. The polished surface intersects the columns of the Crinoids
at different angles, and thus gives rise to varying appearances.
(Original.)]
_Limestone_, like chalk, is composed of carbonate of lime, sometimes
almost pure, but more commonly with a greater or less intermixture
of some foreign material, such as alumina or silica. The varieties
of limestone are almost innumerable, but the great majority can
be clearly proved to agree with chalk in being essentially of
organic origin, and in being more or less largely composed of the
remains of living beings. In many instances the organic remains
which compose limestone are so large as to be readily visible to
the naked eye, and the rock is at once seen to be nothing more
than an agglomeration of the skeletons, generally fragmentary, of
certain marine animals, cemented together by a matrix of carbonate
of lime. This is the case, for example, with the so-called "Crinoidal
Limestones" and "Encrinital Marbles" with which the geologist
is so familiar, especially as occurring in great beds amongst
the older formations of the earth's crust. These are seen, on
weathered or broken surfaces, or still better in polished slabs
(fig. 9), to be composed more or less exclusively of the broken
stems and detached plates of sea-lilies (_Crinoids_). Similarly,
other limestones are composed almost entirely of the skeletons of
corals; and such old coralline limestones can readily be paralleled
by formations which we can find in actual course of production
at the present day. We only need to transport ourselves to the
islands of the Pacific, to the West Indies, or to the Indian
Ocean, to find great masses of lime formed similarly by living
corals, and well known to everyone under the name of "coral-reefs."
Such reefs are often of vast extent, both superficially and in
vertical thickness, and they fully equal in this respect any of
the coralline limestones of bygone ages. Again, we find other
limestones--such as the celebrated "Nummulitic Limestone" (fig. 10),
which sometimes attains a thickness of some thousands of feet--which
are almost entirely made up of the shells of _Foraminifera_. In
the case of the "Nummulitic Limestone," just mentioned, these
shells are of large size, varying from the size of a split pea
up to that of a florin. There are, however, as we shall see,
many other limestones, which are likewise largely made up of
_Foraminifera_, but in which the shells are very much more minute,
and would hardly be seen at all without the microscope.
[Illustration: Fig. 10.--Piece of Nummulitic Limestone from the
Great Pyramid. Of the natural size. (Original.)]
We may, in fact, consider that the great agents in the production
of limestones in past ages have been animals belonging to the
_Crinoids_, the _Corals_, and the _Foraminifera_. At the present
day, the Crinoids have been nearly extinguished, and the few known
survivors seem to have retired to great depths in the ocean; but
the two latter still actively carry on the work of lime-making,
the former being very largely helped in their operations by certain
lime-producing marine plants (_Nullipores_ and _Corallines_). We
have to remember, however, that though the limestones, both ancient
and modern, that we have just spoken of, are truly organic, they
are not necessarily formed out of the remains of animals which
actually lived on the precise spot where we now find the limestone
itself. We may find a crinoidal limestone, which we can show to
have been actually formed by the successive growth of generations
of sea-lilies _in place_; but we shall find many others in which
the rock is made up of innumerable fragments of the skeletons
of these creatures, which have been clearly worn and rubbed by
the sea-waves, and which have been mechanically transported to
their present site. In the same way, a limestone may be shown
to have been an actual coral-reef, by the fact that we find in
it great masses of coral, growing in their natural position,
and exhibiting plain proofs that they were simply quietly buried
by the calcareous sediment as they grew; but other limestones
may contain only numerous rolled and water-worn fragments of
corals. This is precisely paralleled by what we can observe in
our existing coral-reefs. Parts of the modern coral-islands and
coral-reefs are really made up of corals, dead or alive, which
actually grew on the spot where we now find them; but other parts
are composed of a limestone-rock ("coral-rock"), or of a loose
sand ("coral-sand"), which is organic in the sense that it is
composed of lime formed by living beings, but which, in truth,
is composed of fragments of the skeletons of these living beings,
mechanically transported and heaped together by the sea. To take
another example nearer home, we may find great accumulations of
calcareous matter formed _in place_, by the growth of shell-fish,
such as oysters or mussels; but we can also find equally great
accumulations on many of our shores in the form of "shell-sand,"
which is equally composed of the shells of molluscs, but which is
formed by the trituration of these shells by the mechanical power
of the sea-waves. We thus see that though all these limestones are
primarily organic, they not uncommonly become "mechanically-formed"
rocks in a secondary sense, the materials of which they are composed
being formed by living beings, but having been mechanically
transported to the place where we now find them.
[Illustration: Fig. 11.--Section of Carboniferous Limestone from
Spergen Hill, Indiana, U.S., showing numerous large-sized
_Foraminifera_ (_Endothyra_) and a few oolitic grains; magnified.
(Original.)]
[Illustration: Fig 12.--Section of Coniston Limestone (Lower
Silurian) from Keisler, Westmoreland; magnified. The matrix is
very coarsely crystalline, and the included organic remains are
chiefly stems of Crinoids. (Original.)]
Many limestones, as we have seen, are composed of large and
conspicuous organic remains, such as strike the eye at once.
Many others, however, which at first sight appear compact, more
or less crystalline, and nearly devoid of traces of life, are
found, when properly examined, to be also composed of the remains
of various organisms. All the commoner limestones, in fact, from
the Lower Silurian period onwards, can be easily proved to be
thus _organic_ rocks, if we investigate weathered or polished
surfaces with a lens, or, still better, if we cut thin slices
of the rock and grind these down till they are transparent. When
thus examined, the rock is usually found to be composed of
innumerable entire or fragmentary fossils, cemented together
by a granular or crystalline matrix of carbonate of lime (figs.
11 and 12). When the matrix is granular, the rock is precisely
similar to chalk, except that it is harder and less earthy in
texture, whilst the fossils are only occasionally referable to
the _Foraminifera_. In other cases, the matrix is more or less
crystalline, and when this crystallisation has been carried to
a great extent, the original organic nature of the rock may be
greatly or completely obscured thereby. Thus, in limestones which
have been greatly altered or "metamorphosed" by the combined
action of heat and pressure, all traces of organic remains become
annihilated, and the rock becomes completely crystalline throughout.
This, for example, is the case with the ordinary white "statuary
marble," slices of which exhibit under the microscope nothing but
an aggregate of beautifully transparent crystals of carbonate
of lime, without the smallest traces of fossils. There are also
other cases, where the limestone is not necessarily highly
crystalline, and where no metamorphic action in the strict sense
has taken place, in which, nevertheless, the microscope fails
to reveal any evidence that the rock is organic. Such cases are
somewhat obscure, and doubtless depend on different causes in
different instances; but they do not affect the important
generalisation that limestones are fundamentally the product
of the operation of living beings. This fact remains certain;
and when we consider the vast superficial extent occupied by
calcareous deposits, and the enormous collective thickness of
these, the mind cannot fail to be impressed with the immensity of
the period demanded for the formation of these by the agency of
such humble and often microscopic creatures as Corals, Sea-lilies,
Foraminifers, and Shell-fish.
Amongst the numerous varieties of limestone, a few are of such
interest as to deserve a brief notice. _Magnesian limestone_
or _dolomite_, differs from ordinary limestone in containing
a certain proportion of carbonate of magnesia along with the
carbonate of lime. The typical dolomites contain a large proportion
of carbonate of magnesia, and are highly crystalline. The ordinary
magnesian limestones (such as those of Durham in the Permian
series, and the Guelph Limestones of North America in the Silurian
series) are generally of a yellowish, buff, or brown colour,
with a crystalline or pearly aspect, effervescing with acid much
less freely than ordinary limestone, exhibiting numerous cavities
from which fossils have been dissolved out, and often assuming
the most varied and singular forms in consequence of what is
called "concretionary action." Examination with the microscope
shows that these limestones are composed of an aggregate of minute
but perfectly distinct crystals, but that minute organisms of
different kinds, or fragments of larger fossils, are often present
as well. Other magnesian limestones, again, exhibit no striking
external peculiarities by which the presence of magnesia would be
readily recognised, and though the base of the rock is crystalline,
they are replete with the remains of organised beings. Thus many
of the magnesian limestones of the Carboniferous series of the
North of England are very like ordinary limestone to look at,
though effervescing less freely with acids, and the microscope
proves them to be charged with the remains of _Foraminifera_
and other minute organisms.
_Marbles_ are of various kinds, all limestones which are sufficiently
hard and compact to take a high polish going by this name. Statuary
marble, and most of the celebrated foreign marbles, are "metamorphic"
rocks, of a highly crystalline nature, and having all traces
of their primitive organic structure obliterated. Many other
marbles, however, differ from ordinary limestone simply in the
matter of density. Thus, many marbles (such as Derbyshire marble)
are simply "crinoidal limestones" (fig. 9); whilst various other
British marbles exhibit innumerable organic remains under the
microscope. Black marbles owe their colour to the presence of
very minute particles of carbonaceous matter, in some cases at
any rate; and they may either be metamorphic, or they may be
charged with minute fossils such as _Foraminifera_ (_e.g._, the
black limestones of Ireland, and the black marble of Dent, in
Yorkshire).
[Illustration: Fig. 13.--Slice of oolitic limestone from the
Jurassic series (Coral Rag) of Weymouth; magnified. (Original.)]
"_Oolitic_" _limestones_, or "_oolites_," as they are often called,
are of interest both to the palaeontologist and geologist. The
peculiar structure to which they owe their name is that the rock
is more or less entirely composed of spheroidal or oval grains,
which vary in size from the head of a small pin or less up to
the size of a pea, and which may be in almost immediate contact
with one another, or may be cemented together by a more or less
abundant calcareous matrix. When the grains are pretty nearly
spherical and are in tolerably close contact, the rock looks very
like the roe of a fish, and the name of "oolite" or "egg-stone"
is in allusion to this. When the grains are of the size of peas
or upwards, the rock is often called a "pisolite" (Lat. _pisum_,
a pea). Limestones having this peculiar structure are especially
abundant in the Jurassic formation, which is often called the
"Oolitic series" for this reason; but essentially similar limestones
occur not uncommonly in the Silurian, Devonian, and Carboniferous
formations, and, indeed, in almost all rock-groups in which
limestones are largely developed. Whatever may be the age of
the formation in which they occur, and whatever may be the size
of their component "eggs," the structure of oolitic limestones
is fundamentally the same. All the ordinary oolitic limestones,
namely, consist of little spherical or ovoid "concretions," as
they are termed, cemented together by a larger or smaller amount
of crystalline carbonate of lime, together, in many instances,
with numerous organic remains of different kinds (fig. 13). When
examined in polished slabs, or in thin sections prepared for the
microscope, each of these little concretions is seen to consist
of numerous concentric coats of carbonate of lime, which sometimes
simply surround an imaginary centre, but which, more commonly,
have been successively deposited round some foreign body, such as
a little crystal of quartz, a cluster of sand-grains, or a minute
shell. In other cases, as in some of the beds of the Carboniferous
limestone in the North of England, where the limestone is highly
"arenaceous," there is a modification of the oolitic structure.
Microscopic sections of these sandy limestones (fig. 14) show
numerous generally angular or oval grains of silica or flint,
each of which is commonly surrounded by a thin coating of carbonate
of lime, or sometimes by several such coats, the whole being
cemented together along with the shells of _Foraminifera_ and
other minute fossils by a matrix of crystalline calcite. As compared
with typical oolites, the concretions in these limestones are
usually much more irregular in shape, often lengthened out and
almost cylindrical, at other times angular, the central nucleus
being of large size, and the surrounding envelope of lime being
very thin, and often exhibiting no concentric structure. In both
these and the ordinary oolites, the structure is fundamentally
the same. Both have been formed in a sea, probably of no great
depth, the waters of which were charged with carbonate of lime
in solution, whilst the bottom was formed of sand intermixed with
minute shells and fragments of the skeletons of larger marine
animals. The excess of lime in the sea-water was precipitated
round the sand-grams, or round the smaller shells, as so many
nuclei, and this precipitation must often have taken place time
after time, so as to give rise to the concentric structure so
characteristic of oolitic concretions. Finally, the oolitic grains
thus produced were cemented together by a further precipitation
of crystalline carbonate of lime from the waters of the ocean.
[Illustration: Fig. 14.--Slice of arenaceous and oolitic limestone
from the Carboniferous series of Shap, Westmoreland; magnified.
The section also exhibit _Foraminifera_ and other minute fossils.
(Original.)]
_Phosphate of Lime_ is another lime-salt, which is of interest to
the palaeontologist. It does not occur largely in the stratified
series, but it is found in considerable beds [4] in the Laurentian
formation, and less abundantly in some later rock-groups, whilst
it occurs abundantly in the form of nodules in parts of the
Cretaceous (Upper Greensand) and Tertiary deposits. Phosphate
of lime forms the larger proportion of the earthy matters of the
bones of Vertebrate animals, and also occurs in less amount in the
skeletons of certain of the Invertebrates (_e.g._, _Crustacea_). It
is, indeed, perhaps more distinctively than carbonate of lime, an
organic compound; and though the formation of many known deposits
of phosphate of lime cannot be positively shown to be connected
with the previous operation of living beings, there is room for
doubt whether this salt is not in reality always primarily a
product of vital action. The phosphatic nodules of the Upper
Greensand are erroneously called "coprolites," from the belief
originally entertained that they were the droppings or fossilised
excrements of extinct animals; and though this is not the case,
there can be little doubt but that the phosphate of lime which
they contain is in this instance of organic origin.[5] It appears,
in fact, that decaying animal matter has a singular power of
determining the precipitation around it of mineral salts dissolved
in water. Thus, when any animal bodies are undergoing decay at the
bottom of the sea, they have a tendency to cause the precipitation
from the surrounding water of any mineral matters which may be
dissolved in it; and the organic body thus becomes a centre round
which the mineral matters in question are deposited in the form
of a "concretion" or "nodule." The phosphatic nodules in question
were formed in a sea in which phosphate of lime, derived from the
destruction of animal skeletons, was held largely in solution;
and a precipitation of it took place round any body, such as a
decaying animal substance, which happened to be lying on the
sea-bottom, and which offered itself as a favourable nucleus. In
the same way we may explain the formation of the calcareous nodules,
known as "septaria" or "cement stones," which occur so commonly in
the London Clay and Kimmeridge Clay, and in which the principal
ingredient is carbonate of lime. A similar origin is to be ascribed
to the nodules of clay iron-stone (impure carbonate of iron) which
occur so abundantly in the shales of the Carboniferous series and
in other argillaceous deposits; and a parallel modern example
is to be found in the nodules of manganese, which were found
by Sir Wyville Thomson, in the Challenger, to be so numerously
scattered over the floor of the Pacific at great depths. In
accordance with this mode of origin, it is exceedingly common
to find in the centre of all these nodules, both old and new,
some organic body, such as a bone, a shell, or a tooth, which
acted as the original nucleus of precipitation, and was thus
preserved in a shroud of mineral matter. Many nodules, it is
true, show no such nucleus; but it has been affirmed that all of
them can be shown, by appropriate microscopical investigation,
to have been formed round an original organic body to begin with
(Hawkins Johnson).
[Footnote 4: Apart from the occurrence or phosphate of lime in
actual beds in the stratified rocks, as in the Laurentian and
Silurian series, this salt may also occur disseminated through
the rock, when it can only be detected by chemical analysis. It
is interesting to note that Dr Hicks has recently proved the
occurrence of phosphate of lime in this disseminated form in
rocks as old as the Cambrian, and that in quantity quite equal to
what is generally found to be present in the later fossiliferous
rocks. This affords a chemical proof that animal life flourished
abundantly in the Cambrian seas.]
[Footnote 5: It has been maintained, indeed, that the phosphatic
nodules so largely worked for agricultural purposes, are in
themselves actual organic bodies or true fossils. In a few cases
this admits of demonstration, as it can be shown that the nodule
is simply an organism (such as a sponge) infiltrated with phosphate
of lime (Sollas); but there are many other cases in which no actual
structure has yet been shown to exist, and as to the true origin
of which it would be hazardous to offer a positive opinion.]
The last lime-salt which need be mentioned is _gypsum_, or _sulphate
of lime_. This substance, apart from other modes of occurrence, is
not uncommonly found interstratified with the ordinary sedimentary
rocks, in the form of more or less irregular beds; and in these
cases it has a palaeontological importance, as occasionally yielding
well-preserved fossils. Whilst its exact mode of origin is uncertain,
it cannot be regarded as in itself an organic rock, though clearly
the product of chemical action. To look at, it is usually a whitish
or yellowish-white rock, as coarsely crystalline as loaf-sugar,
or more so; and the microscope shows it to be composed entirely
of crystals of sulphate of lime.
We have seen that the _calcareous_ or lime-containing rocks are
the most important of the group of organic deposits; whilst the
_siliceous_ or flint-containing rocks may be regarded as the
most important, most typical, and most generally distributed
of the mechanically-formed rocks. We have, however, now briefly
to consider certain deposits which are more or less completely
formed of flint; but which, nevertheless, are essentially organic
in their origin.
Flint or silex, hard and intractable as it is, is nevertheless
capable of solution in water to a certain extent, and even of
assuming, under certain circumstances, a gelatinous or viscous
condition. Hence, some hot-springs are impregnated with silica
to a considerable extent; it is present in small quantity in
sea-water; and there is reason to believe that a minute proportion
must very generally be present in all bodies of fresh water as
well. It is from this silica dissolved in the water that many
animals and some plants are enabled to construct for themselves
flinty skeletons; and we find that these animals and plants are and
have been sufficiently numerous to give rise to very considerable
deposits of siliceous matter by the mere accumulation of their
skeletons. Amongst the animals which require special mention in
this connection are the microscopic organisms which are known to
the naturalist as _Polycystina_. These little creatures are of the
lowest possible grade of organisation, very closely related to the
animals which we have previously spoken of as _Foraminifera_, but
differing in the fact that they secrete a shell or skeleton composed
of flint instead of lime. The _Polycystina_ occur abundantly in
our present seas; and their shells are present in some numbers
in the ooze which is found at great depths in the Atlantic and
Pacific oceans, being easily recognised by their exquisite shape,
their glassy transparency, the general presence of longer or
shorter spines, and the sieve-like perforations in the walls.
Both in Barbadoes and in the Nicobar islands occur geological
formations which are composed of the flinty skeletons of these
microscopic animals; the deposit in the former locality attaining
a great thickness, and having been long known to workers with
the microscope under the name of "Barbadoes earth" (fig. 15).
[Illustration: Fig. 15.--Shells of _Polycystina_ from "Barbadoes
earth;" greatly magnified. (Original.)]
[Illustration: Fig. 16.--Cases of Diatoms in the Richmond "Infusorial
earth;" highly magnified. (Original.)]
In addition to flint-producing animals, we have also the great
group of fresh-water and marine microscopic plants known as
_Diatoms_, which likewise secrete a siliceous skeleton, often of
great beauty. The skeletons of Diatoms are found abundantly at the
present day in lake-deposits, guano, the silt of estuaries, and in
the mud which covers many parts of the sea-bottom; they have been
detected in strata of great age; and in spite of their microscopic
dimensions, they have not uncommonly accumulated to form deposits
of great thickness, and of considerable superficial extent. Thus
the celebrated deposit of "tripoli" ("Polir-schiefer") of Bohemia,
largely worked as polishing-powder, is composed wholly, or almost
wholly, of the flinty cases of Diatoms, of which it is calculated
that no less than forty-one thousand millions go to make up a
single cubic inch of the stone. Another celebrated deposit is
the so-called "Infusorial earth" of Richmond in Virginia, where
there is a stratum in places thirty feet thick, composed almost
entirely of the microscopic shells of Diatoms.
Nodules or layers of _flint_, or the impure variety of flint
known as _chert_, are found in limestones of almost all ages
from the Silurian upwards; but they are especially abundant in
the chalk. When these flints are examined in thin and transparent
slices under the microscope, or in polished sections, they are
found to contain an abundance of minute organic bodies--such as
_Foraminifera_, sponge-spicules, &c.--embedded in a siliceous
basis. In many instances the flint contains larger organisms--such
as a Sponge or a Sea-urchin. As the flint has completely surrounded
and infiltrated the fossils which it contains, it is obvious
that it must have been deposited from sea-water in a gelatinous
condition, and subsequently have hardened. That silica is capable
of assuming this viscous and soluble condition is known; and
the formation of flint may therefore be regarded as due to the
separation of silica from the sea-water and its deposition round
some organic body in a state of chemical change or decay, just as
nodules of phosphate of lime or carbonate of iron are produced.
The existence of numerous organic bodies in flint has long been
known; but it should be added that a recent observer (Mr Hawkins
Johnson) asserts that the existence of an organic structure can
be demonstrated by suitable methods of treatment, even in the
actual matrix or basis of the flint.[6]
[Footnote 6: It has been asserted that the flints of the chalk
are merely fossil sponges. No explanation of the origin of flint,
however, can be satisfactory, unless it embraces the origin of
chert in almost all great limestones from the Silurian upwards,
as well as the common phenomenon of the silicification of organic
bodies (such as corals and shells) which are known with certainty
to have been originally calcareous.]
In addition to deposits formed of flint itself, there are other
siliceous deposits formed by certain _silicates_, and also of
organic origin. It has been shown, namely--by observations carried
out in our present seas--that the shells of _Foraminifera_ are
liable to become completely infiltrated by silicates (such as
"glauconite," or silicate of iron and potash). Should the actual
calcareous shell become dissolved away subsequent to this
infiltration--as is also liable to occur--then, in place of the
shells of the _Foraminifera_, we get a corresponding number of
green sandy grains of glauconite, each grain being the _cast_
of a single shell. It has thus been shown that the green sand
found covering the sea-bottom in certain localities (as found by
the Challenger expedition along the line of the Agulhas current)
is really organic, and is composed of casts of the shells of
_Foraminifera_. Long before these observations had been made,
it had been shown by Professor Ehrenberg that the green sands of
various geological formations are composed mainly of the internal
casts of the shells of _Foraminifera_, and we have thus another
and a very interesting example how rock-deposits of considerable
extent and of geological importance can be built up by the operation
of the minutest living beings.
As regards _argillaceous_ deposits, containing _alumina_ or _clay_
as their essential ingredient, it cannot be said that any of
these have been actually shown to be of organic origin. A recent
observation by Sir Wyville Thomson would, however, render it not
improbable that some of the great argillaceous accumulations of
past geological periods may be really organic. This distinguished
observer, during the cruise of the Challenger, showed that the
calcareous ooze which has been already spoken of as covering
large areas of the floor of the Atlantic and Pacific at great
depths, and which consists almost wholly of the shells of
_Foraminifera_, gave place at still greater depths to a red ooze
consisting of impalpable clayey mud, coloured by oxide of iron,
and devoid of traces of organic bodies. As the existence of this
widely-diffused red ooze, in mid-ocean, and at such great depths,
cannot be explained on the supposition that it is a sediment
brought down into the sea by rivers, Sir Wyville Thomson came to
the conclusion that it was probably formed by the action of the
sea-water upon the shells of _Foraminifera_. These shells, though
mainly consisting of lime, also contain a certain proportion of
alumina, the former being soluble in the carbonic acid dissolved
in the sea-water, whilst the latter is insoluble. There would
further appear to be grounds for believing that the solvent power
of the sea-water over lime is considerably increased at great
depths. If, therefore, we suppose the shells of _Foraminifera_
to be in course of deposition over the floor of the Pacific, at
certain depths they would remain unchanged, and would accumulate
to form a calcareous ooze; but at greater depths they would be
acted upon by the water, their lime would be dissolved out, their
form would disappear, and we should simply have left the small
amount of alumina which they previously contained. In process
of time this alumina would accumulate to form a bed of clay; and
as this clay had been directly derived from the decomposition
of the shells of animals, it would be fairly entitled to be
considered an organic deposit. Though not finally established,
the hypothesis of Sir Wyville Thomson on this subject is of the
greatest interest to the palaeontologist, as possibly serving to
explain the occurrence, especially in the older formations, of
great deposits of argillaceous matter which are entirely destitute
of traces of life.
It only remains, in this connection, to shortly consider the
rock-deposits in which _carbon_ is found to be present in greater
or less quantity. In the great majority of cases where rocks
are found to contain carbon or carbonaceous matter, it can be
stated with certainty that this substance is of organic origin,
though it is not necessarily derived from vegetables. Carbon
derived from the decomposition of animal bodies is not uncommon;
though it never occurs in such quantity from this source as it
may do when it is derived from plants. Thus, many limestones are
more or less highly bituminous; the celebrated siliceous flags
or so-called "bituminous schists" of Caithness are impregnated
with oily matter apparently derived from the decomposition of the
numerous fishes embedded in them; Silurian shales containing
Graptolites, but destitute of plants, are not uncommonly
"anthracitic," and contain a small percentage of carbon derived
from the decay of these zoophytes; whilst the petroleum so largely
worked in North America has not improbably an animal origin.
That the fatty compounds present in animal bodies should more or
less extensively impregnate fossiliferous rock-masses, is only
what might be expected; but the great bulk of the carbon which
exists stored up in the earth's crust is derived from plants;
and the form in which it principally presents itself is that of
coal. We shall have to speak again, and at greater length, of
coal, and it is sufficient to say here that all the true coals,
anthracites, and lignites, are of organic origin, and consist
principally of the remains of plants in a more or less altered
condition. The bituminous shales which are found so commonly
associated with beds of coal also derive their carbon primarily
from plants; and the same is certainly, or probably, the case
with similar shales which are known to occur in formations younger
than the Carboniferous. Lastly, carbon may occur as a conspicuous
constituent of rock-masses in the form of _graphite_ or _black-lead_.
In this form, it occurs in the shape of detached scales, of veins
or strings, or sometimes of regular layers;[7] and there can be
little doubt that in many instances it has an organic origin,
though this is not capable of direct proof. When present, at any
rate, in quantity, and in the form of layers associated with
stratified rocks, as is often the case in the Laurentian formation,
there can be little hesitation in regarding it as of vegetable
origin, and as an altered coal.
[Footnote 7: In the Huronian formation at Steel River, on the
north shore of Lake Superior, there exists a bed of carbonaceous
matter which is regularly interstratified with the surrounding
rocks, and has a thickness of from 30 to 40 feet. This bed is
shown by chemical analysis to contain about 50 per cent of carbon,
partly in the form of graphite, partly in the form of anthracite;
and there can be little doubt but that it is really a stratum
of "metamorphic" coal.]
CHAPTER III.
CHRONOLOGICAL SUCCESSION OF THE FOSSILIFEROUS ROCKS.
The physical geologist, who deals with rocks simply as rocks,
and who does not necessarily trouble himself about what fossils
they may contain, finds that the stratified deposits which form
so large a portion of the visible part of the earth's crust are
not promiscuously heaped together, but that they have a certain
definite arrangement. In each country that he examines, he finds
that certain groups of strata lie above certain other groups;
and in comparing different countries with one another, he finds
that, in the main, the same groups of rocks are always found in the
same relative position to each other. It is possible, therefore,
for the physical geologist to arrange the known stratified rocks
into a successive series of groups, or "formations," having a
certain definite order. The establishment of this physical order
amongst the rocks introduces, however, at once the element of
_time_, and the physical succession of the strata can be converted
directly into a historical or _chronological_ succession. This
is obvious, when we reflect that any bed or set of beds of
sedimentary origin is clearly and necessarily younger than all
the strata upon which it rests, and older than all those by which
it is surmounted.
It is possible, then, by an appeal to the rocks alone, to determine
in each country the general physical succession of the strata,
and this "stratigraphical" arrangement, when once determined,
gives us the _relative_ ages of the successive groups. The task,
however, of the physical geologist in this matter is immensely
lightened when he calls in palaeontology to his aid, and studies
the evidence of the fossils embedded in the rocks. Not only is
it thus much easier to determine the order of succession of the
strata in any given region, but it becomes now for the first time
possible to compare, with certainty and precision, the order of
succession in one region with that which exists in other regions
far distant. The value of fossils as tests of the relative ages
of the sedimentary rocks depends on the fact that they are not
indefinitely or promiscuously scattered through the crust of the
earth,--as it is conceivable that they might be. On the contrary,
the first and most firmly established law of Palaeontology is, that
_particular kinds of fossils are confined to particular rocks_,
and _particular groups of fossils are confined to particular
groups of rocks_. Fossils, then, are distinctive of the rocks in
which they are found--much more distinctive, in fact, than the
mere mineral character of the rock can be, for _that_ commonly
changes as a formation is traced from one region to another,
whilst the fossils remain unaltered. It would therefore be quite
possible for the palaeontologist, by an appeal to the fossils
alone, to arrange the series of sedimentary deposits into a pile
of strata having a certain definite order. Not only would this
be possible, but it would be found--if sufficient knowledge had
been brought to bear on both sides--that the palaeontological
arrangement of the strata would coincide in its details with the
stratigraphical or physical arrangement.
Happily for science, there is no such division between the
palaeontologist and the physical geologist as here supposed; but
by the combined researches of the two, it has been found possible
to divide the entire series of stratified deposits into a number
of definite _rock-groups_ or _formations_, which have a recognised
order of succession, and each of which is characterised by possessing
an assemblage of organic remains which do not occur in association
in any other formation. Such an _assemblage of fossils_,
characteristic of any given formation, represents the _life_ of
the particular _period_ in which the formation was deposited.
In this way the past history of the earth becomes divided into a
series of successive _life-periods_, each of which corresponds
with the deposition of a particular _formation_ or group of strata.
Whilst particular _assemblages_ of organic forms characterise
particular _groups_ of rocks, it may be further said that, in
a general way, each subdivision of each formation has its own
peculiar fossils, by which it may be recognised by a skilled
worker in Palaeontology. Whenever, for instance, we meet with
examples of the fossils which are known as _Graptolites_, we may
be sure that we are dealing with _Silurian_ rocks (leaving out
of sight one or two forms doubtfully referred to this family).
We may, however, go much farther than this with perfect safety. If
the Graptolites belong to certain genera, we may be quite certain
that we are dealing with _Lower_ Silurian rocks. Furthermore, if
certain special forms are present, we may be even able to say to
what exact subdivision of the Lower Silurian series they belong.
As regards particular fossils, however, or even particular classes
of fossils, conclusions of this nature require to be accompanied
by a tacit but well-understood reservation. So far as our present
observation goes, none of the undoubted Graptolites have ever been
discovered in rocks later than those known upon other grounds
to be Silurian; but it is possible that they might at any time be
detected in younger deposits. Similarly, the species and genera
which we now regard as characteristic of the Lower Silurian, may
at some future time be found to have survived into the Upper
Silurian period. We should not forget, therefore, in determining
the age of strata by palaeontological evidence, that we are always
reasoning upon generalisations which are the result of experience
alone, and which are liable to be vitiated by further and additional
discoveries.
When the palaeontological evidence as to the age of any given
set of strata is corroborated by the physical evidence, our
conclusions may be regarded as almost certain; but there are
certain limitations and fallacies in the palaeontological method
of inquiry which deserve a passing mention. In the first place,
fossils are not always present in the stratified rocks; many
aqueous rocks are unfossiliferous, through a thickness of hundreds
or even thousands of feet of little-altered sediments; and even
amongst beds which do contain fossils, we often meet with strata
of many feet or yards in thickness which are wholly destitute
of any traces of fossils. There are, therefore, to begin with,
many cases in which there is no palaeontological evidence extant
or available as to the age of a given group of strata. In the
second place, palaeontological observers in different parts of
the world are liable to give different names to the same fossil,
and in all parts of the world they are occasionally liable to
group together different fossils under the same title. Both these
sources of fallacy require to be guarded against in reasoning as
to the age of strata from their fossil remains. Thirdly, the mere
fact of fossils being found in beds which are known by physical
evidence to be of different ages, has commonly led palaeontologists
to describe them as different species. Thus, the same fossil,
occurring in successive groups of strata, and with the merely
trivial and varietal differences due to the gradual change in its
environment, has been repeatedly described as a distinct species,
with a distinct name, in every bed in which it was found. We know,
however, that many fossils range vertically through many groups
of strata, and there are some which even pass through several
formations. The mere fact of a difference of physical position
ought never to be taken into account at all in considering and
determining the true affinities of a fossil. Fourthly, the results
of experience, instead of being an assistance, are sometimes
liable to operate as a source of error. When once, namely, a
generalisation has been established that certain fossils occur
in strata of a certain age, palaeontologists are apt to infer
that _all_ beds containing similar fossils must be of the same
age. There is a presumption, of course, that this inference would
be correct; but it is not a conclusion resting upon absolute
necessity, and there might be physical evidence to disprove it.
Fifthly, the physical geologist may lead the palaeontologist astray
by asserting that the physical evidence as to the age and position
of a given group of beds is clear and unequivocal, when such
evidence may be, in reality, very slight and doubtful. In this
way, the observer may be readily led into wrong conclusions as
to the nature of the organic remains--often obscure and
fragmentary--which it is his business to examine, or he may be
led erroneously to think that previous generalisations as to
the age of certain kinds of fossils are premature and incorrect.
Lastly, there are cases in which, owing to the limited exposure
of the beds, to their being merely of local development, or to
other causes, the physical evidence as to the age of a given
group of strata may be entirely uncertain and unreliable, and
in which, therefore, the observer has to rely wholly upon the
fossils which he may meet with.
In spite of the above limitations and fallacies, there can be
no doubt as to the enormous value of palaeontology in enabling us
to work out the historical succession of the sedimentary rocks.
It may even be said that in any case where there should appear
to be a clear and decisive discordance between the physical and
the palaeontological evidence as to the age of a given series
of beds, it is the former that is to be distrusted rather than
the latter. The records of geological science contain not a few
cases in which apparently clear physical evidence of superposition
has been demonstrated to have been wrongly interpreted; but the
evidence of palaeontology, when in any way sufficient, has rarely
been upset by subsequent investigations. Should we find strata
containing plants of the Coal-measures apparently resting upon
other strata with Ammonites and Belemnites, we may be sure that
the physical evidence is delusive; and though the above is an
extreme case, the presumption in all such instances is rather that
the physical succession has been misunderstood or misconstrued,
than that there has been a subversion of the recognised succession
of life-forms.
We have seen, then, that as the collective result of observations
made upon the superposition of rocks in different localities,
from their mineral characters, and from their included fossils,
geologists have been able to divide the entire stratified series into
a number of different divisions or formations, each characterised
by a _general_ uniformity of mineral composition, and by a special
and peculiar _assemblage_ of organic forms. Each of these primary
groups is in turn divided into a series of smaller divisions,
characterised and distinguished in the same way. It is not pretended
for a moment that all these primary rock-groups can anywhere be seen
surmounting one another regularly.[8] There is no region upon the
earth where all the stratified formations can be seen together;
and, even when most of them occur in the same country, they can
nowhere be seen all succeeding each other in their regular and
uninterrupted succession. The reason of this is obvious. There
are many places--to take a single example--where one may see the
the Silurian rocks, the Devonian, and the Carboniferous rocks
succeeding one another regularly, and in their proper order. This
is because the particular region where this occurs was always
submerged beneath the sea while these formations were being
deposited. There are, however, many more localities in which
one would find the Carboniferous rocks resting unconformably upon
the Silurians without the intervention of any strata which could
be referred to the Devonian period. This might arise from one of
two causes: 1. The Silurians might have been elevated above the
sea immediately after their deposition, so as to form dry land
during the whole of the Devonian period, in which case, of course,
no strata of the latter age could possibly be deposited in that
area. 2. The Devonian might have been deposited upon the Silurian,
and then the whole might have been elevated above the sea, and
subjected to an amount of denudation sufficient to remove the
Devonian strata entirely. In this case, when the land was again
submerged, the Carboniferous rocks, or any younger formation,
might be deposited directly upon Silurian strata. From one or
other of these causes, then, or from subsequent disturbances
and denudations, it happens that we can rarely find many of the
primary formations following one another consecutively and in
their regular order.
[Footnote 8: As we have every reason to believe that dry land
and sea have existed, at any rate from the commencement of the
Laurentian period to the present day, it is quite obvious that
no one of the great formations can ever, under any circumstances,
have extended over the entire globe. In other words, no one of
the formations can ever have had a greater geographical extent
than that of the seas of the period in which the formation was
deposited. Nor is there any reason for thinking that the proportion
of dry land to ocean has ever been materially different to what
it is at present, however greatly the areas of sea and land may
have changed as regards their place. It follows from the above,
that there is no sufficient basis for the view that the crust of
the earth is composed of a succession of concentric layers, like
the coats of an onion, each layer representing one formation.]
In no case, however, do we ever find the Devonian resting upon
the Carboniferous, or the Silurian rocks reposing on the Devonian.
We have therefore, by a comparison of many different areas, an
established order of succession of the stratified formations, as
shown in the subjoined ideal section of the crust of the earth
(fig. 17).
The main subdivisions of the stratified rocks are known by the
following names:--
1. Laurentian.
2. Cambrian (with Huronian ?).
3. Silurian.
4. Devonian or Old Red Sandstone.
5. Carboniferous.
6. Permian \_ New Red Sandstone.
7. Triassic /
8. Jurassic or Oolitic.
9. Cretaceous.
10. Eocene.
11. Miocene.
12. Pliocene.
13. Post-tertiary.
[Illustration: Fig. 17. IDEAL SECTION OF THE CRUST OF THE EARTH.]
Of these primary rock divisions, the Laurentian, Cambrian, Silurian,
Devonian, Carboniferous, and Permian are collectively grouped
together under the name of the Primary or _Paloeozoic_ rocks (Gr.
_palaios_, ancient; _zoe_, life). Not only do they constitute the
oldest stratified accumulations, but from the extreme divergence
between their animals and plants and those now in existence, they may
appropriately be considered as belonging to an "Old-Life" period of
the world's history. The Triassic, Jurassic, and Cretaceous systems
are grouped together as the _Secondary_ or _Mesozoic_ formations
(Gr. _mesos_, intermediate; _zoe_, life); the organic remains of
this "Middle-Life" period being, on the whole, intermediate in
their characters between those of the palaeozoic epoch and those
of more modern strata. Lastly, the Eocene, Miocene, and Pliocene
formations are grouped together as the _Tertiary_ or _Kainozoic_
rocks (Gr. _kainos_, new; _zoe_, life); because they constitute
a "New-Life" period, in which the organic remains approximate in
character to those now existing upon the globe. The so-called
_Post-Tertiary_ deposits are placed with the Kainozoic, or may
be considered as forming a separate _Quaternary_ system.
CHAPTER IV.
THE BREAKS IN THE GEOLOGICAL AND PALAEONTOLOGICAL RECORD.
The term "contemporaneous" is usually applied by geologists to
groups of strata in different regions which contain the same
fossils, or an assemblage of fossils in which many identical
forms are present. That is to say, beds which contain identical,
or nearly identical, fossils, however widely separated they may
be from one another in point of actual distance, are ordinarily
believed to have been deposited during the same period of the
earth's history. This belief, indeed, constitutes the keystone
of the entire system of determining the age of strata by their
fossil contents; and if we take the word "contemporaneous" in a
general and strictly geological sense, this belief can be accepted
as proved beyond denial. We must, however, guard ourselves against
too literal an interpretation of the word "contemporaneous,"
and we must bear in mind the enormously-prolonged periods of
time with which the geologist has to deal. When we say that two
groups of strata in different regions are "contemporaneous," we
simply mean that they were formed during the same geological
period, and perhaps at different stages of that period, and we
do not mean to imply that they were formed at precisely the same
instant of time.
A moment's consideration will show us that it is only in the former
sense that we can properly speak of strata being "contemporaneous;"
and that, in point of fact, beds containing the same fossils, if
occurring in widely distant areas, can hardly be "contemporaneous"
in any literal sense; but that the very identity of their fossils
is proof that they were deposited one after the other. If we find
strata containing identical fossils within the limits of a single
geographical region--say in Europe--then there is a reasonable
probability that these beds are strictly contemporaneous, in the
sense that they were deposited at the same time. There is a
reasonable probability of this, because there is no improbability
involved in the idea of an ocean occupying the whole area of
Europe, and peopled throughout by many of the same species of
marine animals. At the present day, for example, many identical
species of animals are found living on the western coasts of
Britain and the eastern coasts of North America, and beds now
in course of deposition off the shores of Ireland and the seaboard
of the state of New York would necessarily contain many of the
same fossils. Such beds would be both literally and geologically
contemporaneous; but the case is different if the distance between
the areas where the strata occur be greatly increased. We find,
for example, beds containing identical fossils (the Quebec or
Skiddaw beds) in Sweden, in the north of England, in Canada,
and in Australia. Now, if all these beds were contemporaneous,
in the literal sense of the term, we should have to suppose that
the ocean at one time extended uninterruptedly between all these
points, and was peopled throughout the vast area thus indicated
by many of the same animals. Nothing, however, that we see at
the present day would justify us in imagining an ocean of such
enormous extent, and at the same time so uniform in its depth,
temperature, and other conditions of marine life, as to allow the
same animals to flourish in it from end to end; and the example
chosen is only one of a long and ever-recurring series. It is
therefore much more reasonable to explain this, and all similar
cases, as owing to the _migration_ of the fauna, in whole or in
part, from one marine area to another. Thus, we may suppose an
ocean to cover what is now the European area, and to be peopled
by certain species of animals. Beds of sediment--clay, sands,
and limestones--will be deposited over the sea-bottom, and will
entomb the remains of the animals as fossils. After this has
lasted for a certain length of time, the European area may undergo
elevation, or may become otherwise unsuitable for the perpetuation
of its fauna; the result of which would be that some or all of the
marine animals of the area would migrate to some more suitable
region. Sediments would then be accumulated in the new area to
which they had betaken themselves, and they would then appear,
for the second time, as fossils in a set of beds widely separated
from Europe. The second set of beds would, however, obviously
not be strictly or literally contemporaneous with the first, but
would be separated from them by the period of time required for
the migration of the animals from the one area into the other.
It is only in a wide and comprehensive sense that such strata
can be said to be contemporaneous.
It is impossible to enter further into this subject here; but it
may be taken as certain that beds in widely remote geographical
areas can only come to contain the same fossils by reason of a
migration having taken place of the animals of the one area to
the other. That such migrations can and do take place is quite
certain, and this is a much more reasonable explanation of the
observed facts than the hypothesis that in former periods the
conditions of life were much more uniform than they are at present,
and that, consequently, the same organisms were able to range over
the entire globe at the same time. It need only be added, that
taking the evidence of the present as explaining the phenomena
of the past--the only safe method of reasoning in geological
matters--we have abundant proof that deposits which _are_ actually
contemporaneous, in the strict sense of the term, _do not contain
the same fossils, if far removed from one another in point of
distance_. Thus, deposits of various kinds are now in process of
formation in our existing seas, as, for example, in the Arctic
Ocean, the Atlantic, and the Pacific, and many of these deposits
are known to us by actual examination and observation with the
sounding-lead and dredge. But it is hardly necessary to add that
the animal remains contained in these deposits--the fossils of some
future period--instead of being identical, are widely different
from one another in their characters.
We have seen, then, that the entire stratified series is capable of
subdivision into a number of definite rock-groups or "formations,"
each possessing a peculiar and characteristic assemblage of fossils,
representing the "life" of the "period" in which the formation
was deposited. We have still to inquire shortly how it came to
pass that two successive formations _should_ thus be broadly
distinguished by their life-forms, and why they should not rather
possess at any rate a majority of identical fossils. It was
originally supposed that this could be explained by the hypothesis
that the close of each formation was accompanied by a general
destruction of all the living beings of the period, and that
the commencement of each new formation was signalised by the
creation of a number of brand-new organisms, destined to figure
as the characteristic fossils of the same. This theory, however,
ignores the fact that each formation--as to which we have any
sufficient evidence--contains a few, at least, of the life-forms
which existed in the preceding period; and it invokes forces
and processes of which we know nothing, and for the supposed
action of which we cannot account. The problem is an undeniably
difficult one, and it will not be possible here to give more than
a mere outline of the modern views upon the subject. Without
entering into the at present inscrutable question as to the manner
in which new life-forms are introduced upon the earth, it may be
stated that almost all modern geologists hold that the living
beings of any given formation are in the main modified forms of
others which have preceded them. It is not believed that any
general or universal destruction of life took place at the
termination of each geological period, or that a general introduction
of new forms took place at the commencement of a new period.
It is, on the contrary, believed that the animals and plants
of any given period are for the most part (or exclusively) the
lineal but modified descendants of the animals and plants of
the immediately preceding period, and that some of them, at any
rate, are continued into the next succeeding period, either
unchanged, or so far altered as to appear as new species. To
discuss these views in detail would lead us altogether too far,
but there is one very obvious consideration which may advantageously
receive some attention. It is obvious, namely, that the great
discordance which is found to subsist between the animal life of
any given formation and that of the next succeeding formation,
and which no one denies, would be a fatal blow to the views just
alluded to, unless admitting of some satisfactory explanation.
Nor is this discordance one purely of life-forms, for there is
often a physical break in the successions of strata as well.
Let us therefore briefly consider how far these interruptions
and breaks in the geological and palaeontological record can be
accounted for, and still allow us to believe in some theory of
continuity as opposed to the doctrine of intermittent and occasional
action.
In the first place, it is perfectly clear that if we admit the
conception above mentioned of a continuity of life from the
Laurentian period to the present day, we could never _prove_ our
view to be correct, unless we could produce in evidence fossil
examples of _all_ the kinds of animals and plants that have lived
and died during that period. In order to do this, we should require,
to begin with, to have access to an absolutely unbroken and perfect
succession of all the deposits which have ever been laid down
since the beginning. If, however, we ask the physical geologist
if he is in possession of any such uninterrupted series, he will
at once answer in the negative. So far from the geological series
being a perfect one, it is interrupted by numerous gaps of unknown
length, many of which we can never expect to fill up. Nor are
the proofs of this far to seek. Apart from the facts that we
have hitherto examined only a limited portion of the dry land,
that nearly two-thirds of the entire area of the globe is
inaccessible to geological investigation in consequence of its
being covered by the sea, that many deposits can be shown to
have been more or less completely destroyed subsequent to their
deposition, and that there may be many areas in which living
beings exist where no rock is in process of formation, we have
the broad fact that rock-deposition only goes on to any extent
in water, and that the earth must have always consisted partly of
dry land and partly of water--at any rate, so far as any period
of which we have geological knowledge is concerned. There _must_,
therefore, always have existed, at some part or another of the
earth's surface, areas where no deposition of rock was going on,
and the proof of this is to be found in the well-known phenomenon
of "_unconformability_." Whenever, namely, deposition of sediment
is continuously going on within the limits of a single ocean, the
beds which are laid down succeed one another in uninterrupted
and regular sequence. Such beds are said to be "conformable," and
there are many rock-groups known where one may pass through fifteen
or twenty thousand feet of strata without a break--indicating
that the beds had been deposited in an area which remained
continuously covered by the sea. On the other hand, we commonly
find that there is no such regular succession when we pass from
one great formation to another, but that, on the contrary, the
younger formation rests "unconformably," as it is called, either
upon the formation immediately preceding it in point of time,
or upon some still older one. The essential physical feature of
this unconformability is that the beds of the younger formation
rest upon a worn and eroded surface formed by the beds of the
older series (fig. 18); and a moment's consideration will show
us what this indicates. It indicates, beyond the possibility of
misconception, that there was an interval between the deposition
of the older series and that of the newer series of strata; and
that during this interval the older beds were raised above the
sea-level, so as to form dry land, and were subsequently depressed
again beneath the waters, to receive upon their worn and wasted
upper surface the sediments of the later group. During the interval
thus indicated, the deposition of rock must of necessity have
been proceeding more or less actively in other areas. Every
unconformity, therefore, indicates that at the spot where it
occurs, a more or less extensive series of beds must be actually
missing; and though we may sometimes be able to point to these
missing strata in other areas, there yet remains a number of
unconformities for which we cannot at present supply the deficiency
even in a partial manner.
[Illustration: Fig. 18.--Section showing strata of Tertiary age
(a) resting upon a worn and eroded surface of White Chalk (b),
the stratification of which is marked by lines of flint.]
It follows from the above that the series of stratified deposits
is to a greater or less extent irremediably imperfect; and in
this imperfection we have one great cause why we can never obtain
a perfect series of all the animals and plants that have lived
upon the globe. Wherever one of these great physical gaps occurs,
we find, as we might expect, a corresponding break in the series
of life-forms. In other words, whenever we find two formations
to be unconformable, we shall always find at the same time that
there is a great difference in their fossils, and that many of
the fossils of the older formation do not survive into the newer,
whilst many of those in the newer are not known to occur in the
older. The cause of this is, obviously, that the lapse of time,
indicated by the unconformability, has been sufficiently great
to allow of the dying out or modification of many of the older
forms of life, and the introduction of new ones by immigration.
Apart, however, altogether, from these great physical breaks
and their corresponding breaks in life, there are other reasons
why we can never become more than partially acquainted with the
former denizens of the globe. Foremost amongst these is the fact
that an enormous number of animals possess no hard parts of the
nature of a skeleton, and are therefore incapable, under any
ordinary circumstances, of leaving behind them any traces of
their existence. It is true that there are cases in which animals
in themselves completely soft-bodied are nevertheless able to leave
marks by which their former presence can be detected: Thus every
geologist is familiar with the winding and twisting "trails" formed
on the surface of the strata by sea-worms; and the impressions
left by the stranded carcases of Jelly-fishes on the fine-grained
lithographic slates of Solenhofen supply us with an example of how
a creature which is little more than "organised sea-water" may
still make an abiding mark upon the sands of time. As a general
rule, however, animals which have no skeletons are incapable of
being preserved as fossils, and hence there must always have
been a vast number of different kinds of marine animals of which
we have absolutely no record whatever. Again, almost all the
fossiliferous rocks have been laid down in water; and it is a
necessary result of this that the great majority of fossils are
the remains of aquatic animals. The remains of air-breathing
animals, whether of the inhabitants of the land or of the air
itself, are comparatively rare as fossils, and the record of
the past existence of these is much more imperfect than is the
case with animals living in water. Moreover, the fossiliferous
deposits are not only almost exclusively aqueous formations, but
the great majority are marine, and only a comparatively small
number have been formed by lakes and rivers. It follows from the
foregoing that the palaeontological record is fullest and most
complete so far as sea-animals are concerned, though even here we
find enormous gaps, owing to the absence of hard structures in
many great groups; of animals inhabiting fresh waters our knowledge
is rendered still further incomplete by the small proportion
that fluviatile and lacustrine deposits bear to marine; whilst
we have only a fragmentary acquaintance with the air-breathing
animals which inhabited the earth during past ages.
Lastly, the imperfection of the palaeontological record, due to
the causes above enumerated, is greatly aggravated, especially
as regards the earlier portion of the earth's history, by the
fact that many rocks which contained fossils when deposited have
since been rendered barren of organic remains. The principal cause
of this common phenomenon is what is known as "metamorphism"--that
is, the subjection of the rock to a sufficient amount of heat to
cause a rearrangement of its particles. When at all of a pronounced
character, the result of metamorphic action is invariably the
obliteration of any fossils which might have been originally
present in the rock. Metamorphism may affect rocks of any age,
though naturally more prevalent in the older rocks, and to this
cause must be set down an irreparable loss of much fossil evidence.
The most striking example which is to be found of this is the
great Laurentian series, which comprises some 30,000 feet of
highly-metamorphosed sediments, but which, with one not wholly
undisputed exception, has as yet yielded no remains of living
beings, though there is strong evidence of the former existence
in it of fossils.
Upon the whole, then, we cannot doubt that the earth's crust, so
far as yet deciphered by us, presents us with but a very imperfect
record of the past. Whether the known and admitted imperfections
of the geological and palaeontological records are sufficiently
serious to account satisfactorily for the deficiency of direct
evidence recognisable in some modern hypotheses, may be a matter
of individual opinion. There can, however, be little doubt that
they are sufficiently extensive to throw the balance of evidence
decisively in favour of some theory of _continuity_, as opposed
to any theory of intermittent and occasional action. The apparent
breaks which divide the great series of the stratified rocks
into a number of isolated formations, are not marks of mighty
and general convulsions of nature, but are simply indications
of the imperfection of our knowledge. Never, in all probability,
shall we be able to point to a complete series of deposits, or a
complete succession of life linking one great geological period
to another. Nevertheless, we may well feel sure that such deposits
and such an unbroken succession must have existed at one time.
We are compelled to believe that nowhere in the long series of
the fossiliferous rocks has there been a total break, but that
there must have been a complete continuity of life, and a more
or less complete continuity of sedimentation, from the Laurentian
period to the present day. One generation hands on the lamp of
life to the next, and each system of rocks is the direct offspring
of those which preceded it in time. Though there has not been
continuity in any given area, still the geological chain could
never have been snapped at one point, and taken up again at a
totally different one. Thus we arrive at the conviction that
_continuity_ is the fundamental law of geology, as it is of the
other sciences, and that the lines of demarcation between the
great formations are but gaps in our own knowledge.
CHAPTER V.
CONCLUSIONS TO BE DRAWN FROM FOSSILS.
We have already seen that geologists have been led by the study
of fossils to the all-important generalisation that the vast
series of the Fossiliferous or Sedimentary Rocks may be divided
into a number of definite groups or "formations," each of which is
characterised by its organic remains. It may simply be repeated here
that these formations are not properly and strictly characterised
by the occurrence in them of any one particular fossil. It may be
that a formation contains some particular fossil or fossils not
occurring out of that formation, and that in this way an observer
may identify a given group with tolerable certainty. It very often
happens, indeed, that some particular stratum, or sub-group of a
series, contains peculiar fossils, by which its existence may
be determined in various localities. As before remarked, however,
the great formations are characterised properly by the association
of certain fossils, by the predominance of certain families or
orders, or by an _assemblage_ of fossil remains representing
the "life" of the period in which the formation was deposited.
Fossils, then, enable us to determine the _age_ of the deposits
in which they occur. Fossils further enable us to come to very
important conclusions as to the mode in which the fossiliferous
bed was deposited, and thus as to the condition of the particular
district or region occupied by the fossiliferous bed at the time
of the formation of the latter. If, in the first place, the bed
contain the remains of animals such as now inhabit rivers, we
know that it is "fluviatile" in its origin, and that it must at
one time have either formed an actual riverbed, or been deposited
by the overflowing of an ancient stream. Secondly, if the bed
contain the remains of shellfish, minute crustaceans, or fish,
such as now inhabit lakes, we know that it is "lacustrine," and
was deposited beneath the waters of a former lake. Thirdly, if
the bed contain the remains of animals such as now people the
ocean, we know that it is "marine" in its origin, and that it
is a fragment of an old sea-bottom.
We can, however, often determine the conditions under which a bed
was deposited with greater accuracy than this. If, for example, the
fossils are of kinds resembling the marine animals now inhabiting
shallow waters, if they are accompanied by the detached relics
of terrestrial organisms, or if they are partially rolled and
broken, we may conclude that the fossiliferous deposit was laid
down in a shallow sea, in the immediate vicinity of a coast-line,
or as an actual shore-deposit. If, again, the remains are those
of animals such as now live in the deeper parts of the ocean,
and there is a very sparing intermixture of extraneous fossils
(such as the bones of birds or quadrupeds, or the remains of
plants), we may presume that the deposit is one of deep water.
In other cases, we may find, scattered through the rock, and
still in their natural position, the valves of shells such as
we know at the present day as living buried in the sand or mud
of the sea-shore or of estuaries. In other cases, the bed may
obviously have been an ancient coral-reef, or an accumulation of
social shells, like Oysters. Lastly, if we find the deposit to
contain the remains of marine shells, but that these are dwarfed
of their fair proportions and distorted in figure, we may conclude
that it was laid down in a brackish sea, such as the Baltic, in
which the proper saltness was wanting, owing to its receiving
an excessive supply of fresh water.
In the preceding, we have been dealing simply with the remains
of aquatic animals, and we have seen that certain conclusions
can be accurately reached by an examination of these. As regards
the determination of the conditions of deposition from the remains
of aerial and terrestrial animals, or from plants, there is not
such an absolute certainty. The remains of land-animals would,
of course, occur in "sub-aerial" deposits--that is, in beds,
like blown sand, accumulated upon the land. Most of the remains
of land-animals, however, are found in deposits which have been
laid down in water, and they owe their present position to the
fact that their former owners were drowned in rivers or lakes,
or carried out to sea by streams. Birds, Flying Reptiles, and
Flying Mammals might also similarly find their way into aqueous
deposits; but it is to be remembered that many birds and mammals
habitually spend a great part of their time in the water, and
that these might therefore be naturally expected to present
themselves as fossils in Sedimentary Rocks. Plants, again, even
when undoubtedly such as must have grown on land, do not prove
that the bed in which they occur was formed on land. Many of the
remains of plants known to us are extraneous to the bed in which
they are now found, having reached their present site by falling
into lakes or rivers, or being carried out to sea by floods or
gales of wind. There are, however, many cases in which plants
have undoubtedly grown on the very spot where we now find them.
Thus it is now generally admitted that the great coal-fields
of the Carboniferous age are the result of the growth _in situ_
of the plants which compose coal, and that these grew on vast
marshy or partially submerged tracts of level alluvial land. We
have, however, distinct evidence of old land-surfaces, both in
the Coal-measures and in other cases (as, for instance, in the
well-known "dirt-bed" of the Purbeck series). When, for example,
we find the erect stumps of trees standing at right angles to
the surrounding strata, we know that the surface through which
these send their roots was at one time the surface of the dry
land, or, in other words, was an ancient soil (fig. 19).
[Illustration: Fig. 19.--Erect Tree containing Reptilian remains.
Coal-measures, Nova Scotia. (After Dawson.)
In many cases fossils enable us to come to important conclusions
as to the climate of the period in which they lived but only a
few instances of this can be here adduced. As fossils in the
majority of instances are the remains of marine animals, it is
mostly the temperature of the sea which can alone be determined
in this way; and it is important to remember that, owing to the
existence of heated currents, the marine climate of a given area
does not necessarily imply a correspondingly warm climate in
the neighbouring land. Land-climates can only be determined by
the remains of land-animals or land-plants, and these are
comparatively rare as fossils. It is also important to remember
that all conclusions on this head are really based upon the present
distribution of animal and vegetable life on the globe, and are
therefore liable to be vitiated by the following considerations:--
a. Most fossils are extinct, and it is not certain that the
habits and requirements of any extinct animal were exactly similar
to those of its nearest living relative.
b. When we get very far back in time, we meet with groups of
organisms so unlike anything we know at the present day as to
render all conjectures as to climate founded upon their supposed
habits more or less uncertain and unsafe.
c. In the case of marine animals, we are as yet very far from
knowing the exact limits of distribution of many species within
our present seas; so that conclusions drawn from living forms
as to extinct species are apt to prove incorrect. For instance,
it has recently been shown that many shells formerly believed to
be confined to the Arctic Seas have, by reason of the extension
of Polar currents, a wide range to the south; and this has thrown
doubt upon the conclusions drawn from fossil shells as to the
Arctic conditions under which certain beds were supposed to have
been deposited.
d. The distribution of animals at the present day is certainly
dependent upon other conditions beside climate alone; and the causes
which now limit the range of given animals are certainly such as
belong to the existing order of things. But the establishment of
the present order of things does not date back in many cases to
the introduction of the present species of animals. Even in the
case, therefore, of existing species of animals, it can often
be shown that the past distribution of the species was different
formerly to what it is now, not necessarily because the climate
has changed, but because of the alteration of other conditions
essential to the life of the species or conducing to its extension.
Still, we are in many cases able to draw completely reliable
conclusions as to the climate of a given geological period, by
an examination of the fossils belonging to that period. Among
the more striking examples of how the past climate of a region
may be deduced from the study of the organic remains contained in
its rocks, the following may be mentioned: It has been shown that
in Eocene times, or at the commencement of the Tertiary period,
the climate of what is now Western Europe was of a tropical or
sub-tropical character. Thus the Eocene beds are found to contain
the remains of shells such as now inhabit tropical seas, as, for
example, Cowries and Volutes; and with these are the fruits of
palms, and the remains of other tropical plants. It has been
shown, again, that in Miocene times, or about the middle of the
Tertiary period, Central Europe was peopled with a luxuriant
flora resembling that of the warmer parts of the United States,
and leading to the conclusion that the mean annual temperature
must have been at least 30 deg. hotter than it is at present. It has
been shown that, at the same time, Greenland, now buried beneath
a vast ice-shroud, was warm enough to support a large number of
trees, shrubs, and other plants, such as inhabit temperate regions
of the globe. Lastly, it has been shown upon physical as well as
palaeontological evidence, that the greater part of the North
Temperate Zone, at a comparatively recent geological period, has
been visited with all the rigours of an Arctic climate, resembling
that of Greenland at the present day. This is indicated by the
occurrence of Arctic shells in the superficial deposits of this
period, whilst the Musk-ox and the Reindeer roamed far south of
their present limits.
Lastly, it was from the study of fossils that geologists learnt
originally to comprehend a fact which may be regarded as of cardinal
importance in all modern geological theories and
speculations--namely, that the crust of the earth is liable to
local elevations and subsidences. For long after the remains of
shells and other marine animals were for the first time observed
in the solid rocks forming the dry land, and at great heights
above the sea-level, attempts were made to explain this almost
unintelligible phenomenon upon the hypothesis that the fossils
in question were not really the objects they represented, but
were in truth mere _lusus naturoe_, due to some "plastic virtue
latent in the earth." The common-sense of scientific men, however,
soon rejected this idea, and it was agreed by universal consent
that these bodies really were remains of animals which formerly
lived in the sea. When once this was admitted, the further steps
were comparatively easy, and at the present day no geological
doctrine stands on a firmer basis than that which teaches us
that our present continents and islands, fixed and immovable as
they appear, have been repeatedly sunk beneath the ocean.
CHAPTER VI.
THE BIOLOGICAL RELATIONS OF FOSSILS.
Not only have fossils, as we have seen, a most important bearing
upon the sciences of Geology and Physical Geography, but they
have relations of the most complicated and weighty character with
the numerous problems connected with the study of living beings,
or in other words, with the science of Biology. To such an extent
is this the case, that no adequate comprehension of Zoology and
Botany, in their modern form, is so much as possible without
some acquaintance with the types of animals and plants which have
passed away. There are also numerous speculative questions in
the domain of vital science, which, if soluble at all, can only
hope to find their key in researches carried out on extinct
organisms. To discuss fully the biological relations of fossils
would, therefore, afford matter for a separate treatise; and all
that can be done here is to indicate very cursorily the principal
points to which the attention of the palaeontological student
ought to be directed.
In the first place, the great majority of fossil animals and
plants are "extinct"--that is to say, they belong to species
which are no longer in existence at the present day. So far,
however, from there being any truth in the old view that there
were periodic destructions of all the living beings in existence
upon the earth, followed by a corresponding number of new creations
of animals and plants, the actual facts of the case show that
the extinction of old forms and the introduction of new forms
have been processes constantly going on throughout the whole
of geological time. Every species seems to come into being at
a certain definite point of time, and to finally disappear at
another definite point; though there are few instances indeed,
if there are any, in which our present knowledge would permit
us safely to fix with precision the times of entrance and exit.
There are, moreover, marked differences in the actual time during
which different species remained in existence, and therefore
corresponding differences in their "vertical range," or, in other
words, in the actual amount and thickness of strata through which
they present themselves as fossils. Some species are found to
range through two or even three formations, and a few have an
even more extended life. More commonly the species which begin
in the commencement of a great formation die out at or before its
close, whilst those which are introduced for the first time near
the middle or end of the formation may either become extinct, or
may pass on into the next succeeding formation. As a general rule,
it is the animals which have the lowest and simplest organisation
that have the longest range in time, and the additional possession
of microscopic or minute dimensions seems also to favour longevity.
Thus some of the _Foraminifera_ appear to have survived, with
little or no perceptible alteration, from the Silurian period
to the present day; whereas large and highly-organised animals,
though long-lived as _individuals_, rarely seem to live long
_specifically_, and have, therefore, usually a restricted vertical
range. Exceptions to this, however, are occasionally to be found
in some "persistent types," which extend through a succession
of geological periods with very little modification. Thus the
existing Lampshells of the genus _Lingula_ are little changed
from the _Linguloe_ which swarmed in the Lower Silurian seas; and
the existing Pearly Nautilus is the last descendant of a clan
nearly as ancient. On the other hand, some forms are singularly
restricted in their limits, and seem to have enjoyed a comparatively
brief lease of life. An example of this is to be found in many of
the _Ammonites_--close allies of the Nautilus--which are often
confined strictly to certain zones of strata, in some cases of
very insignificant thickness.
Of the _causes_ of extinction amongst fossil animals and plants,
we know little or nothing. All we can say is, that the attributes
which constitute a _species_ do not seem to be intrinsically
endowed with permanence, any more than the attributes which
constitute an _individual_, though the former may endure whilst
many successive generations of the latter have disappeared. Each
species appears to have its own life-period, its commencement,
its culmination, and its gradual decay; and the life-periods
of different species may be of very different duration.
From what has been said above, it may be gathered that our existing
species of animals and plants are, for the most part, quite of modern
origin, using the term "modern" in its geological acceptation.
Measured by human standards, the majority of existing animals
(which are capable of being preserved as fossils) are known to
have a high antiquity; and some of them can boast of a pedigree
which even the geologist may regard with respect. Not a few of
our shellfish are known to have commenced their existence at
some point of the Tertiary period; one Lampshell (_Terebratulina
caput-serpentis_) is believed to have survived since the Chalk; and
some of the _Foraminifera_ date, at any rate, from the Carboniferous
period. We learn from this the additional fact that our existing
animals and plants do not constitute an assemblage of organic
forms which were introduced into the world collectively and
simultaneously, but that they commenced their existence at very
different periods, some being extremely old, whilst others may be
regarded as comparatively recent animals. And this introduction of
the existing fauna and flora was a slow and _gradual_ process, as
shown admirably by the study of the fossil shells of the Tertiary
period. Thus, in the earlier Tertiary period, we find about 95
per cent of the known fossil shells to be species that are no
longer in existence, the remaining 5 per cent being forms which are
known to live in our present seas. In the middle of the Tertiary
period we find many more recent and still existing species of
shells, and the extinct types are much fewer in number; and this
gradual introduction of forms now living goes on steadily, till,
at the close of the Tertiary period, the proportions with which
we started may be reversed, as many as 90 or 95 per cent of the
fossil shells being forms still alive, while not more than 5 per
cent may have disappeared.
All known animals at the present day may be divided into some
five or six primary divisions, which are known technically as
"_sub-kingdoms_." Each of these sub-kingdoms [9] may be regarded
as representing a certain type or plan of structure, and all
the animals comprised in each are merely modified forms of this
common type. Not only are all known living animals thus reducible
to some five or six fundamental plans of structure, but amongst
the vast series of fossil forms no one has yet been found--however
unlike any existing animal--to possess peculiarities which would
entitle it to be placed in a new sub-kingdom. All fossil animals,
therefore, are capable of being referred to one or other of the
primary divisions of the animal kingdom. Many fossil groups have
no closely-related group now in existence; but in no case do
we meet with any grand structural type which has not survived
to the present day.
[Footnote 9: In the Appendix a brief definition is given of the
sub-kingdoms, and the chief divisions of each are enumerated.]
The old types of life differ in many respects from those now
upon the earth; and the further back we pass in time, the more
marked does this divergence become. Thus, if we were to compare
the animals which lived in the Silurian seas with those inhabiting
our present oceans, we should in most instances find differences
so great as almost to place us in another world. This divergence
is the most marked in the Palaeozoic forms of life, less so in
those of the Mesozoic period, and less still in the Tertiary
period. Each successive formation has therefore presented us
with animals becoming gradually more and more like those now in
existence; and though there is an immense and striking difference
between the Silurian animals and those of to-day, this difference
is greatly reduced if we compare the Silurian fauna with the
Devonian; _that_ again with the Carboniferous; and so on till
we reach the present.
It follows from the above that the animals of any given formation
are more like those of the next formation below, and of the next
formation above, than they are to any others; and this fact of
itself is an almost inexplicable one, unless we believe that
the animals of any given formation are, in part at any rate, the
lineal descendants of the animals of the preceding formation,
and the progenitors, also in part at least, of the animals of the
succeeding formation. In fact, the palaeontologist is so commonly
confronted with the phenomenon of closely-allied forms of animal
life succeeding one another in point of time, that he is compelled
to believe that such forms have been developed from some common
ancestral type by some process of "_evolution_." On the other
hand, there are many phenomena, such as the apparently sudden
introduction of new forms throughout all past time, and the common
occurrence of wholly isolated types, which cannot be explained
in this way. Whilst it seems certain, therefore, that many of
the phenomena of the succession of animal life in past periods
can only be explained by some law of evolution, it seems at the
same time certain that there has always been some other deeper
and higher law at work, on the nature of which it would be futile
to speculate at present.
Not only do we find that the animals of each successive formation
become gradually more and more like those now existing upon the
globe, as we pass from the older rocks into the newer, but we also
find that there has been a gradual progression and development
in the _types_ of animal life which characterise the geological
ages. If we take the earliest-known and oldest examples of any
given group of animals, it can sometimes be shown that these
primitive forms, though in themselves highly organised, possessed
certain characters such as are now only seen in the _young_ of
their existing representatives. In technical language, the early
forms of life in some instances possess "_embryonic_" characters,
though this does not prevent them often attaining a size much
more gigantic than their nearest living relatives. Moreover, the
ancient forms of life are often what is called "comprehensive
types"--that is to say, they possess characters in combination
such as we nowadays only find separately developed in different,
groups of animals. Now, this permanent retention of embryonic
characters and this "comprehensiveness" of structural type are
signs of what a zoologist considers to be a comparatively low
grade of organisation; and the prevalence of these features in
the earlier forms of animals is a very striking phenomenon, though
they are none the less perfectly organised so far as their own
type is concerned. As we pass upwards in the geological scale,
we find that these features gradually disappear, higher and ever
higher forms are introduced, and "specialisation" of type takes
the place of the former comprehensiveness. We shall have occasion
to notice many of the facts on which these views are based at
a later period, and in connection with actual examples. In the
meanwhile, it is sufficient to state, as a widely-accepted
generalisation of palaeontology, that there has been in the past
a general progression of organic types, and that the appearance
of the lower forms of life has in the main preceded that of the
higher forms in point of time.
PART II
HISTORICAL PALAEONTOLOGY
CHAPTER VII.
THE LAURENTIAN AND HURONIAN PERIODS.
The _Laurentian Rocks_ constitute the base of the entire stratified
series, and are, therefore, the oldest sediments of which we have
as yet any knowledge. They are more largely and more typically
developed in North America, and especially in Canada, than in
any known part of the world, and they derive their title from
the range of hills which the old French geographers named the
"Laurentides." These hills are composed of Laurentian Rocks, and
form the watershed between the valley of the St Lawrence river
on the one hand, and the great plains which stretch northwards
to Hudson Bay on the other hand. The main area of these ancient
deposits forms a great belt of rugged and undulating country,
which extends from Labrador westwards to Lake Superior, and then
bends northwards towards the Arctic Sea. Throughout this extensive
area the Laurentian Rocks for the most part present themselves
in the form of low, rounded, ice-worn hills, which, if generally
wanting in actual sublimity, have a certain geological grandeur
from the fact that they "have endured the battles and the storms
of time longer than any other mountains" (Dawson). In some places,
however, the Laurentian Rocks produce scenery of the most magnificent
character, as in the great gorge cut through them by the river
Saguenay, where they rise at times into vertical precipices 1500
feet in height. In the famous group of the Adirondack mountains,
also, in the state of New York, they form elevations no less than
6000 feet above the level of the sea. As a general rule, the
character of the Laurentian region is that of a rugged, rocky,
rolling country, often densely timbered, but rarely well fitted
for agriculture, and chiefly attractive to the hunter and the
miner.
As regards its mineral characters, the Laurentian series is composed
throughout of metamorphic and highly crystalline rocks, which
are in a high degree crumpled, folded, and faulted. By the late
Sir William Logan the entire series was divided into two great
groups, the _Lower Laurentian_ and the _Upper Laurentian_, of
which the latter rests unconformably upon the truncated edges
of the former, and is in turn unconformably overlaid by strata
of Huronian and Cambrian age (fig. 20).
[Illustration: Fig. 20.--Diagrammatic section of the Laurentian
Rocks in Lower Canada. a Lower Laurentian; b Upper Laurentian,
resting unconformably upon the lower series; c Cambrian strata
(Potsdam Sandstone), resting unconformably on the Upper Laurentian.]
The _Lower Laurentian_ series attains the enormous thickness of
over 20,000 feet, and is composed mainly of great beds of gneiss,
altered sandstones (quartzites), mica-schist, hornblende-schist,
magnetic iron-ore, and haematite, together with masses of limestone.
The limestones are especially interesting, and have an extraordinary
development--three principal beds being known, of which one is
not less than 1500 feet thick; the collective thickness of the
whole being about 3500 feet.
The _Upper Laurentian_ series, as before said, reposes unconformably
upon the Lower Laurentian, and attains a thickness of at least
10,000 feet. Like the preceding, it is wholly metamorphic, and
is composed partly of masses of gneiss and quartzite; but it
is especially distinguished by the possession of great beds of
felspathic rock, consisting principally of "Labrador felspar."
Though typically developed in the great Canadian area already
spoken of, the Laurentian Rocks occur in other localities, both
in America and in the Old World. In Britain, the so-called
"fundamental gneiss" of the Hebrides and of Sutherlandshire is
probably of Lower Laurentian age, and the "hypersthene rocks"
of the Isle of Skye may, with great probability, be regarded
as referable to the Upper Laurentian. In other localities in
Great Britain (as in St David's, South Wales; the Malvern Hills;
and the North of Ireland) occur ancient metamorphic deposits
which also are probably referable to the Laurentian series. The
so-called "primitive gneiss" of Norway appears to belong to the
Laurentian, and the ancient metamorphic rocks of Bohemia and
Bavaria may be regarded as being approximately of the same age.
[Illustration: Fig. 21.--Section of Lower Laurentian Limestone
from Hull, Ottawa; enlarged five diameters. The rock is very
highly crystalline, and contains mica and other minerals. The
irregular black masses in it are graphite. (Original.)]
By some geological writers the ancient and highly metamorphosed
sediments of the Laurentian and the succeeding Huronian series
have been spoken of as the "Azoic rocks" (Gr. _a_, without; _zoe_,
life); but even if we were wholly destitute of any evidence of
life during these periods, this name would be objectionable upon
theoretical grounds. If a general name be needed, that of "Eozoic"
(Gr. _eos_, dawn; _zoe_, life), proposed by Principal Dawson, is the
most appropriate. Owing to their metamorphic condition, geologists
long despaired of ever detecting any traces of life in the vast pile
of strata which constitute the Laurentian System. Even before any
direct traces were discovered, it was, however, pointed out that
there were good reasons for believing that the Laurentian seas had
been tenanted by an abundance of living beings. These reasons are
briefly as follows:--(1) Firstly, the Laurentian series consists,
beyond question, of marine sediments which originally differed
in no essential respect from those which were subsequently laid
down in the Cambrian or Silurian periods. (2) In all formations
later than the Laurentian, any limestones which are present can
be shown, with few exceptions, to be _organic_ rocks, and to be
more or less largely made up of the comminuted debris of marine
or fresh-water animals. The Laurentian limestones, in consequence
of the metamorphism to which they have been subjected, are so
highly crystalline (fig. 21) that the microscope fails to detect
any organic structure in the rock, and no fossils beyond those
which will be spoken of immediately have as yet been discovered in
them. We know, however, of numerous cases in which limestones,
of later age, and undoubtedly organic to begin with, have been
rendered so intensely crystalline by metamorphic action that
all traces of organic structure have been obliterated. We have
therefore, by analogy, the strongest possible ground for believing
that the vast beds of Laurentian limestone have been originally
organic in their origin, and primitively composed, in the main,
of the calcareous skeletons of marine animals. It would, in fact,
be a matter of great difficulty to account for the formation
of these great calcareous masses on any other hypothesis. (3)
The occurrence of phosphate of lime in the Laurentian Rocks in
great abundance, and sometimes in the form of irregular beds,
may very possibly be connected with the former existence in the
strata of the remains of marine animals of whose skeleton this
mineral is a constituent. (4) The Laurentian Rocks contain a
vast amount of carbon in the form of black-lead or _graphite_.
This mineral is especially abundant in the limestones, occurring
in regular beds, in veins or strings, or disseminated through
the body of the limestone in the shape of crystals, scales, or
irregular masses. The amount of graphite in some parts of the
Lower Laurentian is so great that it has been calculated as equal
to the quantity of carbon present in an equal thickness of the
Coal-measures. The general source of solid carbon in the crust
of the earth is, however, plant-life; and it seems impossible to
account for the Laurentian graphite, except upon the supposition
that it is metamorphosed vegetable matter. (5) Lastly, the great
beds of iron-ore (peroxide and magnetic oxide) which occur in the
Laurentian series interstratified with the other rocks, point
with great probability to the action of vegetable life; since
similar deposits in later formations can commonly be shown to
have been formed by the deoxidising power of vegetable matter
in a state of decay.
In the words of Principal Dawson, "anyone of these reasons might,
in itself, be held insufficient to prove so great and, at first
sight, unlikely a conclusion as that of the existence of abundant
animal and vegetable life in the Laurentian; but the concurrence
of the whole in a series of deposits unquestionably marine, forms
a chain of evidence so powerful that it might command belief
even if no fragment of any organic or living form or structure
had ever been recognised in these ancient rocks." Of late years,
however, there have been discovered in the Laurentian Rocks certain
bodies which are believed to be truly the remains of animals,
and of which by far the most important is the structure known
under the now celebrated name of _Eozooen_. If truly organic, a
very special and exceptional interest attaches itself to _Eozooen_,
as being the most ancient fossil animal of which we have any
knowledge; but there are some who regard it really a peculiar
form of mineral structure, and a severe, protracted, and still
unfinished controversy has been carried on as to its nature. Into
this controversy it is wholly unnecessary to enter here; and it
will be sufficient to briefly explain the structure of _Eozooen_,
as elucidated by the elaborate and masterly investigations of
Carpenter and Dawson, from the standpoint that it is a genuine
organism--the balance of evidence up to this moment inclining
decisively to this view.
[Illustration: Fig. 22.--Fragment of _Eozooen_, of the natural
size, showing alternate laminae of loganite and dolomite. (After
Dawson.)]
The structure known as _Eozooen_ is found in various localities
in the Lower Laurentian limestones of Canada, in the form of
isolated masses or spreading layers, which are composed of thin
alternating laminae, arranged more or less concentrically (fig.
22). The laminae of these masses are usually of different colours
and composition; one series being white, and composed of carbonate
of lime--whilst the laminae of the second series alternate with
the preceding, are green in colour, and are found by chemical
analysis to consist of some silicate, generally serpentine or the
closely-related "loganite." In some instances, however, all the
laminae are calcareous, the concentric arrangement still remaining
visible in consequence of the fact that the laminae are composed
alternately of lighter and darker coloured limestone.
When first discovered, the masses of _Eozooen_ were supposed to
be of a mineral nature; but their striking general resemblance
to the undoubted fossils which will be subsequently spoken of
under the name of _Stromatopora_ was recognised by Sir William
Logan, and specimens were submitted for minute examination, first
to Principal Dawson, and subsequently to Dr W. B. Carpenter.
After a careful microscopic examination, these two distinguished
observers came to the conclusion that _Eozooen_ was truly organic,
and in this opinion they were afterwards corroborated by other
high authorities (Mr W. K. Parker, Professor Rupert Jones, Mr H.
B. Brady, Professor Guembel, &c.) Stated briefly, the structure
of _Eozooen_, as exhibited by the microscope, is as follows:--
[Illustration: Fig. 23.--Diagram of a portion of _Eozooen_ cut
vertically. A, B, C, Three tiers of chambers communicating with
one another by slightly constricted apertures: _a a_, The true
shell-wall, perforated by numerous delicate tubes; _b b_. The
main calcareous skeleton ("intermediate skeleton"); c, Passage
of communication ("stolon-passage") from one tier of chambers
to another; d, Ramifying tubes in the calcareous skeleton.
(After Carpenter.)]
The concentrically-laminated mass of _Eozooen_ is composed of
numerous calcareous layers, representing the original skeleton
of the organism (fig. 23, b). These calcareous layers serve to
separate and define a series of chambers arranged in successive
tiers, one above the other (fig. 23, A, B, C); and they are
perforated not only by passages (fig. 23, c), which serve to
place successive tiers of chambers in communication, but also by
a system of delicate branching canals (fig. 23, d). Moreover,
the central and principal portion of each calcareous layer, with
the ramified canal-system just spoken of, is bounded both above
and below by a thin lamina which has a structure of its own, and
which may be regarded as the proper shell-wall (fig. 23, a a).
This proper wall forms the actual lining of the chambers, as well
as the outer surface of the whole mass; and it is perforated with
numerous fine vertical tubes (fig. 24, a a), opening into the
chambers and on to the surface by corresponding fine pores. From
the resemblance of this tubulated layer to similar structures
in the shell of the Nummulite, it is often spoken of as the
"Nummuline layer." The chambers are sometimes piled up one above
the other in an irregular manner; but they are more commonly
arranged in regular tiers, the separate chambers being marked
off from one another by projections of the wall in the form of
partitions, which are so far imperfect as to allow of a free
communication between contiguous chambers. In the original condition
of the organism, all these chambers, of course, must have been
filled with living-matter; but they are found in the present
state of the fossil to be generally filled with some silicate,
such as serpentine, which not only fills the actual chambers,
but has also penetrated the minute tubes of the proper wall and
the branching canals of the intermediate skeleton. In some cases
the chambers are simply filled with crystalline carbonate of
lime. When the originally porous fossil has been permeated by
a silicate, it is possible to dissolve away the whole of the
calcareous skeleton by means of acids, leaving an accurate and
beautiful cast of the chambers and the tubes connected with them
in the insoluble silicate.
[Illustration: Fig. 24.--Portion of one of the calcareous layers
of _Eozooen_, magnified 100 diameters. a a, The proper wall
("Nummuline layer") of one of the chambers, showing the fine
vertical tubuli with which it is penetrated, and which are slightly
bent along the line a' a'. c c, The intermediate skeleton,
with numerous branched canals. The oblique lines are the cleavage
planes of the carbonate of lime, extending across both the
intermediate skeleton and the proper wall. (After Carpenter.)]
The above are the actual appearances presented by _Eozooen_ when
examined microscopically, and it remains to see how far they
enable us to decide upon its true position in the animal kingdom.
Those who wish to study this interesting subject in detail must
consult the admirable memoirs by Dr W. B. Carpenter and Principal
Dawson: it will be enough here to indicate the results which
have been arrived at. The only animals at the present day which
possess a continuous calcareous skeleton, perforated by pores
and penetrated by canals, are certain organisms belonging to
the group of the _Foraminifera_. We have had occasion before
to speak of these animals, and as they are not conspicuous or
commonly-known forms of life, it may be well to say a few words
as to the structure of the living representatives of the group.
The _Foraminifera_ are all inhabitants of the sea, and are mostly
of small or even microscopic dimensions. Their bodies are composed
of an apparently structureless animal substance of an albuminous
nature ("sarcode"), of a gelatinous consistence, transparent, and
exhibiting numerous minute granules or rounded particles. The
body-substance cannot be said in itself to possess any definite
form, except in so far as it may be bounded by a shell; but it
has the power, wherever it may be exposed, of emitting long
thread-like filaments ("pseudopodia"), which interlace with one
another to form a network (fig. 25, b). These filaments can be
thrown out at will, and to considerable distances, and can be
again retracted into the soft mass of the general body-substance,
and they are the agents by which the animal obtains its food.
The soft bodies of the _Foraminifera_ are protected by a shell,
which is usually calcareous, but may be composed of sand-grains
cemented together; and it may consist of a single chamber (fig.
26, a), or of many chambers arranged in different ways (fig.
26, _b-f_). Sometimes the shell has but one large opening into
it--the mouth; and then it is from this aperture that the animal
protrudes the delicate net of filaments with which it seeks its
food. In other cases the entire shell is perforated with minute
pores (fig. 26, e), through which the soft body-substance gains
the exterior, covering the whole shell with a gelatinous film
of animal matter, from which filaments can be emitted at any
point. When the shell consists of many chambers, all of these are
placed in direct communication with one another, and the actual
substance of the shell is often traversed by minute canals filled
with living matter (e.g., in _Calcarina_ and _Nummulina_). The
shell, therefore, may be regarded, in such cases, as a more or
less completely porous calcareous structure, filled to its minutest
internal recesses with the substance of the living animal, and
covered externally with a layer of the same substance, giving
off a network of interlacing filaments.
[Illustration: Fig. 25.--The animal of _Nonionina_, one of the
_Foraminifera_, after the shell has been removed by a weak acid;
b, _Gromia_, a single-chambered Foraminifer (after Schultze),
showing the shell surrounded by a network of filaments derived
from the body substance.]
[Illustration: Fig 26.--Shells of living _Foraminifera_. a,
_Orbulina universa_, in its perfect condition, showing the tubular
spines which radiate from the surface of the shell; b, _Globigerina
bulloides_, in its ordinary condition, the thin hollow spines
which are attached to the shell when perfect having been broken
off; c, Textularia variabilis; d, Peneroplis planatus; e, Rotalia
concamerata; f, _Cristellaria subarcuatula._ [Fig. a is after
Wyville Thomson; the others are after Williamson. All the figures
are greatly enlarged.]]
Such, in brief, is the structure of the living _Foraminifera_;
and it is believed that in _Eozooen_ we have an extinct example of
the same group, not only of special interest from its immemorial
antiquity, but hardly less striking from its gigantic dimensions.
In its original condition, the entire chamber-system of _Eozooen_
is believed to have been filled with soft structureless living
matter, which passed from chamber to chamber through the wide
apertures connecting these cavities, and from tier to tier by
means of the tubuli in the shell-wall and the branching canals
in the intermediate skeleton. Through the perforated shell-wall
covering the outer surface the soft body-substance flowed out,
forming a gelatinous investment, from every point of which radiated
an interlacing net of delicate filaments, providing nourishment
for the entire colony. In its present state, as before said,
all the cavities originally occupied by the body-substance have
been filled with some mineral substance, generally with one of
the silicates of magnesia; and it has been asserted that this
fact militates strongly against the organic nature of _Eozooen_,
if not absolutely disproving it. As a matter of fact, however--as
previously noticed--it is by no means very uncommon at the present
day to find the shells of living species of _Foraminifera_ in which
all the cavities primitively occupied by the body-substance, down
to the minutest pores and canals, have been similarly injected
by some analogous silicate, such as glauconite.
Those, then, whose opinions on such a subject deservedly carry the
greatest weight, are decisively of opinion that we are presented
in the _Eozooen_ of the Laurentian Rocks of Canada with an ancient,
colossal, and in some respects abnormal type of the _Foraminifera_.
In the words of Dr Carpenter, it is not pretended that "the doctrine
of the Foraminiferal nature of _Eozooen_ can be _proved_ in the
demonstrative sense;" but it may be affirmed "that the _convergence
of a number of separate and independent probabilities_, all accordant
with that hypothesis, while a separate explanation must be invented
for each of them on any other hypothesis, gives it that _high
probability_ on which we rest in the ordinary affairs of life, in
the verdicts of juries, and in the interpretation of geological
phenomena generally."
It only remains to be added, that whilst _Eozooen_ is by far the
most important organic body hitherto found in the Laurentian, and
has been here treated at proportionate length, other traces of life
have been detected, which may subsequently prove of great interest
and importance. Thus, Principal Dawson has recently described
under the name of _Archoeosphoerinoe_ certain singular rounded
bodies which he has discovered in the Laurentian limestones, and
which he believes to be casts of the shells of _Foraminifera_
possibly somewhat allied to the existing _Globigerinoe_. The same
eminent palaeontologist has also described undoubted worm-burrows
from rocks probably of Laurentian age. Further and more extended
researches, we may reasonably hope, will probably bring to light
other actual remains of organisms in these ancient deposits.
THE HURONIAN PERIOD.
The so-called _Huronian Rocks_, like the Laurentian, have their
typical development in Canada, and derive their name from the
fact that they occupy an extensive area on the borders of Lake
Huron. They are wholly metamorphic, and consist principally of
altered sandstones or quartzites, siliceous, felspathic, or talcose
slates, conglomerates, and limestones. They are largely developed
on the north shore of Lake Superior, and give rise to a broken
and hilly country, very like that occupied by the Laurentians,
with an abundance of timber, but rarely with sufficient soil
of good quality for agricultural purposes. They are, however,
largely intersected by mineral veins, containing silver, gold,
and other metals, and they will ultimately doubtless yield a rich
harvest to the miner. The Huronian Rocks have been identified,
with greater or less certainty, in other parts of North America,
and also in the Old World.
The total thickness of the Huronian Rocks in Canada is estimated
as being not less than 18,000 feet, but there is considerable
doubt as to their precise geological position. In their typical
area they rest unconformably on the edges of strata of _Lower_
Laurentian age; but they have never been seen in direct contact
with the _Upper_ Laurentian, and their exact relations to this
series are therefore doubtful. It is thus open to question whether
the Huronian Rocks constitute a distinct formation, to be
intercalated in point of time between the Laurentian and the
Cambrian groups; or whether, rather, they should not be considered
as the metamorphosed representatives of the Lower Cambrian Rocks
of other regions.
As regards the fossils of the Huronian Rocks, little can be said.
Some of the specimens of _Eozooen Canadense_ which have been
discovered in Canada are thought to come from rocks which are
probably of Huronian age. In Bavaria, Dr Guembel has described a
species of _Eozooen_ under the name of _Eozooen Bavaricum_, from
certain metamorphic limestones which he refers to the Huronian
formation. Lastly, the late Mr Billings described, from rocks
in Newfoundland apparently referable to the Huronian, certain
problematical limpet-shaped fossils, to which he gave the name
of _Aspidella_.
LITERATURE.
Amongst the works and memoirs which the student may consult with
regard to the Laurentian and Huronian deposits may be mentioned
the following:[10]--
(1) 'Report of Progress of the Geological Survey of Canada from its
Commencement to 1863,' pp. 38-49, and pp. 50-66.
(2) 'Manual of Geology.' Dana. 2d Ed. 1875.
(3) 'The Dawn of Life.' J. W, Dawson. 1876.
(4) "On the Occurrence of Organic Remains in the Laurentian Rocks
of Canada." Sir W. E. Logan. 'Quart. Journ. Geol. Soc.,'
xxi. 45-50.'
(5) "On the Structure of Certain Organic Remains in the Laurentian
Limestones of Canada." J. W. Dawson. 'Quart. Journ. Geol.
Soc.,' xxi. 51-59.
(6) "Additional Note on the Structure and Affinities of Eozooen
Canadense." W. B, Carpenter. 'Quart. Journ. Geol. Soc.,' xxi.
59-66.
(7) "Supplemental Notes on the Structure and Affinities of Eozooen'
Canadense," W. B. Carpenter, 'Quart. Journ. Geol. Soc.,'
xxii. 219-228.
(8) "On the So-Called Eozooenal Rocks." King & Rowney. 'Quart.
Journ. Geol. Soc.,' xxii. 185-218.
(9) 'Chemical and Geological Essays.' Sterry Hunt.
The above list only includes some of the more important memoirs
which may be consulted as to the geological and chemical features
of the Laurentian and Huronian Rocks, and as to the true nature
of _Eozooen_. Those who are desirous of studying the later phases
of the controversy with regard to _Eozooen_ must consult the papers
of Carpenter, Carter, Dawson, King & Rowney, Hahn, and others, in
the 'Quart. Journ. of the Geological Society,' the 'Proceedings
of the Royal Irish Academy,' the 'Annals of Natural History,'
the 'Geological Magazine,' &c. Dr Carpenter's 'Introduction to
the Study of the Foraminifera' should also be consulted.
[Footnote 10: In this and in all subsequently following
bibliographical lists, not only is the selection of works and
memoirs quoted necessarily extremely limited; but only such have,
as a general rule, been chosen for mention as are easily accessible
to students who are in the position of being able to refer to a good
library. Exceptions, however, are occasionally made to this rule,
in favour of memoirs or works of special historical interest. It
is also unnecessary to add that it has not been thought requisite
to insert in these lists the well-known handbooks of geological
and palaeontological science; except in such instances as where
they contain special information on special points.]
CHAPTER VIII.
THE CAMBRIAN PERIOD.
The traces of life in the Laurentian period, as we have seen,
are but scanty; but the _Cambrian Rocks_--so called from their
occurrence in North Wales and its borders ("Cambria ")--have
yielded numerous remains of animals and some dubious plants.
The Cambrian deposits have thus a special interest as being the
oldest rocks in which occur any number of well-preserved and
unquestionable organisms. We have here the remains of the first
_fauna_, or assemblage of animals, of which we have at present
knowledge. As regards their geographical distribution, the Cambrian
Rocks have been recognised in many parts of the world, but there
is some question as to the precise limits of the formation, and
we may consider that their most typical area is in South Wales,
where they have been carefully worked out, chiefly by Dr Henry
Hicks. In this region, in the neighbourhood of the promontory
of St David's, the Cambrian Rocks are largely developed, resting
upon an ancient ridge of Pre-Cambrian (Laurentian?) strata, and
overlaid by the lowest beds of the Lower Silurian. The subjoined
sketch-section (fig. 27) exhibits in a general manner the succession
of strata in this locality.
From this section it will be seen that the Cambrian Rocks in
Wales are divided in the first place into a lower and an upper
group. The _Lower Cambrian_ is constituted at the base by a great
series of grits, sandstones, conglomerates, and slates, which
are known as the "Longmynd group," from their vast development
in the Longmynd Hills in Shropshire, and which attain in North
Wales a thickness of 8000 feet or more. The Longmynd beds are
succeeded by the so-called "Menevian group," a series of sandstones,
flags, and grits, about 600 feet in thickness, and containing
a considerable number of fossils. The _Upper Cambrian_ series
consists in its lower portion of nearly 5000 feet of strata,
principally shaly and slaty, which are known as the "Lingula
Flags," from the great abundance in them of a shell referable
to the genus _Lingula_. These are followed by 1000 feet of dark
shales and flaggy sandstones, which are known as the "Tremadoc
slates," from their occurrence near Tremadoc in North Wales;
and these in turn are surmounted, apparently quite conformably,
by the basement beds of the Lower Silurian.
[Illustration: Fig 27. GENERALIZED SECTION OF THE CAMBRIAN ROCKS
IN WALES.]
The above may be regarded as giving a typical series of the Cambrian
Rocks in a typical locality; but strata of Cambrian age are known in
many other regions, of which it is only possible here to allude to
a few of the most important. In Scandinavia occurs a well-developed
series of Cambrian deposits, representing both the lower and
upper parts of the formation. In Bohemia, the Upper Cambrian, in
particular, is largely developed, and constitutes the so-called
"Primordial zone" of Barrande. Lastly, in North America, whilst the
Lower Cambrian is only imperfectly developed, or is represented by
the Huronian, the Upper Cambrian formation has a wide extension,
containing fossils similar in character to the analogous strata
in Europe, and known as the "Potsdam Sandstone." The subjoined
table shows the chief areas where Cambrian Rocks are developed,
and their general equivalency:
TABULAR VIEW OF THE CAMBRIAN FORMATION.
_Britain._ | _Europe._ | _America._
| |
/a. Tremadoc Slates. | a. Primordial zone | a. Potsdam
| | of Bohemia. | Sandstone.
| b. Lingula Flags. | b. Paradoxides | b. Acadian
Upper < | Schists, Olenus | group of New
Cambrian. | | Schists, and | Brunswick.
| | Dictyonema schists |
\ | of Sweden. |
| |
/a. Longmynd Beds. | a. Fucoidal | Huronian
| | Sandstone of Sweden | Formation?
| b. Llanberis Slates.| b. _Eophyton_ |
| | Sandstone of Sweden.|
Lower < c. Harlech Grits. | |
Cambrian. | d. _Oldhamia_ | |
| Slates of Ireland.| |
| e. Conglomerates and| |
| and Sandstones of | |
| Sutherlandshire? | |
\f. Menevian Beds. | |
Like all the older Palaeozoic deposits, the Cambrian Rocks, though
by no means necessarily what would be called actually "metamorphic,"
have been highly cleaved, and otherwise altered from their original
condition. Owing partly to their indurated state, and partly to
their great antiquity, they are usually found in the heart of
mountainous districts, which have undergone great disturbance,
and have been subjected to an enormous amount of denudation. In
some cases, as in the Longmynd Hills in Shropshire, they form
low rounded elevations, largely covered by pasture, and with few
or no elements of sublimity. In other cases, however, they rise
into bold and rugged mountains, girded by precipitous cliffs.
Industrially, the Cambrian Rocks are of interest, if only for
the reason that the celebrated Welsh slates of Llanberis are
derived from highly-cleaved beds of this age. Taken as a whole,
the Cambrian formation is essentially composed of arenaceous
and muddy sediments, the latter being sometimes red, but more
commonly nearly black in colour. It has often been supposed that
the Cambrians are a deep-sea deposit, and that we may thus account
for the few fossils contained in them; but the paucity of fossils
is to a large extent imaginary, and some of the Lower Cambrian
beds of the Longmynd Hills would appear to have been laid down
in shallow water; as they exhibit rain-prints, sun-cracks, and
ripple-marks--incontrovertible evidence of their having been a
shore-deposit. The occurrence, of innumerable worm-tracks and
burrows in many Cambrian strata is also a proof of shallow-water
conditions; and the general absence of limestones, coupled with
the coarse mechanical nature of many of the sediments of the
Lower Cambrian, maybe taken as pointing in the same direction.
The _life_ of the Cambrian, though not so rich as in the succeeding
Silurian period, nevertheless consists of representatives of
most of the great classes of invertebrate animals. The coarse
sandy deposits of the formation, which abound more particularly
towards its lower part, naturally are to a large extent barren
of fossils; but the muddy sediments, when not too highly cleaved,
and especially towards the summit of the group, are replete with
organic remains. This is also the case, in many localities at any
rate, with the finer beds of the Potsdam Sandstone in America.
Limestones are known to occur in only a few areas (chiefly in
America), and this may account for the apparent total absence
of corals. It is, however, interesting to note that, with this
exception, almost all the other leading groups of Invertebrates
are known to have come into existence during the Cambrian period.
Fig. 28.--Fragment of _Eophyton Linneanum_, a supposed land-plant.
Lower Cambrian, Sweden, of the natural size.
Of the land-surfaces of the Cambrian period we know nothing;
and there is, therefore, nothing surprising in the fact that
our acquaintance with the Cambrian vegetation is confined to
some marine plants or sea-weeds, often of a very obscure and
problematical nature. The "Fucoidal Sandstone" of Sweden, and the
"Potsdam Sandstone" of North America, have both yielded numerous
remains which have been regarded as markings left by sea-weeds or
"Fucoids;" but these are highly enigmatical in their characters,
and would, in many instances, seem to be rather referable to the
tracks and burrows of marine worms. The first-mentioned of these
formations has also yielded the curious, furrowed and striated
stems which have been described as a kind of land-plant under
the name of _Eopkyton_ (fig. 28). It cannot be said, however,
that the vegetable origin of these singular bodies has been
satisfactorily proved. Lastly, there are found in certain green
and purple beds of Lower Cambrian age at Bray Head, Wicklow,
Ireland, some very remarkable fossils, which are well known under
the name of _Oldhamia_, but the true nature of which is very
doubtful. The commonest form of _Oldhamia_ (fig. 29) consists of
a thread-like stem or axis, from which spring at regular intervals
bundles of short filamentous branches in a fan-like manner. In
the locality where it occurs, the fronds of _Oldhamia_ are very
abundant, and are spread over the surfaces of the strata in tangled
layers. That it is organic is certain, and that it is a calcareous
sea-weed is probable; but it may possibly belong to the sea-mosses
(_Polyzoa_), or to the sea-firs (_Sertularians_).
Amongst the lower forms of animal life (_Protozoa_), we find the
Sponges represented by the curious bodies, composed of netted
fibres, to which the name of _Protospongia_ has been given (fig.
32, a); and the comparatively gigantic, conical, or cylindrical
fossils termed _Archoeocyathus_ by Mr Billings are certainly
referable either to the _Foraminifera_ or to the Sponges. The
almost total absence of limestones in the formation may be regarded
as a sufficient explanation of the fact that the _Foraminifera_
are not more largely and unequivocally represented; though the
existence of greensands in the Cambrian beds of Wisconsin and
Tennessee may be taken as an indication that this class of animals
was by no means wholly wanting. The same fact may explain the
total absence of corals, so far as at present known.
[Illustration: Fig. 29.--A portion of _Oldhamia antiqua_, Lower
Cambrian, Wicklow, Ireland, of the natural size. (After Salter.)]
The group of the _Echinodermata_ (Sea-lilies, Sea-urchins, and
their allies) is represented by a few forms, which are principally
of interest as being the earliest-known examples of the class.
It is also worthy of note that these precursors of a group which
subsequently attains such geological importance, are referable to
no less than three distinct _orders_--the Crinoids or Sea-lilies,
represented by a species of _Dendrocrinus_; the Cystideans by
_Protocystites_; and the Star-fishes by _Palasterina_ and some
other forms. Only the last of these groups, however, appears
to occur in the Lower Cambrian.
[Illustration: Fig. 30.--Annelide-burrows (_Scolithus linearus_)
from the Potsdam Sandstone of Canada, of the natural size. (After
Billings.)]
The Ringed-worms (_Annelida_), if rightly credited with all the
remains usually referred to them, appear to have swarmed in the
Cambrian seas. Being soft-bodied, we do not find the actual worms
themselves in the fossil condition, but we have, nevertheless,
abundant traces of their existence. In some cases we find vertical
burrows of greater or less depth, often expanded towards their
apertures, in which the worm must have actually lived (fig. 30),
as various species do at the present day. In these cases, the
tube must have been rendered more or less permanent by receiving
a coating of mucus, or perhaps a genuine membranous secretion,
from the body of the animal; and it may be found quite empty,
or occupied by a cast of sand or mud. Of this nature are the
burrows which have been described under the names of _Scolithus_
and _Scolecoderma_, and probably the _Histioderma_ of the Lower
Cambrian of Ireland. In other cases, as in _Arenicolites_ (fig.
32, b), the worm seems to have inhabited a double burrow, shaped
like the letter U, and having two openings placed close together
on the surface of the stratum. Thousands of these twin-burrows
occur in some of the strata of the Longmynd, and it is supposed
that the worm used one opening to the burrow as an aperture of
entrance, and the other as one of exit. In other cases, again,
we find simply the meandering trails caused by the worm dragging
its body over the surface of the mud. Markings of this kind are
commoner in the Silurian Rocks, and it is generally more or less
doubtful whether they may not have been caused by other marine
animals, such as shellfish, whilst some of them have certainly
nothing whatever to do with the worms. Lastly, the Cambrian beds
often show twining cylindrical bodies, commonly more or less
matted together, and not confined to the surfaces of the strata,
but passing through them. These have often been regarded as the
remains of sea-weeds, but it is more probable that they represent
casts of the underground burrows of worms of similar habits to
the common lob-worm (_Arenicola_) of the present day.
The _Articulate_ animals are numerously represented in the Cambrian
deposits, but exclusively by the class of _Crustaceans_. Some
of these are little double-shelled creatures, resembling our
living water-fleas (_Ostracoda_). A few are larger forms, and
belong to the same group as the existing brine-shrimps and
fairy-shrimps (_Phyllopoda_). One of the most characteristic of
these is the _Hymenocaris vermicauda_ of the Lingula Flags (fig.
32, d). By far the larger number of the Cambrian _Crustacea_
belong, however, to the remarkable and wholly extinct group of
the _Trilobites_. These extraordinary animals must have literally
swarmed in the seas of the later portion of this and the whole of
the succeeding period; and they survived in greatly diminished
numbers till the earlier portion of the Carboniferous period.
They died out, however, wholly before the close of the Palaeozoic
epoch, and we have no Crustaceans at the present day which can be
considered as their direct representatives. They have, however,
relationships of a more or less intimate character with the existing
groups of the Phyllopods, the King-crabs (_Limulus_), and the
Isopods ("Slaters," Wood-lice, &c.) Indeed, one member of the
last-mentioned order, namely, the _Serolis_ of the coasts of
Patagonia, has been regarded as the nearest living ally of the
Trilobites. Be this as it may, the Trilobites possessed a skeleton
which, though capable of undergoing almost endless variations,
was wonderfully constant in its pattern of structure, and we
may briefly describe here the chief features of this.
[Illustration: Fig. 31.--Cambrian Trilobites: a, _Paradoxides
Bohemicus_, reduced in size; b, _Ellipsocephalus Hoffi_; c, _Sao
hirsuta_; d, _Conocorypke Sultzeri_ (all the above, together with
fig. g, are from the Upper Cambrian or "Primordial Zone" of
Bohemia); e, Head-shield of _Dikellocephalus Celticus_, from the
Lingula Flags of Wales; f, Head-shield of _Conocoryphe Matthewi_,
from the Upper Cambrian (Acadian Group) of New Brunswick; g,
_Agnostus rex_, Bohemia; h, Tail-shield of _Dikellocephalus
Minnesotensis_, from the Upper Cambrian (Potsdam Sandstone) of
Minnesota. (After Barrande, Dawson, Salter, and Dale Owen.)]
The upper surface of the body of a Trilobite was defended by a
strong shell or "crust," partly horny and partly calcareous in
its composition. This shell (fig. 31) generally exhibits a very
distinct "trilobation" or division into three longitudinal lobes,
one central and two lateral. It also exhibits a more important and
more fundamental division into three transverse portions, which
are so loosely connected with one another as very commonly to be
found separate. The first and most anterior of these divisions
is a shield or buckler which covers the head; the second or middle
portion is composed of movable rings covering the trunk ("thorax
"); and the third is a shield which covers the tailor "abdomen." The
head-shield (fig. 31, e) is generally more or less semicircular
in shape; and its central portion, covering the stomach of the
animal, is usually strongly elevated, and generally marked by
lateral furrows. A little on each side of the head are placed
the eyes, which are generally crescentic in shape, and resemble
the eyes of insects and many existing Crustaceans in being
"compound," or made up of numerous simple eyes aggregated together.
So excellent is the state of preservation of many specimens of
Trilobites, that the numerous individual lenses of the eyes have
been uninjured, and as many as four hundred have been counted
in each eye of some forms. The eyes may be supported upon
prominences, but they are never carried on movable stalks (as
they are in the existing lobsters and crabs); and in some of the
Cambrian Trilobites, such as the little _Agnosti_ (fig. 31 g),
the animal was blind. The lateral portions of the head-shield
are usually separated from the central portion by a peculiar
line of division (the so-called "facial suture") on each side;
but this is also wanting in some of the Cambrian species. The
backward angles of the head-shield, also, are often prolonged
into spines, which sometimes reach a great length. Following
the head-shield behind, we have a portion of the body which is
composed of movable segments or "body-rings," and which is
technically called the "thorax," Ordinarily, this region is strongly
trilobed, and each ring consists of a central convex portion,
and of two flatter side-lobes. The number of body-rings in the
thorax is very variable (from two to twenty-six), but is fixed
for the adult forms of each group of the Trilobites. The young
forms have much fewer rings than the full-grown ones; and it
is curious to find that the Cambrian Trilobites very commonly
have either a great many rings (as in _Paradoxides_, fig. 31,
a), or else very few (as in _Agnostus_, fig. 31, g). In some
instances, the body-rings do not seem to have been so constructed
as to allow of much movement, but in other cases this region of
the body is so flexible that the animal possessed the power of
rolling itself up completely, like a hedgehog; and many individuals
have been permanently preserved as fossils in this defensive
condition. Finally, the body of the Trilobite was completed by
a tail-shield (technically termed the "pygidium"), which varies
much in size and form, and is composed of a greater or less number
of rings, similar to those which form the thorax, but immovably
amalgamated with one another (fig. 31, h).
The under surface of the body in the Trilobites appears to have
been more or less entirely destitute of hard structures, with the
exception of a well-developed upper lip, in the form of a plate
attached to the inferior side of the head-shield in front. There
is no reason to doubt that the animal possessed legs; but these
structures seem to have resembled those of many living Crustaceans
in being quite soft and membranous. This, at any rate, seems to
have been generally the case; though structures which have been
regarded as legs have been detected on the under surface of one
of the larger species of Trilobites. There is also, at present,
no direct evidence that the Trilobites possessed the two pairs
of jointed feelers ("antennae") which are so characteristic of
recent Crustaceans.
The Trilobites vary much in size, and the Cambrian formation
presents examples of both the largest and the smallest members
of the order. Some of the young forms may be little bigger than
a millet-seed, and some adult examples of the smaller species
(such as _Agnostus_) may be only a few lines in length; whilst
such giants of the order as _Paradoxides_ and _Asaphus_ may reach
a length of from one to two feet. Judging from what we actually
know as to the structure of the Trilobites, and also from analogous
recent forms, it would seem that these ancient Crustaceans were
mud-haunting creatures, denizens of shallow seas, and affecting
the soft silt of the bottom rather than the clear water above.
Whenever muddy sediments are found in the Cambrian and Silurian
formations, there we are tolerably sure to find Trilobites, though
they are by no means absolutely wanting in limestones. They appear
to have crawled out upon the sea-bottom, or burrowed in the yielding
mud, with the soft under surface directed downwards; and it is
probable that they really derived their nutriment from the organic
matter contained in the ooze amongst which they lived. The vital
organs seem to have occupied the central lobe of the skeleton,
by which they were protected; and a series of delicate leaf-like
paddles, which probably served as respiratory organs, would appear
to have been carried on the under surface of the thorax. That
they had their enemies may be regarded as certain; but we have
no evidence that they were furnished with any offensive weapons,
or, indeed, with any means of defence beyond their hard crust,
and the power, possessed by so many of them, of rolling themselves
into a ball. An additional proof of the fact that they for the
most part crawled along the sea-bottom is found in the occurrence
of tracks and markings of various kinds, which can hardly be
ascribed to any other creatures with any show of probability.
That this is the true nature of some of the markings in question
cannot be doubted at all; and in other cases no explanation so
probable has yet been suggested. If, however, the tracks which have
been described from the Potsdam Sandstone of North America under
the name of _Protichnites_ are really due to the peregrinations
of some Trilobite, they must have been produced by one of the
largest examples of the order.
As already said, the Cambrian Rocks are very rich in the remains
of Trilobites. In the lowest beds of the series (Longmynd Rocks),
representatives of some half-dozen genera have now been detected,
including the dwarf _Agnostus_ and the giant _Paradoxides_. In
the higher beds, the number both of genera and species is largely
increased; and from the great comparative abundance of individuals,
the Trilobites have every right to be considered as the most
characteristic fossils of the Cambrian period,--the more so as
the Cambrian species belong to peculiar types, which, for the
most part, died out before the commencement of the Silurian epoch.
All the remaining Cambrian fossils which demand any notice here
are members of one or other division of the great class of the
_Mollusca_, or "Shell-fish" properly so called. In the Lower
Cambrian Rocks the Lamp-shells (_Brachiopoda_) are the principal
or sole representatives of the class, and appear chiefly in three
interesting and important types--namely, _Lingulella, Discina,_
and _Obolella_. Of these the last (fig. 32, i) is highly
characteristic of these ancient deposits; whilst _Discina_ is
one of those remarkable persistent types which, commencing at
this early period, has continued to be represented by varying
forms through all the intervening geological formations up to the
present day. _Lingulella_ (fig. 32, c), again, is closely allied
to the existing "Goose-bill" Lamp-shell (_Lingula anatina_), and
thus presents us with another example of an extremely long-lived
type. The _Lingulelloe_ and their successors; the _Linguloe_, are
singular in possessing a shell which is of a horny texture, and
contains but a small proportion of calcareous matter. In the Upper
Cambrian Rocks, the _Lingulelloe_ become much more abundant, the
broad satchel-shaped species known as _L. Davisii_ (fig. 32,
e) being so abundant that one of the great divisions of the
Cambrian is termed the "Lingula Flags." Here, also, we meet for
the first time with examples of the genus Orthis (fig. 32, f,
k, l) a characteristic Palaeozoic type of the Brachiopods, which
is destined to undergo a vast extension in later ages.
[Illustration: Fig 32.--Cambrian Fossils: a, _Protospongia
fenestrata_, Menevian Group; b, _Arenicolites didymus_, Longmynd
Group; c, _Lingulella ferruginea_, Longmynd and Menevian, enlarged;
d, _Hymenocaris vermicauda_, Lingula Flags; e, _Lingulella Davisii_,
Lingula Flags; f, _Orthis lenticularis_, Lingula Flags; g, _Theca
Davidii_, Tremadoc Slates; h, _Modiolopsis Solvensis_, Tremadoc
Slates; i, _Obolela sagittalis_, interior of valve, Menevian;
j, Exterior of the same; k, _Orthis Hicksii_, Menevian; l,
Cast of the same; m, _Olenus micrurus_, Lingula Flags. (Alter
Salter, Hicks, and Davidson.)]
Of the higher groups of the _Mollusca_ the record is as yet but
scanty. In the Lower Cambrian, we have but the thin, fragile,
dagger-shaped shells of the free-swimming oceanic Molluscs or
"Winged-snails" (_Pteropoda_), of which the most characteristic
is the genus _Theca_ (fig. 32, g). In the Upper Cambrian, in
addition to these, we have a few Univalves (_Gasteropoda_), and,
thanks to the researches of Dr Hicks, quite a small assemblage
of Bivalves (_Lamellibranchiata_), though these are mostly of no
great dimensions (fig. 32, h). Of the chambered _Cephalopoda_
(Cuttle-fishes and their allies), we have but few traces; and these
wholly confined to the higher beds of the formation. We meet,
however, with examples of the wonderful genus _Orthoceras_, with
its straight, partitioned shell, which we shall find in an immense
variety of forms in the Silurian rocks. Lastly, it is worthy of
note that the lowest of all the groups of the _Mollusca_--namely,
that of the Sea-mats, Sea-mosses, and Lace-corals (_Polyzoa_)--is
only doubtfully known to have any representatives in the Cambrian,
though undergoing a large and varied development in the Silurian
deposits.
[Illustration: Fig. 33.--Fragment of _Dictyonema sociale_,
considerably enlarged, showing the horny branches, with their
connecting cross-bars, and with a row of cells on each side.
(Original.)]
An exception, however, may with much probability be made to this
statement in favour of the singular genus _Dictyonema_ (fig.
33), which is highly characteristic of the highest Cambrian beds
(Tremadoc Slates). This curious fossil occurs in the form of
fan-like or funnel-shaped expansions, composed of slightly-diverging
horny branches, which are united in a net-like manner by numerous
delicate cross-bars, and exhibit a row of little cups or cells,
in which the animals were contained, on each side. _Dictyonema_
has generally been referred to the _Graptolites_; but it has a
much greater affinity with the plant-like Sea-firs (_Sertularians_)
or the Sea-mosses (_Polyzoa_), and the balance of evidence is
perhaps in favour of placing it with the latter.
LITERATURE.
The following are the more important and accessible works and
memoirs which may be consulted in studying the stratigraphical
and palaeontological relations of the Cambrian Rocks:--
(1) 'Siluria.' Sir Roderick Murchison. 5th ed., pp. 21-46.
(2) 'Synopsis of the Classification of the British Palaeozoic Rocks.'
Sedgwick. Introduction to the 3d Fasciculus of the 'Descriptions
of British Palaeozoic Fossils in the Woodwardian Museum,'
by F. M'Coy, pp. i-xcviii, 1855.
(3) 'Catalogue of the Cambrian and Silurian Fossils in the Geological
Museum of the University of Cambridge.' Salter. With a Preface
by Prof. Sedgwick. 1873.
(4) 'Thesaurus Siluricus.' Bigsby. 1868.
(5) "History of the Names Cambrian and Silurian." Sterry
Hunt.--'Geological Magazine.' 1873.
(6) 'Systeme Silurien du Centre de la Boheme.' Barrande. Vol. I.
(7) 'Report of Progress of the Geological Survey of Canada, from its
Commencement to 1863,' pp. 87-109.
(8) 'Acadian Geology.' Dawson. Pp. 641-657.
(9) "Guide to the Geology of New York," Lincklaen; and "Contributions
to the Palaeontology of New York," James Hall.--'Fourteenth
Report on the State Cabinet.' 1861.
(10) 'Palaeozoic Fossils of Canada.' Billings. 1865.
(11) 'Manual of Geology.' Dana. Pp. 166-182. 2d ed. 1875.
(12) "Geology of North Wales," Ramsay; with Appendix on the
Fossils, Salter.--'Memoirs of the Geological Survey of Great
Britain,' vol. iii. 1866.
(13) "On the Ancient Rocks of the St David's Promontory, South Wales,
and their Fossil Contents." Harkness and Hicks.--' Quart.
Journ. Geol. Soc.,' xxvii. 384-402. 1871.
(14) "On the Tremadoc Rocks in the Neighbourhood of St David's,
South Wales, and their Fossil Contents." Hicks.--'Quart.
Journ. Geol. Soc.,' xxix. 39-52. 1873.
In the above list, allusion has necessarily been omitted to numerous
works and memoirs on the Cambrian deposits of Sweden and Norway,
Central Europe, Russia, Spain, and various parts of North America,
as well as to a number of important papers on the British Cambrian
strata by various well-known observers. Amongst these latter
may be mentioned memoirs by Prof. Phillips, and Messrs Salter,
Hicks, Belt, Plant, Homfray, Ash, Holl, &c.
CHAPTER IX.
THE LOWER SILURIAN PERIOD.
The great system of deposits to which Sir Roderick Murchison
applied the name of "Silurian Rocks" reposes directly upon the
highest Cambrian beds, apparently without any marked unconformity,
though with a considerable change in the nature of the fossils. The
name "Silurian" was originally proposed by the eminent geologist
just alluded to for a great series of strata lying below the Old
Red Sandstone, and occupying districts in Wales and its borders
which were at one time inhabited by the "Silures," a tribe of
ancient Britons. Deposits of a corresponding age are now known
to be largely developed in other parts of England, in Scotland,
and in Ireland, in North America, in Australia, in India, in
Bohemia, Saxony, Bavaria, Russia, Sweden and Norway, Spain, and
in various other regions of less note. In some regions, as in
the neighbourhood of St Petersburg, the Silurian strata are found
not only to have preserved their original horizontality, but
also to have retained almost unaltered their primitive soft and
incoherent nature. In other regions, as in Scandinavia and many
parts of North America, similar strata, now consolidated into
shales, sandstones, and limestones, may be found resting with
a very slight inclination on still older sediments. In a great
many regions, however, the Silurian deposits are found to have
undergone more or less folding, crumpling, and dislocation,
accompanied by induration and "cleavage" of the finer and softer
sediments; whilst in some regions, as in the Highlands of Scotland,
actual "metamorphism" has taken place. In consequence of the
above, Silurian districts usually present the bold, rugged, and
picturesque outlines which are characteristic of the older
"Primitive" rocks of the earth's crust in general. In many instances,
we find Silurian strata rising into mountain-chains of great
grandeur and sublimity, exhibiting the utmost diversity of which
rock-scenery is capable, and delighting the artist with endless
changes of valley, lake, and cliff. Such districts are little
suitable for agriculture, though this is often compensated for
by the valuable mineral products contained in the rocks. On the
other hand, when the rocks are tolerably soft and uniform in
their nature, or when few disturbances of the crust of the earth
have taken place, we may find Silurian areas to be covered with
an abundant pasturage or to be heavily timbered.
Under the head of "Silurian Rocks," Sir Roderick Murchison included
all the strata between the summit of the "Longmynd." beds and the
Old Red Sandstone, and he divided these into the two great groups
of the _Lower_ Silurian and _Upper_ Silurian. It is, however, now
generally admitted that a considerable portion of the basement
beds of Murchison's Silurian series must be transferred---if only
upon palaeontological grounds--to the Upper Cambrian, as has here
been done; and much controversy has been carried on as to the proper
nomenclature of the Upper Silurian and of the remaining portion
of Murchison's Lower Silurian. Thus, some would confine the name
"Silurian" exclusively to the Upper Silurian, and would apply the
name of "Cambro-Silurian" to the Lower Silurian, or would include
all beds of the latter age in the "Cambrian" series of Sedgwick.
It is not necessary to enter into the merits of these conflicting
views. For our present purpose, it is sufficient to recognise
that there exist two great groups of rocks between the highest
Cambrian beds, as here defined, and the base of the Devonian or
Old Red Sandstone. These two great groups are so closely allied
to one another, both physically and palaeontologically, that many
authorities have established a third or intermediate group (the
"Middle Silurian"), by which a passage is made from one into
the other. This method of procedure involves disadvantages which
appear to outweigh its advantages; and the two groups in question
are not only generally capable of very distinct stratigraphical
separation, but at the same time exhibit, together with the alliances
above spoken of, so many and such important palaeontological
differences, that it is best to consider them separately. We
shall therefore follow this course in the present instance; and
pending the final solution of the controversy as to Cambrian and
Silurian nomenclature, we shall distinguish these two groups
of strata as the "Lower Silurian" and the "Upper Silurian."
The _Lower Silurian Rocks_ are known already to be developed
in various regions; and though their _general_ succession in
these areas is approximately the same, each area exhibits
peculiarities of its own, whilst the subdivisions of each are
known by special names. All, therefore, that can be attempted
here, is to select two typical areas--such as Wales and North
America and to briefly consider the grouping and divisions of
the Lower Silurian in each.
In Wales, the line between the Cambrian and Lower Silurian is
somewhat ill-defined, and is certainly not marked by any strong
unconformity. There are, however; grounds for accepting the line
proposed, for palaeontological reasons, by Dr Hicks, in accordance
with which the Tremadoc Slates ("Lower Tremadoc" of Salter) become
the highest of the Cambrian deposits of Britain. If we take this
view, the Lower Silurian rocks of Wales and adjoining districts
are found to have the following _general_ succession from below
upwards (fig. 34):--
1. The _Arenig Group_.--This group derives its name from the
Arenig mountains, where it is extensively developed. It consists
of about 4000 feet of slates, shales, and flags, and is divisible
into a lower, middle, and upper division, of which the former
is often regarded as Cambrian under the name of "Upper Tremadoc
Slates."
2. The _Llandeilo Group_.--The thickness of this group varies
from about 4000 to as much as 10,000 feet; but in this latter
case a great amount of the thickness is made up of volcanic ashes
and interbedded traps. The sedimentary beds of this group are
principally slates and flags, the latter occasionally with calcareous
bands; and the whole series can be divided into a lower, middle,
and upper Llandeilo division, of which the last is the most
important. The name of "Llandeilo" is derived from the town of
the same name in Wales, where strata of this age were described
by Murchison.
3. The _Caradoc_ or _Bala Group_.--The alternative names of this
group are also of local origin, and are derived, the one from
Caer Caradoc in Shropshire, the other from Bala in Wales, strata
of this age occurring in both localities. The series is divided
into a lower and upper group, the latter chiefly composed of
shales and flags, and the former of sandstones and shales, together
with the important and interesting calcareous band known as the
"Bala Limestone." The thickness of the entire series varies from
4000 to as much as 12,000 feet, according as it contains more
or less of interstratified igneous rocks.
4. The _Llandovery Group_ (Lower Llandovery of Murchison).--This
series, as developed near the town of Llandovery, in
Caermarthenshire, consists of less than 1000 feet of conglomerates,
sandstones, and shales. It is probable, however, that the little
calcareous band known as the "Hirnant Limestone," together with
certain pale-coloured slates which lie above the Bala Limestone,
though usually referred to the Caradoc series, should in reality
be regarded as belonging to the Llandovery group.
The general succession of the Lower Silurian strata of Wales
and its borders, attaining a maximum thickness (along with
contemporaneous igneous matter) of nearly 30,000 feet, is
diagramatically represented in the annexed sketch-section (fig.
34):--
[Illustration: Fig 34. GENERALIZED SECTION OF THE LOWER SILURIAN
ROCKS OF WALES.]
In North America, both in the United States and in Canada, the
Silurian rocks are very largely developed, and may be regarded
as constituting an exceedingly full and typical series of the
deposits of this period. The chief groups of the Silurian rocks
of North America are as follows, beginning, as before, with the
lowest strata, and proceeding upwards (fig. 35):--
1. _Quebec Group_.--This group is typically developed in the
vicinity of Quebec, where it consists of about 5000 feet of strata,
chiefly variously-coloured shales, together with some sandstones
and a few calcareous bands. It contains a number of peculiar
Graptolites, by which it can be identified without question with
the Arenig group of Wales and the corresponding Skiddaw Slates
of the North of England. It is also to be noted that numerous
Trilobites of a distinct Cambrian _facies_ have been obtained in
the limestones of the Quebec group, near Quebec. These fossils,
however, have been exclusively obtained from the limestones of
the group; and as these limestones are principally calcareous
breccias or conglomerates, there is room for believing that these
primordial fossils are really derived, in part at any rate, from
fragments of an upper Cambrian limestone. In the State of New
York, the Graptolitic shales of Quebec are wanting; and the base
of the Silurian is constituted by the so-called "Calciferous
Sand-rock" and "Chazy Limestone."[11] The first of these is
essentially and typically calcareous, and the second is a genuine
limestone.
[Footnote 11: The precise relations of the Quebec shales with
Graptolites (Levis Formation) to the Calciferous and Chazy beds
are still obscure, though there seems little doubt but that the
Quebec Shales are superior to the Calciferous Sand-rock.]
2. The _Trenton Group_.--This is an essentially calcareous group,
the various limestones of which it is composed being known as
the "Bird's-eye," "Black River," and "Trenton" Limestones, of
which the last is the thickest and most important. The thickness
of this group is variable, and the bands of limestone in it are
often separated by beds of shale.
3. The _Cincinnati Group_ (Hudson River Formation[12]).--This
group consists essentially of a lower series of shales, often
black in colour and highly charged with bituminous matter (the
"Utica Slates "), and of an upper series of shales, sandstones, and
limestones (the "Cincinnati" rocks proper). The exact parallelism
of the Trenton and Cincinnati groups with the subdivisions of the
Welsh Silurian series can hardly be stated positively. Probably
no precise equivalency exists; but there can be no doubt but that
the Trenton and Cincinnati groups correspond, as a whole, with the
Llandeilo and Caradoc groups of Britain. The subjoined diagrammatic
section (fig. 35) gives a general idea of the succession of the
Lower Silurian rocks of North America:--
[Illustration: Fig 35. GENERALIZED SECTION OF THE LOWER SILURIAN
ROCKS OF NORTH AMERICA.]
[Illustration: Fig. 36.--_Licrophycus Ottawaensis_ a "Fucoid,"
from the Trenton Limestone (Lower Silurian) of Canada. (After
Billings.)]
[Footnote 12: There is some difficulty about the precise nomenclature
of this group. It was originally called the "Hudson River Formation;"
but this name is inappropriate, as rocks of this age hardly touch
anywhere the actual Hudson River itself, the rocks so called
formerly being now known to be of more ancient date. There is
also some want of propriety in the name of "Cincinnati Group,"
since the rocks which are known under this name in the vicinity of
Cincinnati itself are the representatives of the Trenton Limestone,
Utica Slates, and the old Hudson River group, inseparably united
in what used to be called the "Blue Limestone Series."].
Of the _life_ of the Lower Silurian period we have record in
a vast number of fossils, showing that the seas of this period
were abundantly furnished with living denizens. We have, however,
in the meanwhile, no knowledge of the land-surfaces of the period.
We have therefore no means of speculating as to the nature of
the terrestrial animals of this ancient age, nor is anything
known with certainty of any land-plants which may have existed.
The only relics of vegetation upon which a positive opinion can
be expressed belong to the obscure group of the "Fucoids," and
are supposed to be the remains of sea-weeds. Some of the fossils
usually placed under this head are probably not of a vegetable
nature at all, but others (fig. 36) appear to be unquestionable
plants. The true affinities of these, however, are extremely
dubious. All that can be said is, that remains which appear to
be certainly vegetable, and which are most probably due to marine
plants, have been recognised nearly at the base of the Lower
Silurian (Arenig), and that they are found throughout the series
whenever suitable conditions recur.
The Protozoans appear to have flourished extensively in the Lower
Silurian seas, though to a large extent under forms which are
still little understood. We have here for the first time the
appearance of Foraminifera of the ordinary type--one of the most
interesting observations in this collection being that made by
Ehrenberg, who showed that the Lower Silurian sandstones of the
neighbourhood of St Petersburg contained casts in glauconite of
Foraminiferous shells, some of which are referable to the existing
genera _Rotalia_ and _Texularia_. True _Sponges_, belonging to
that section of the group in which the skeleton is calcareous,
are also not unknown, one of the most characteristic genera being
_Astylospongia_ (fig. 37). In this genus are included more or
less globular, often lobed sponges, which are believed not to
have been attached to foreign bodies. In the form here figured
there is a funnel-shaped cavity at the summit; and the entire
mass of the sponge is perforated, as in living examples, by a
system of canals which convey the sea-water to all parts of the
organism. The canals by which the sea-water gains entrance open
on the exterior of the sphere, and those by which it again escapes
from the sponge open into the cup-shaped depression at the summit.
[Illustration: Fig. 37.--_Astylospongia proemorsa_, cut vertically
so as to exhibit the canal-system in the interior. Lower Silurian,
Tennessee. (After Ferdinand Roemer.)]
The most abundant, and at the same time the least understood,
of Lower Silurian Protozoans belong, however, to the genera
_Stromatopora_ and _Receptaculites_, the structure of which can
merely be alluded to here. The specimens of _Stromatopora_ (fig.
38) occur as hemispherical, pear-shaped, globular, or irregular
masses, often of very considerable size, and sometimes demonstrably
attached to foreign bodies. In their structure these masses consist
of numerous thin calcareous laminae, usually arranged concentrically,
and separated by narrow interspaces. These interspaces are generally
crossed by numerous vertical calcareous pillars, giving the vertical
section of the fossil a lattice-like appearance. There are also
usually minute pores in the concentric laminae, by which the
successive interspaces are placed in communication; and sometimes
the surface presents large rounded openings, which appear to
correspond with the water-canals of the Sponges. Upon the whole,
though presenting some curious affinities to the calcareous Sponges,
_Stromatopora_ is perhaps more properly regarded as a gigantic
_Foraminifer_. If this view be correct, it is of special interest
as being probably the nearest ally of _Eozooen_, the general
appearance of the two being strikingly similar, though their
minute structure is not at all the same. Lastly, in the fossils
known as _Receptaculites_ and _Ischadites_ we are also presented
with certain singular Lower Silurian Protozoans, which may with
great probability be regarded as gigantic _Foraminifera_. Their
structure is very complex; but fragments are easily recognised
by the fact that the exterior is covered with numerous rhomboidal
calcareous plates, closely fitting together, and arranged in
peculiar intersecting curves, presenting very much the appearance
of the engine-turned case of a watch.
[Illustration: Fig. 38.--A small and perfect specimen of
_Stromatopora rugosa_, of the natural size, from the Trenton
Limestone of Canada. (After Billings.)]
Passing next to the sub-kingdom of _Coelenterate_ animals (Zoophytes,
Corals, &c.), we find that this great group, almost or wholly
absent in the Cambrian, is represented in Lower Silurian deposits
by a great number of forms belonging on the one hand to the true
Corals, and en the other hand to the singular family of the
_Graptolites_. If we except certain plant-like fossils which
probably belong rather to the Sertularians or the Polyzoans (e.g.,
_Dictyonema, Dendrograptus_, &c.), the family of the _Graptolites_
may be regarded as exclusively Silurian in its distribution. Not
only is this the case, but it attained its maximum development
almost upon its first appearance, in the Arenig Rocks; and whilst
represented by a great variety of types in the Lower Silurian;
it only exists in the Upper Silurian in a much diminished form.
The _Graptolites_ (Gr. _grapho_, I write; _lithos_, stone) were
so named by Linnaeus, from the resemblance of some of them to
written or pencilled marks upon the stone, though the great
naturalist himself did not believe them to be true fossils at
all. They occur as linear or leaf-like bodies, sometimes simple,
sometimes compound and branched; and no doubt whatever can be
entertained as to their being the skeletons of composite organisms,
or colonies of semi-independent animals united together by a common
fleshy trunk, similar to what is observed in the colonies of the
existing Sea-firs (Sertularians). This fleshy trunk or common
stem of the colony was protected by a delicate horny sheath, and
it gave origin to the little flower-like "polypites," which
constituted the active element of the whole assemblage. These
semi-independent beings were, in turn, protected each by a little
horny cup or cell, directly connected with the common sheath
below, and terminating above in an opening through which the
polypite could protrude its tentacled head or could again withdraw
itself for safety. The entire skeleton, again, was usually, if
not universally, supported by a delicate horny rod or "axis,"
which appears to have been hollow, and which often protrudes to
a greater or less extent beyond one or both of the extremities
of the actual colony.
The above gives the elementary constitution of any _Graptolite_,
but there are considerable differences as to the manner in which
these elements are arranged and combined. In some forms the common
stem of the colony gives origin to but a single row of cells
on one side. If the common stem is a simple, straight, or
slightly-curved linear body, then we have the simplest form of
Graptolite known (the genus _Monograptus_); and it is worthy of
note that these simple types do not come into existence till
comparatively late (Llandeilo), and last nearly to the very close
of the Upper Silurian. In other cases, whilst there is still but
a single row of cells, the colony may consist of two of these
simple stems springing from a common point, as in the so-called
"twin Graptolites" (_Didymograptus_, fig. 40). This type is entirely
confined to the earlier portion of the Lower Silurian period
(Arenig and Llandeilo). In other cases, again, there may be four
of such stems springing from a central point (_Tetragraptus_).
Lastly, there are numerous complex forms (such as _Dichograptus,
Loganograptus_, &c.) in which there are eight or more of these
simple branches, all arising from a common centre (fig. 39),
which is sometimes furnished with a singular horny disc. These
complicated branching forms, as well as the _Tetragrapti_, are
characteristic of the horizon of the Arenig group. Similar forms,
often specifically identical, are found at this horizon in Wales,
in the great series of the Skiddaw Slates of the north of England,
in the Quebec group in Canada, in equivalent beds in Sweden, and
in certain gold-bearing slates of the same age in Victoria in
Australia.
[Illustration: Fig. 39.--_Dichograptus octobrachiatus_, a branched,
"unicellular" Graptolite from the Skiddaw and Quebec Groups (Arenig).
(After Hall.)]
In another great group of Graptolites (including the genera
_Diplograptus, Dicranograptus, Climacograptus_, &c.) the common
stem of the colony gives origin, over part or the whole or its
length, to _two_ rows of cells, one on each side (fig. 41). These
"double-celled" Graptolites are highly characteristic of the Lower
Silurian deposits; and, with an exception more apparent than real
in Bohemia, they are exclusively confined to strata of Lower
Silurian age, and are not known to occur in the Upper Silurian.
Lastly, there is a group of Graptolites (_Phyllograptus_, fig.
42) in which the colony is leaf-like in form, and is composed
of _four_ rows of cells springing in a cross-like manner from
the common stem. These forms are highly characteristic of the
Arenig group.
[Illustration: Fig. 40.--Central portion of the colony of
_Didymegraptus divaricatus_, Upper Llandeilo, Dumfresshire.
(Original.)]
[Illustration: Fig. 41.--Examples of _Diplograptus pristis_,
showing variations in the appendages at the base. Upper Llandeilo,
Dumfriesshire. (Original.)]
[Illustration: Fig. 42.--Group of individuals of _Phyllograptus
typus_, from the Quebec group of Canada. (After Hall.) One of
the four rows of cells is hidden on the under surface.]
The Graptolites are usually found in dark-coloured, often black
shales, which sometimes contain so much carbon as to become
"anthracitic." They may be simply carbonaceous; but they are
more commonly converted into iron-pyrites, when they glitter
with the brilliant lustre of silver as they lie scattered on the
surface of the rock, fully deserving in their metallic tracery
the name of "written stones." They constitute one of the most
important groups of Silurian fossils, and are of the greatest
value in determining the precise stratigraphical position of
the beds in which they occur. They present, however, special
difficulties in their study; and it is still a moot point as
to their precise position in the zoological scale. The balance
of evidence is in favour of regarding them as an ancient and
peculiar group of the Sea-firs (Hydroid Zoophytes), but some
regard them as belonging rather to the Sea-mosses (_Polyzoa_).
Under any circumstances, they cannot be directly compared either
with the ordinary Sea-firs or the ordinary Sea-mosses; for these
two groups consist of fixed organisms, whereas the Graptolites
were certainly free-floating creatures, living at large in the
open sea. The only Hydroid Zoophytes or Polyzoans which have
a similar free mode of existence, have either no skeleton at
all, or have hard structures quite unlike the horny sheaths of
the Graptolites.
The second great group of Coelenterate animals (_Actinozoa_)
is represented in the Lower Silurian rocks by numerous Corals.
These, for obvious reasons, are much more abundant in regions
where the Lower Silurian series is largely calcareous (as in
North America) than in districts like Wales, where limestones
are very feebly developed. The Lower Silurian Corals, though
the first of their class, and presenting certain peculiarities,
may be regarded as essentially similar in nature to existing
Corals. These, as is well known, are the calcareous skeletons of
animals--the so-called "Coral-Zoophytes"--closely allied to the
common Sea-anemones in structure and habit. A _simple_ coral (fig.
43) consists of a calcareous cup embedded in the soft tissues of
the flower-like polype, and having at its summit a more or less
deep depression (the "calice") in which the digestive organs
are contained. The space within the coral is divided into
compartments by numerous vertical calcareous plates (the "septa"),
which spring from the inside of the wall of the cup, and of which
some generally reach the centre. _Compound_ corals, again (fig.
44), consist of a greater or less number of structures similar
in structure to the above, but united together in different ways
into a common mass. _Simple_ corals, therefore, are the skeletons
of _single_ and independent polypes; whilst _compound_ corals
are the skeletons of assemblages or colonies of similar polypes,
living united with one another another as an organic community.
[Illustration: Fig. 43.--_Zaphrentis Stokesi_, a simple "cup-coral,"
Upper Silurian, Canada. (After Billings.)]
[Illustration: Fig. 44.--Upper surface of a mass of _Strombodes
pentagonus_. Upper Silurian, Canada. (After Billings.)]
In the general details of their structure, the Lower Silurian
Corals do not differ from the ordinary Corals of the present
day. The latter, however, have the vertical calcareous plates
of the coral ("septa") arranged in multiples of six or five;
whereas the former have these structures arranged in multiples
of four, and often showing a cross-like disposition. For this
reason, the common Lower Silurian Corals are separated to form
a distinct group under the name of _Rugose_ Corals or _Rugosa_.
They are further distinguished by the fact that the cavity of
the coral ("visceral chamber") is usually subdivided by more
or less numerous horizontal calcareous plates or partitions,
which divide the coral into so many tiers or storeys, and which
are known as the "tabulae" (fig. 45).
[Illustration: Fig. 45.--_Columnaria alveolata_, a Rugose compound
coral, with imperfect septa, but having the corallites partitioned
off into storeys by "tabulae." Lower Silurian, Canada. (After
Billings.)]
In addition to the Rugose Corals, the Lower Silurian rocks contain
a number of curious compound corals, the tubes of which have
either no septa at all or merely rudimentary ones, but which
have the transverse partitions or "tabulae" very highly developed.
These are known as the _Tabulate Corals_; and recent researches
on some of their existing allies (such as _Heliopora_) have shown
that they are really allied to the modern Sea-pens, Organ-pipe
Corals, and Red Coral, rather than to the typical stony Corals.
Amongst the characteristic Rugose Corals of the Lower Silurian
may be mentioned species belonging to the genera _Columnaria,
Favistella, Streptelasma_, and _Zaphrentis_; whilst amongst the
"Tabulate" Corals, the principal forms belong to the genera
_Choetetes, Halysites_ (the Chain-coral), _Constellaria_, and
_Heliolites_. These groups of the Corals, however, attain a greater
development at a later period, and they will be noticed more
particularly hereafter.
[Illustration: Fig. 46.--Group of Cystideans. A, _Caryocrinus
ornatus_,[13] Upper Silurian, America; B, _Pleurocystites squamosus_,
showing two short "arms," Lower Silurian, Canada; C, _Pseudocrinus
bifasciatus_, Upper Silurian, England; D, _Lepadocrinus Gebhartii_,
Upper Silurian, America. (After Hall, Billings, and Salter.)]
[Footnote 13: The genus _Caryocrinus_ is sometimes regarded as
properly belonging to the _Crinoids_, but there seem to be good
reasons for rather considering it as an abnormal form of
_Cystidean_.]
Passing onto higher animals, we find that the class of the
_Echinodermata_ is represented by examples of the Star-fishes
(_Asteroidea_), the Sea-lilies (_Crinoidea_), and the peculiar
extinct group of the Cystideans (_Cystoidea_), with one or two of
the Brittle-stars (_Ophiuroidea_)--the Sea-urchins (_Echinoidea_)
being still wanting. The Crinoids, though in some places extremely
numerous, have not the varied development that they possess in
the Upper Silurian, in connection with which their structure will
be more fully spoken of. In the meanwhile, it is sufficient to
note that many of the calcareous deposits of the Lower Silurian
are strictly entitled to the name of "Crinoidal limestones,"
being composed in great part of the detached joints, and plates,
and broken stems, of these beautiful but fragile organisms (see
fig. 12). Allied to the Crinoids are the singular creatures which
are known as _Cystideans_ (fig. 46). These are generally composed
of a globular or ovate body (the "calyx"), supported upon a short
stalk (the "column"), by which the organism was usually attached
to some foreign body. The body was enclosed by closely-fitting
calcareous plates, accurately jointed together; and the stem was
made up of numerous distinct pieces or joints, flexibly united
to each other by membrane. The chief distinction which strikes
one in comparing the Cystideans with the Crinoids is, that the
latter are always furnished, as will be subsequently seen, with
a beautiful crown of branched and feathery appendages, springing
from the summit of the calyx, and which are composed of innumerable
calcareous plates or joints, and are known as the "arms." In the
Cystideans, on the other hand, there are either no "arms" at all,
or merely short, unbranched, rudimentary arms. The Cystideans are
principally, and indeed nearly exclusively, Silurian fossils;
and though occurring in the Upper Silurian in no small numbers,
they are pre-eminently characteristic of the Llandeilo-Caradoc
period of Lower Silurian time. They commenced their existence,
so far as known, in the Upper Cambrian; and though examples are
not absolutely unknown in later periods, they are pre-eminently
characteristic of the earlier portion of the Palaeozoic epoch.
[Illustration: Fig. 47.--Lower Silurian Crustaceans. a, _Asaphus
tyrannus_, Upper Llandeilo; b. _Ogygia Buchii_, Upper Llandeilo;
c, _Trinucleus concentricus_, Caradoc; d, _Caryocaris Wrightii_,
Arenig (Skiddaw Slates); e, _Beyrichia complicata_, natural size and
enlarged, Upper Llandeilo and Caradoc; f, _Primitia strangulata_,
Caradoc: g. Head-shield of _Calymene Blumenbachii_, var.
_brevicapitata_, Caradoc; h, Head-shield of _Triarthrus Becki_
(Utica Slates), United States: i, Shield of _Leperditia
Canadensis_, var. _Josephiana_, of the natural size, Trenton
Limestone, Canada; j, The same, viewed from the front. (After
Salter, M'Coy, Rupert Jones, and Dana.)]
The Ringed Worms (_Annelides_) are abundantly represented in the
Lower Silurian, but principally by tracks and burrows similar
in essential respects to those which occur so commonly in the
Cambrian formation, and calling for no special comment. Much more
important are the _Articulate_ animals, represented as heretofore,
wholly by the remains of the aquatic group of the _Crustaceans_.
Amongst these are numerous little bivalved forms--such as species
of _Primitia_ (fig. 47, f), _Beyrichia_ (fig. 47, e), and
_Leperditia_ (fig. 47, i and j). Most of these are very small,
varying from the size of a pin's head up to that of a hemp seed;
but they are sometimes as large as a small bean (fig. 47, i),
and they are commonly found in myriads together in the rock. As
before said, they belong to the same great group as the living
Water-fleas (_Ostracoda_). Besides these, we find the pod-shaped
head-shields of the shrimp-like Phyllopods--such as _Caryocaris_
(fig. 47, d) and _Ceratiocaris_. More important, however, than
any of these are the _Trilobites_, which may be considered as
attaining their maximum development in the Lower Silurian. The
huge _Paradoxides_ of the Cambrian have now disappeared, and with
them almost all the principal and characteristic "primordial"
genera, save _Olenus_ and _Agnostus_. In their place we have a
great number of new forms--some of them, like the great _Asaphus
tyrannus_ of the Upper Llandeilo (fig. 47, a), attaining a
length of a foot or more, and thus hardly yielding in the matter
of size to their ancient rivals. Almost every subdivision of the
Lower Silurian series has its own special and characteristic
species of Trilobites; and the study of these is therefore of
great importance to the geologist. A few widely-dispersed and
characteristic species have been here figured (fig. 47); and
the following may be considered as the principal Lower Silurian
genera--_Asaphus, Ogygia, Cheirurus, Ampyx, Caiymene, Trinucleus,
Lichas, Illoenus, AEglina, Harpes, Remopleurides, Phacops, Acidaspis_,
and _Homalonotus_, a few of them passing upwards under new forms
into the Upper Silurian.
Coming next to the _Mollusca_, we find the group of the Sea-mosses
and Sea-mats (_Polyzoa_) represented now by quite a number of forms.
Amongst these are examples of the true Lace-corals (_Retepora_
and _Fenestella_), with their netted fan-like or funnel-shaped
fronds; and along with these are numerous delicate encrusting
forms, which grew parasitically attached to shells and corals
(_Hippothoa, Alecto_, &c.); but perhaps the most characteristic
forms belong to the genus _Ptilodictya_ (figs. 48 and 49). In
this group the frond is flattened, with thin striated edges,
sometimes sword-like or scimitar-shaped, but often more or less
branched; and it consists of two layers of cells, separated by
a delicate membrane, and opening upon opposite sides. Each of
these little chambers or "cells" was originally tenanted by a
minute animal, and the whole thus constituted a compound organism
or colony.
[Illustration: Fig. 48.--_Ptilodictya falciformis_. a, Small
specimen of the natural size; b, Cross-section, showing the
shape of the frond; c, Portion of the surface, enlarged. Trenton
Limestone and Cincinnati Group, America. (Original.)]
[Illustration: Fig. 49.--A, _Ptilodictya acuta_; B, _Ptilodictya
Schafferi_. a, Fragment, of the natural size; b, Portion,
enlarged to show the cells. Cincinnati Group of Ohio and Canada.
(Original.)]
[Illustration: Fig. 50.--Lower Silurian Brachiopods. a and
a', _Orthis biforata_, Llandeilo-Caradoc, Britain and America:
b, _Orthis flabellulum_, Caradoc, Britain: c, _Orthis subquadrata_,
Cincinnati Group, America; c', Interior of the dorsal valve of
the same: d, _Strophomena deltoidea_, Llandeilo-Caradoc, Britain
and America. (After Meek, Hall, and Salter.)]
The Lamp-shells or _Brachiopods_ are so numerous, and present
such varied types, both in this and the succeeding period of
the Upper Silurian, that the name of "Age of Brachiopods" has
with justice been applied to the Silurian period as a whole. It
would be impossible here to enter into details as to the many
different forms of Brachiopods which present themselves in the
Lower Silurian deposits; but we may select the three genera _Orthis,
Strophomena_, and _Leptoena_ for illustration, as being specially
characteristic of this period, though not exclusively confined to it.
The numerous shells which belong to the extensive and cosmopolitan
genus _Orthis_ (fig. 50, a, b, c, and fig. 51, c and d),
are usually more or less transversely-oblong or subquadrate, the
two valves (as more or less in all the Brachiopods) of unequal
sizes, generally more or less convex, and marked with radiating
ribs or lines. The valves of the shell are united to one another
by teeth and sockets, and there is a straight hinge-line. The beaks
are also separated by a distinct space ("hinge-area"), formed in
part by each valve, which is perforated by a triangular opening,
through which, in the living condition, passed a muscular cord
attaching the shell to some foreign object. The genus _Strophomena_
(fig. 50, d, and 51, a and b) is very like _Orthis_ in
general character; but the shell is usually much flatter, one
or other valve often being concave, the hinge-line is longer,
and the aperture for the emission of the stalk of attachment is
partially closed by a calcareous plate. In _Leptoena_, again
(fig. 51, e), the shell is like _Strophomena_ in many respects,
but generally comparatively longer, often completely semicircular,
and having one valve convex and the other valve concave. Amongst
other genera of Brachiopods which are largely represented in the
Lower Silurian rocks may be mentioned _Lingula, Crania, Discina,
Trematis, Siphonotreta, Acrotreta, Rhynchonella_, and _Athyris_;
but none of these can claim the importance to which the three
previously-mentioned groups are entitled.
[Illustration: Fig. 51.--Lower Silurian Brachiopods, a, _Strophomena
alternata_, Cincinnati Group, America; b, _Strophomena filitexta,
Trenton and Cincinnati Groups, America; c, _Orthis testudinaria_,
Caradoc, Europe, and America; d, d', _Orthis plicateila_, Cincinnati
Group, America; e, e', e'', _Leptoena sericea_, Llandeilo and
Caradoc, Europe and America. (After Meek, Hall, and the Author.)]
The remaining Lower Silurian groups of _Mollusca_ can be but
briefly glanced at here. The Bivalves (_Lamellibranchiata_) find
numerous representatives, belonging to such genera as _Modiolopsis,
Ctenodonta, Orthonota, Paloearca, Lyrodesma, Ambonychia_, and
_Cleidophorus_. The Univalves (_Gasteropoda_) are also very numerous,
the two most important genera being _Murchisonia_ (fig. 52) and
_Pleurotomaria_. In both these groups the outer lip of the shell
is notched; but the shell in the former is elongated and turreted,
whilst in the latter it is depressed. The curious oceanic Univalves
known as the _Heteropods_ are also very abundant, the principal
forms belonging to _Bellerophon_ and _Maclurea_. In the former
(fig. 53) there is a symmetrical convoluted shell, like that of
the Pearly Nautilus in shape, but without any internal partitions,
and having the aperture often expanded and notched behind. The
species of _Maclurea_ (fig. 54) are found both in North America
and in Scotland, and are exclusively confined to the Lower Silurian
period, so far as known. They have the shell coiled into a flat
spiral, the mouth being furnished with a very curious, thick,
and solid lid or "operculum." The Lower Silurian _Pteropods_,
or "Winged snails," are numerous, and belong principally to the
genera _Theca, Conularia_, and _Tentaculites_, the last-mentioned
of these often being extremely abundant in certain strata.
[Illustration: Fig. 52.--_Murchisonia gracilis_, Trenton Limestone,
America. (After Billings.)]
[Illustration: Fig. 53.--Different views of _Bellerophon Argo_,
Trenton Limestone, Canada. (After Billings.)]
[Illustration: Fig. 54.--Different views of _Maclurea crenulata_,
Quebec Group, Newfoundland. (After Billings.)]
[Illustration: Fig. 55.--Fragment of _Orthoceras crebriseptum_,
Cincinnati Group, North America, of the natural size. The lower
figure section showing the air-chambers, and the form and position
of the siphuncle. (After Billings.)]
[Illustration: Fig. 56.--[14] Restoration of Orthoceras, the shell
being supposed to be divided vertically, and only its upper part
being shown. a, Arms; f, Muscular tube ("funnel") by which
water is expelled from the mantle-chamber; c, Air-chambers;
s, Siphuncle.]
[Footnote 14: This illustration is taken from a rough sketch
made by the author many years ago, but he is unable to say from
what original source it was copied.]
Lastly, the Lower Silurian Rocks have yielded a vast number of
chambered shells, referable to animals which belong to the same
great division as the Cuttle-fishes (the _Cephalopoda_), and
of which the Pearly Nautilus is the only living representative
at the present day. In this group of _Cephalopods_ the animal
possesses a well-developed external shell, which is divided into
chambers by shelly partitions ("septa"). The animal lives in
the last-formed and largest chamber of the shell, to which it
is organically connected by muscular attachments. The head is
furnished with long muscular processes or "arms," and can be
protruded from the mouth of the shell at will, or again withdrawn
within it. We learn, also, from the Pearly Nautilus, that these
animals must have possessed two pairs of breathing organs or
"gills;" hence all these forms are grouped together under the
name of the "Tetrabranchiate" Cephalopods (Gr. _tetra_, four;
_bragchia_, gill). On the other hand, the ordinary Cuttle-fishes
and Calamaries either possess an internal skeleton, or if they
have an external shell, it is not chambered; their "arms" are
furnished with powerful organs of adhesion in the form of suckers;
and they possess only a single pair of gills. For this last reason
they are termed the "Dibranchiate" Cephalopods (Gr. _dis_, twice;
_bragchia_, gill). No trace of the true Cuttle-fishes has yet
been found in Lower Silurian deposits; but the Tetrabranchiate
group is represented by a great number of forms, sometimes of
great size. The principal Lower Silurian genus is the well-known
and widely-distributed _Orthoceras_ (fig. 55). The shell in this
genus agrees with that of the existing _Pearly Nautilus_, in
consisting of numerous chambers separated by shelly partitions
(or septa), the latter being perforated by a tube which runs the
whole length of the shell after the last chamber, and is known
as the "siphuncle" (fig. 56, s). The last chamber formed is the
largest, and in it the animal lives. The chambers behind this
are apparently filled with some gas secreted by the animal itself;
and these are supposed to act as a kind of float, enabling the
creature to move with ease under the weight of its shell. The
various air-chambers, though the siphuncle passes through them,
have no direct connection with one another; and it is believed
that the animal has the power of slightly altering its specific
gravity, and thus of rising or sinking in the water by driving
additional fluid into the siphuncle or partially emptying it.
The _Orthoceras_ further agrees with the Pearly Nautilus in the
fact that the partitions or septa separating the different
air-chambers are simple and smooth, concave in front and convex
behind, and devoid of the elaborate lobation which they exhibit
in the Ammonites; whilst the siphuncle pierces the septa either
in the centre or near it. In the Nautilus, however, the shell is
coiled into a flat spiral; whereas in _Orthoceras_ the shell is
a straight, longer or shorter cone, tapering behind, and gradually
expanding towards its mouth in front. The chief objections to
the belief that the animal of the _Orthoceras_ was essentially
like that of the Pearly Nautilus are--the comparatively small
size of the body-chamber, the often contracted aperture of the
mouth, and the enormous size of some specimens of the shell.
Thus, some _Orthocerata_ have been discovered measuring ten or
twelve feet in length, with a diameter of a foot at the larger
extremity. These colossal dimensions certainly make it difficult
to imagine that the comparatively small body-chamber could have
held an animal large enough to move a load so ponderous as its
own shell. To some, this difficulty has appeared so great that
they prefer to believe that the _Orthoceras_ did not live in
its shell at all, but that its shell was an internal skeleton
similar to what we shall find to exist in many of the true
Cuttle-fishes. There is something to be said in favour of this
view, but it would compel us to believe in the existence in Lower
Silurian times of Cuttle-fishes fully equal in size to the giant
"Kraken" of fable. It need only be added in this connection that
the Lower Silurian rocks have yielded the remains of many other
Tetrabranchiate Cephalopods besides _Orthoceras_. Some of these
belong to _Cyrtoceras_, which only differs from _Orthoceras_ in
the bow-shaped form of the shell; others belong to _Phragmoceras_,
_Lituites_, &c.; and, lastly; we have true _Nautili_, with their
spiral shells, closely resembling the existing Pearly Nautilus.
Whilst all the sub-kingdoms of the Invertebrate animals are
represented in the Lower Silurian rocks, no traces of Vertebrate
animals have ever been discovered in these ancient deposits,
unless the so-called "Conodonts" found by Pander in vast numbers
in strata of this age [15] in Russia should prove to be really
of this nature. These problematical bodies are of microscopic
size, and have the form of minute, conical, tooth-shaped spines,
with sharp edges, and hollow at the base. Their original discoverer
regarded them as the horny teeth of fishes allied to the Lampreys;
but Owen came to the conclusion that they probably belonged to
Invertebrates. The recent investigation of a vast number of similar
but slightly larger bodies, of very various forms, in the
Carboniferous rocks of Ohio, has led Professor Newberry to the
conclusion that these singular fossils really are, as Pander
thought, the teeth of Cyclostomatous fishes. The whole of this
difficult question has thus been reopened, and we may yet have
to record the first advent of Vertebrate animals in the Lower
Silurian.
[Footnote 15: According to Pander, the "Conodonts" are found not
only in the Lower Silurian beds, but also in the "Ungulite Grit"
(Upper Cambrian), as well as in the Devonian and Carboniferous
deposits of Russia. Should the Conodonts prove to be truly the
remains of fishes, we should thus have to transfer the first
appearance of vertebrates to, at any rate, as early a period as
the Upper Cambrian.]
CHAPTER X.
THE UPPER SILURIAN PERIOD.
Having now treated of the Lower Silurian period at considerable
length, it will not be necessary to discuss the succeeding group
of the _Upper Silurian_ in the same detail--the more so, as with a
general change of _species_ the Upper Silurian animals belong for
the most part to the same great types as those which distinguish
the Lower Silurian. As compared, also, as regards the total bulk of
strata concerned, the thickness of the Upper Silurian is generally
very much below that of the Lower Silurian, indicating that they
represent a proportionately shorter period of time. In considering
the general succession of the Upper Silurian beds, we shall,
as before, select Wales and America as being two regions where
these deposits are typically developed.
In Wales and its borders the general succession of the Upper
Silurian rocks may be taken to be as follows, in ascending order
(fig. 57):--
(1) The base of the Upper Silurian series is constituted by a
series of arenaceous beds, to which the name of "May Hill Sandstone"
was applied by Sedgwick. These are succeeded by a series of
greenish-grey or pale-grey slates ("Tarannon Shales"), sometimes
of great thickness; and these two groups of beds together form
what may be termed the "_May Hill Group_" (Upper Llandovery of
Murchison). Though not very extensively developed in Britain, this
zone is one very well marked by its fossils; and it corresponds
with the "Clinton Group" of North America, in which similar fossils
occur. In South Wales this group is clearly unconformable to the
highest member of the subjacent Lower Silurian (the Llandovery
group); and there is reason to believe that a similar, though
less conspicuous, physical break occurs very generally between
the base of the Upper and the summit of the Lower Silurian.
(2) The _Wenlock Group_ succeeds the May Hill group, and constitutes
the middle member of the Upper Silurian. At its base it may have
an irregular limestone ("Woolhope Limestone"), and its summit may
be formed by a similar but thicker calcareous deposit ("Wenlock
Limestone"); but the bulk of the group is made up of the argillaceous
and shaly strata known as the "Wenlock Shale." In North Wales
the Wenlock group is, represented by a great accumulation of
flaggy and gritty strata (the "Denbighshire Flags and Grits"),
and similar beds (the "Coniston Flags" and "Coniston Grits")
take the same place in the north of England.
(3) The _Ludlow Group_ is the highest member of the Upper Silurian,
and consists typically of a lower arenaceous and shaly series (the
"Lower Ludlow Rock") a middle calcareous member (the "Aymestry
Limestone"), and an upper shaly and sandy series (the "Upper
Ludlow Rock" and "Downton Sandstone"). At the summit, or close
to the summit, of the Upper Ludlow, is a singular stratum only a
few inches thick (varying from an inch to a foot), which contains
numerous remains of crustaceans and fishes, and is well known
under the name of the "bone-bed." Finally, the Upper Ludlow rock
graduates invariably into a series of red sandy deposits, which,
when of a flaggy character, are known locally as the "Tile-stones."
These beds are probably to be regarded as the highest member
of the Upper Silurian; but they are sometimes looked upon as
passage-beds into the Old Red Sandstone, or as the base of this
formation. It is, in fact, apparently impossible to draw any
actual line of demarcation between the Upper Silurian and the
overlying deposits of the Devonian or Old Red Sandstone series.
Both in Britain and in America the Lower Devonian beds repose
with perfect conformity upon the highest Silurian beds, and the
two formations appear to pass into one another by a gradual and
imperceptible transition.
The Upper Silurian strata of Britain vary from perhaps 3000 or
4000 feet in thickness up to 8000 or 10,000 feet. In North America
the corresponding series, though also variable, is generally of
much smaller thickness, and may be under 1000 feet. The general
succession of the Upper Silurian deposits of North America is
as follows:--
(1) _Medina Sandstone_.--This constitutes the base of the Upper
Silurian, and consists of sandy strata, singularly devoid of life,
and passing below in some localities into a conglomerate ("Oneida
Conglomerate"), which is stated to contain pebbles derived from
the older beds, and which would thus indicate an unconformity
between the Upper and Lower Silurian.
(2) _Clinton Group_.--Above the Medina sandstone are beds of
sandstone and shale, sometimes with calcareous bands, which
constitute what is known as the "Clinton Group." The Medina and
Clinton groups are undoubtedly the equivalent of the "May Hill
Group" of Britain, as shown by the identity of their fossils.
[Illustration: Fig. 57. GENERALIZED SECTION OF THE UPPER SILURIAN
STRATA OF WALES AND SHROPSHIRE.]
(3) _Niagara Group_.--This group consists typically of a series of
argillaceous beds ("Niagara Shale") capped by limestones ("Niagara
Limestone"); and the name of the group is derived from the fact
that it is over limestones of this age that the Niagara river
is precipitated to form the great Falls. In places the Niagara
group is wholly calcareous, and it is continued upwards into a
series of marls and sandstones, with beds of salt and masses
of gypsum (the "Salina Group"), or into a series of magnesian
limestones ("Guelph Limestones"). The Niagara group, as a whole,
corresponds unequivocally with the Wenlock group of Britain.
(4) _Lower Helderberg Group_.--The Upper Silurian period in North
America was terminated by the deposition of a series of calcareous
beds, which derive the name of "Lower Helderberg" from the Helderberg
mountains, south of Albany, and which are divided into several zones,
capable of recognition by their fossils, and known by local names
(Tentaculite Limestone, Water-lime, Lower Pentamerus Limestone,
Delthyris Shaly Limestone, and Upper Pentamerus Limestone). As
a whole, this series may be regarded as the equivalent of the
Ludlow group of Britain, though it is difficult to establish any
precise parallelism. The summit of the Lower Heiderberg group
is constituted by a coarse-grained sandstone (the "Oriskany
Sandstone"), replete with organic remains, which have to a large
extent a Silurian _facies_. Opinions differ as to whether this
sandstone is to be regarded as the highest bed of the Upper Silurian
or the base of the Devonian. We thus see that in America, as
in Britain, no other line than an artificial one can be drawn
between the Upper Silurian and the overlying Devonian.
As regards the _life_ of the Upper Silurian period, we have,
as before, a number of so-called "Fucoids," the true vegetable
nature of which is in many instances beyond doubt. In addition
to these, however, we meet for the first time, in deposits of
this age, with the remains of genuine land-plants, though our
knowledge of these is still too scanty to enable us to construct
any detailed picture of the terrestrial vegetation of the period.
Some of these remains indicate the existence of the remarkable genus
_Lepidodendron_--a genus which played a part of great importance
in the forests of the Devonian and Carboniferous periods, and
which may be regarded as a gigantic and extinct type of the
Club-mosses (_Lycopodiaceoe_). Near the summit of the Ludlow
formation in Britain there have also been found beds charged
with numerous small globular bodies, which Dr Hooker has shown
to be the seed-vessels or "sporangia" of Club-mosses. Principal
Dawson further states that he has seen in the same formation
fragments of wood with the structure of the singular Devonian
Conifer known as _Prototaxites_. Lastly, the same distinguished
observer has described from the Upper Silurian of North America
the remains of the singular land-plants belonging to the genus
_Psilophyton_, which will be referred to at greater length hereafter.
The marine life of the Upper Silurian is in the main constituted
by types of animals similar to those characterising the Lower
Silurian, though for the most part belonging to different species.
The _Protozoans_ are represented principally by _Stromatopora_ and
_Ischadites_, along with a number of undoubted sponges (such as
_Amphispongia, Astroeospongia, Astylospongia_, and _Paloeomanon_).
Amongst the _Coelenterates_, we find the old group of _Graptolites_
now verging on extinction. Individuals still remain numerous,
but the variety of generic and specific types has now become
greatly reduced. All the branching and complex forms of the Arenig,
the twin-Graptolites and _Dicranograpti_ of the Llandeilo, and
the double-celled _Diplograpti_ and _Climacograpti_ of the Bala
group, have now disappeared. In their place we have the singular
_Retiolites_, with its curiously-reticulated skeleton; and several
species of the single-celled genus _Monograptus_, of which a
characteristic species (_M. Priodon_) is here figured. If we
remove from this group the plant-like _Dictyonemoe_, which are
still present, and which survive into the Devonian, no known
species of _Graptolite_ has hitherto been detected in strata
higher in geological position than the Ludlow. This, therefore,
presents us with the first instance we have as yet met with of
the total disappearance and extinction of a great and important
series of organic forms.
[Illustration: Fig. 58.--A, _Monograptus priodon_, slightly enlarged.
B, Fragment of the same viewed from behind. C, Fragment of the
same viewed in front, showing the mouths of the cellules. D,
Cross-section of the same. From the Wenlock Group (Coniston Flags
of the North of England). (Original.)]
The _Corals_ are very numerously represented in the Upper Silurian
rocks some of the limestones (such as the Wenlock Limestone)
being often largely composed of the skeletons of these animals.
Almost all the known forms of this period belong to the two great
divisions of the Rugose and Tabulate corals, the former being
represented by species of _Zaphrentis, Omphyma, Cystiphyllum,
Strombodes, Acervularia, Cyathophyllum_, &c.; whilst the latter
belong principally to the genera _Favosites, Choetetes, Halysites,
Syringopora, Heliolites_, and _Plasmopora_. Amongst the _Rugosa_, the
first appearance of the great and important genus _Cyathophyllum_,
so characteristic of the Palaeozoic period, is to be noted; and
amongst the _Tabulata_ we have similarly the first appearance,
in force at any rate, of the widely-spread genus _Favosites_--the
"Honeycomb-corals." The "Chain-corals" (_Halysites_), figured
below (fig. 59), are also very common examples of the Tabulate
corals during this period, though they occur likewise in the
Lower Silurian.
[Illustration: Fig. 59.--a, _Halysites catenularia_, small variety,
of the natural size; b, Fragment of a large variety of the same,
of the natural size; c, Fragment of limestone with the tubes
of _Halysites agglomerata_, of the natural size; d, Vertical
section of two tubes of the same, showing the tabulae, enlarged.
Niagara Limestone (Wenlock), Canada. (Original.)]
[Illustration: Fig. 60.--Upper Silurian Star-fishes. 1, _Palasterina
primoeva_, Lower Ludlow; 2, _Paloeaster Ruthveni_, Lower Ludlow;
3, _Paloeocoma Colvini_, Lower Ludlow. (After Salter.)]
[Illustration: Fig. 61.--A, _Protaster Sedgwickii_, showing the
disc and bases of the arms; B, Portion of an arm, greatly enlarged.
Lower Ludlow. (After Salter.)]
Amongst the _Echinodermata_, all those orders which have hard parts
capable of ready preservation are more or less largely represented.
We have no trace of the Holothurians or Sea-cucumbers; but this
is not surprising, as the record of the past is throughout almost
silent as to the former existence of these soft-bodied creatures,
the scattered plates and spicules in their skin offering a very
uncertain chance of preservation in the fossil condition. The
Sea-urchins (_Echinoids_) are said to be represented by examples
of the old genus _Paloechinus_. The Star-fishes (_Asteroids_) and
the Brittle-stars (_Ophiuroids_) are, comparatively speaking,
largely represented; the former by species of _Palasterina_ (fig.
60), _Paloeaster_ (fig. 60), _Paloeocoma_ (fig. 60), _Petraster,
Glyptaster_, and _Lepidaster_--and the latter by species of
_Protaster_ (fig. 61), _Paloeodiscus, Acroura_, and _Eucladia_.
The singular _Cystideans_, or "Globe Crinoids," with their globular
or ovate, tesselated bodies (fig. 46, A, C, D,), are also not
uncommon in the Upper Silurian; and if they do not become finally
extinct here, they certainly survive the close of this period by
but a very brief time. By far the most important, however, of
the Upper Silurian Echinodenns, are the Sea-lilies or _Crinoids_.
The limestones of this period are often largely composed of the
fragmentary columns and detached plates of these creatures, and
some of them (such as the Wenlock Limestone of Dudley) have yielded
perhaps the most exquisitely-preserved examples of this group
with which we are as yet acquainted. However varied in their
forms, these beautiful organisms consist of a globular, ovate,
or pear-shaped body (the "calyx"), supported upon a longer or
shorter jointed stem (or "column"). The body is covered externally
with an armour of closely-fitting calcareous plates (fig. 62),
and its upper surface is protected by similar but smaller plates
more loosely connected by a leathery integument. From the upper
surface of the body, round its margin, springs a series of longer
or shorter flexible processes, composed of innumerable calcareous
joints or pieces, movably united with one another. The arms are
typically five in number; but they generally subdivide at least
once, sometimes twice, and they are furnished with similar but
more slender lateral branches or "pinnules," thus giving rise
to a crown of delicate feathery plumes. The "column" is the stem
by which the animal is attached permanently to the bottom of the
sea; and it is composed of numerous separate plates, so jointed
together that whilst the amount of movement between any two pieces
must be very limited, the entire column acquires more or less
flexibility, allowing the organism as a whole to wave backwards and
forwards on its stalk. Into the exquisite _minutioe_ of structure
by which the innumerable parts entering into the composition
of a single Crinoid are adapted for their proper purposes in
the economy of the animal, it is impossible to enter here. No
period, as before said, has yielded examples of greater beauty
than the Upper Silurian, the principal genera represented being
_Cyathocrinus, Platycrinus, Marsupiocrinus, Taxocrinus,
Eucalyptocrinus, Ichthyocrinus, Mariacrinus, Periechocrinus,
Glyptocrinus, Crotalocrinus_, and _Edriocrinus_.
[Illustration: Fig 62.--Upper Silurian Crinoids. a, Calyx and
arms of _Eucalyptocrinus polydactylus_, Wenlock Limestone; b,
_Ichthyocrinus loevis_, Niagara Limestone, America; c, _Taxocrinus
tuberculatus_, Wenlock Limestone. (After M'Coy and Hall.)]
[Illustration: Fig. 63.--_Planolites vulgaris_, the filled-up
burrows of a marine worm. Upper Silurian (Clinton Group), Canada.
(Original.)]
The tracks and burrows of _Annelides_ are as abundant in the
Upper Silurian strata as in older deposits, and have just as
commonly been regarded as plants. The most abundant forms are the
cylindrical, twisted bodies (Planolites), which are so frequently
found on the surfaces of sandy beds, and which have been described
as the stems of sea-weeds. These fossils (fig. 63), however,
can be nothing more, in most cases, than the filled-up burrows
of marine worms resembling the living Lob-worms. There are also
various remains which belong to the group of the tube-inhabiting
Annelides (_Tubicola_). Of this nature are the tubes of _Serpulites_
and _Cornultites_, and the little spiral discs of _Spirorbis
Lewisii_.
[Illustration: Fig. 64.--Upper Silurian Trilobites. a, _Cheirurus
bimucronatus_, Wenlock and Caradoc; b, _Phacops longicaudatus_,
Wenlock, Britain, and America; c, _Phacops Downingioe_, Wenlock
and Ludlow; d, _Harpes ungula_, Upper Silurian, Bohemia. (After
Salter and Barrande.)]
Amongst the _Articulates_, we still meet only with the remains of
_Crustaceans_. Besides the little bivalved _Ostracoda_--which here
are occasionally found of the size of beans--and various _Phyllopods_
of different kinds, we have an abundance of _Trilobites_. These
last-mentioned ancient types, however, are now beginning to show
signs of decadence; and though still individually numerous, there
is a great diminution in the number of generic types. Many of
the old genera, which flourished so abundantly in Lower Silurian
seas, have now died out; and the group is represented chiefly
by species of _Cheirurus, Encrinurus, Harpes, Proetus, Lichas,
Acidaspis, Illoenus, Calymene, Homalonotus_, and _Phacops_--the
last of these, one of the highest and most beautiful of the groups
of Trilobites, attaining here its maximum of development. In the
annexed illustration (fig. 64) some of the characteristic Upper
Silurian Trilobites are represented--all, however, belonging
to genera which have their commencement in the Lower Silurian
period. In addition to the above, the Ludlow rocks of Britain
and the Lower Helderberg beds of North America have yielded the
remains of certain singular Crustaceans belonging to the extinct
order of the _Eurypterida_. Some of these wonderful forms are
not remarkable for their size; but others, such as _Pterygotus
Anglicus_ (fig. 65), attain a length of six feet or more, and
may fairly be considered as the giants of their class. The
Eurypterids are most nearly allied to the existing King-crabs
(_Limuli_), and have the anterior end of the body covered with
a great head-shield, carrying two pairs of eyes, the one simple
and the other compound. The feelers are converted into pincers,
whilst the last pair of limbs have their bases covered with spiny
teeth so as to act as jaws, and are flattened and widened out
towards their extremities so as to officiate as swimming-paddles.
The hinder extremity of the body is composed of thirteen rings,
which have no legs attached to them; and the last segment of
the tail is either a flattened plate or a narrow, sword-shaped
spine. Fragments of the skeleton are easily recognised by the
peculiar scale-like markings with which the surface is adorned,
and which look not at all unlike the scales of a fish. The most
famous locality for these great Crustaceans is Lesmahagow, in
Lanarkshire, where many different species have been found. The
true King-crabs (_Limuli_) of existing seas also appear to have
been represented by at least one form (_Neolimulus_) in the Upper
Silurian.
[Illustration: Fig. 65.--_Pterygotus Anglicus_, viewed from the
under side, reduced in size, and restored. c c, The feelers
(antennae), terminating in nipping-claws; o o, Eyes; m m,
Three pairs of jointed limbs, with pointed extremities; n n,
Swimming-paddles, the bases of which are spiny and act as jaws.
Upper Silurian, Lanarkshire. (After Henry Woodward.)]
Coming to the _Mollusca_, we note the occurrence of the same
great groups as in the Lower Silurian. Amongst the Sea-mosses
(_Polyzoa_), we have the ancient Lace-corals (_Fenestella_ and
_Retepora_), with the nearly-allied _Glauconome_, and species
of _Ptilodictya_ (fig. 66); whilst many forms often referred
here may probably have to be transferred to the Corals, just as
some so-called Corals will ultimately be removed to the present
group.
[Illustration: Fig. 66.--Upper Silurian Polyzoa. 1, Fan-shaped
frond of _Rhinopora verrucosa_; 1a, Portion of the surface of
the same, enlarged; 2 and 2a, _Phoenopora ensiformis_, of the
natural size and enlarged; 3 and 3a, _Helopora fragilis_, of
the natural size and enlarged; 4 and 4a, _Ptilodictya raripora_,
of the natural size and enlarged. The specimens are all from the
Clinton Formation (May Hill Group) of Canada. (Original.)]
[Illustration: Fig. 67.--_Spirifera hysterica_. The right-hand
figure shows the interior of the dorsal valve with the calcareous
spires for the support of the arms.]
[Illustration: Fig. 68.--Upper Silurian Brachiopods. a a',
_Leptocoelia plano-convexa_, Clinton Group, America; b b',
_Rhynchonella neglecta_, Clinton Group, America; c, _Rhynchonella
cuneata_, Niagara Group, America, and Wenlock Group, Britain;
d d', _Orthis elelgantula_, Llandeilo to Ludlow, America and
Europe; e e', _Atrypa hemispherica_, Clinton Group, America, and
Llandovery and May Hill Groups, Britain; f f', _Atrypa congesta_,
Clinton Group, America; g g', _Orthis Davidsoni_, Clinton Group,
America. (After Hall, Billings, and the Author.)]
The Brachiopods continued to flourish during the Upper Silurian
Period in immense numbers and under a greatly increased variety
of forms. The three prominent Lower Silurian genera _Orthis,
Strophomena_, and _Leptoena_ are still well represented, though
they have lost their former preeminence. Amongst the numerous
types which have now come upon the scene for the first time,
or which have now a special development, are _Spirifera_ and
_Pentamerus_. In the first of these (fig. 69. b, c), one of
the valves of the shell (the dorsal) is furnished in its interior
with a pair of great calcareous spires, which served for the
support of the long and fringed fleshy processes or "arms" which
were attached to the sides of the mouth.[16] In the genus
_Pentamerus_ (fig. 70) the shell is curiously subdivided in its
interior by calcareous plates. The _Pentameri_ commenced their
existence at the very close of the Lower Silurian (Llandovery),
and survived to the close of the Upper Silurian; but they are
specially characteristic of the May Hill and Wenlock groups,
both in Britain and in other regions. One species, _Pentamerus
galeatus_, is common to Sweden, Britain, and America. Amongst
the remaining Upper Silurian Brachiopods are the extraordinary
_Trimerellids_; the old and at the same time modern _Linguloe,
Discinoe_, and _Cranioe_; together with many species of _Atrypa_
(fig. 68, e), _Leptocoelia_ (fig. 68, a), _Rhynchonella_
(fig. 68, b, c), _Meristella_ (fig. 69, a, e, f), _Athyris,
Retzia, Chonetes_, &c.
[Footnote 16: In all the Lamp-shells the mouth is provided with
two long fleshy organs, which carry delicate filaments on their
sides, and which are usually coiled into a spiral. These organs
are known as the "arms," and it is from their presence that the
name of "_Brachiopoda_" is derived (Gr. _brachion_, arm; _podes_,
feet). In some cases the arms are merely coiled away within the
shell, without any support; but in other cases they are carried
upon a more or less elaborate shelly loop, often spoken of as the
"carriage-spring apparatus." In the _Spirifers_, and in other ancient
genera, this apparatus is coiled up into a complicated spiral (fig.
67). It is these "arms," with or without the supporting loops or
spires, which serve as one of the special characters distinguishing
the _Brachiopods_ from the true Bivalves (_Lamellibranchiata_).]
[Illustration: Fig. 69.-a a', Meristella intermedia_, Niagara
Group, America; b, _Spirifera Niagarensis_, Niagara Group, America;
c c', _Spirifera crispa_, May Hill to Ludlow, Britain, and Niagara
Group, America; d, _Strophomena (Streptorhynchus) subplana_,
Niagara Group, America; e, _Meristella naviformis_, Niagara Group,
America; f, _Meristella cylindrica_, Niagara Group, America.
(After Hall, Billings, and the Author.)]
[Illustration: Fig. 70.--_Pentamerus Knightii_. Wenlock and Ludlow.
The right-hand figure shows the internal partitions of the shell.]
[Illustration: Fig. 71.--Upper Silurian Bivalves. A, _Cardiola
interrupta_, Wenlock and Ludlow; B, _Pterinea subfalcata_, Wenlock;
C, _Cardiola fibrosa_, Ludlow. (After Salter and M'Coy.)]
[Illustration: Fig. 72.--Upper Silurian Gasteropods. a, _Platyceras
ventricosum_, Lower Helderberg, America; b, _Euomphalus discors_,
Wenlock, Britain; c, _Holopella obsoleta_ Ludlow, Britain; d,
_Platyschisma helicites_, Upper Ludlow, Britain; e, _Holopella
gracilior_, Wenlock, Britain; f, _Platyceras multisinuatum_, Lower
Helderberg, America; g, _Holopea subconica_, Lower Helderberg,
America; h, h', _Platyostoma Niagarense_, Niagara Group, America.
(After Hall, M'Coy, and Salter.)]
[Illustration: Fig 73.--_Tentaculites ornatus_. Upper Silurian
of Europe and North America.]
The higher groups of the _Mollusca_ are also largely represented
in the Upper Silurian. Apart from some singular types, such as
the huge and thick-shelled _Megalomi_ of the American Wenlock
formation, the Bivalves (_Lamellibranchiata_) present little of
special interest; for though sufficiently numerous, they are rarely
well preserved, and their true affinities are often uncertain.
Amongst the most characteristic genera of this period may be
mentioned _Cardiola_ (fig. 71, A and C) and _Pterinea_ (fig. 71,
B), though the latter survives to a much later date. The Univalves
(_Gasteropoda_) are very numerous, and a few characteristic forms
are here figured (fig. 72). Of these, no genus is perhaps more
characteristic than _Euomphalus_ (fig. 72, b), with its flat
discoidal shell, coiled up into an oblique spiral, and deeply
hollowed out on one side; but examples of this group are both
of older and of more modern date. Another very extensive genus,
especially in America, is Platyceras (fig. 72, a and f),
with its thin fragile shell--often hardly coiled up at all--its
minute spire, and its widely-expanded, often sinuated mouth. The
British _Acroculioe_ should probably be placed here, and the
group has with reason been regarded as allied to the Violet-snails
(_Ianthina_) of the open Atlantic. The species of _Platyostoma_
(fig. 72, h) also belong to the same family; and the entire
group is continued throughout the Devonian into the Carboniferous.
Amongst other well-known Upper Silurian Gasteropods are species
of the genera _Holopea_ (fig. 72, g), _Holopella_ (fig. 72.
e), _Platyschisma_ (fig. 72, d), _Cyclonema, Pleurotomaria,
Murchisonia, Trochonema_, &c. The oceanic Univalves (_Heteropods_)
are represented mainly by species of _Bellerophon_; and the Winged
Snails, or _Pteropods_, can still boast of the gigantic _Thecoe_
and _Conularioe_, which characterise yet older deposits. The
commonest genus of _Pteropoda_, however, is _Tentaculites_ (fig.
73), which clearly belongs here, though it has commonly been
regarded as the tube of an Annelide. The shell in this group
is a conical tube, usually adorned with prominent transverse
rings, and often with finer transverse or longitudinal striae as
well; and many beds of the Upper Silurian exhibit myriads of
such tubes scattered promiscuously over their surfaces.
The last and highest group of the _Mollusca_--that of the
_Cephalopoda_--is still represented only by _Tetrabranchiate_
forms; but the abundance and variety of these is almost beyond
belief. Many hundreds of different species are known, chiefly
belonging to the straight _Orthoceratites_, but the slightly-curved
_Cyrtoceras_ is only little less common. There are also numerous
forms of the genera _Phragmoceras, Ascoceras, Gyroteras, Lituites_,
and _Nautilus_. Here, also, are the first-known species of the
genus _Goniatites_--a group which attains considerable importance
in later deposits, and which is to be regarded as the precursor
of the _Ammonites_ of the Secondary period.
[Illustration: Fig. 74.--Head-shield of _Pteraspis Banksii_, Ludlow
rocks. (After Murchison.)]
[Illustration: Fig. 75.--A, Spine of _Onchus tenuistriatus_;
B, Shagreen-scales of _Thelodus_. Both from the "bone-bed" of
the Upper Ludlow rocks. (After Murchison.)]
Finally, we find ourselves for the first time called upon to
consider the remains of undoubted vertebrate animals, in the
form of _Fishes_. The oldest of these remains, so far as yet
known, are found in the Lower Ludlow rocks, and they consist of
the bony head-shields or bucklers of certain singular armoured
fishes belonging to the group of the _Ganoids_, represented at
the present day by the Sturgeons, the Gar-pikes of North America,
and a few other less familiar forms. The principal Upper Silurian
genus of these is _Pteraspis_, and the annexed illustration (fig.
74) will give some idea of the extraordinary form of the shield
covering the head in these ancient fishes. The remarkable stratum
near the top of the Ludlow formation known as the "bone-bed" has
also yielded the remains of shark-like fishes. Some of these,
for which the name of _Onchus_ has been proposed, are in the form
of compressed, slightly-curved spines (fig. 75, A), which would
appear to be of the nature of the strong defensive spines implanted
in front of certain of the fins in many living fishes. Besides
these, have been found fragments of prickly skin or shagreen
(_Sphagodus_), along with minute cushion-shaped bodies (_Thelodus_,
fig. 75, B), which are doubtless the bony scales of some fish
resembling the modern Dog-fishes. As the above mentioned remains
belong to two distinct, and at the same time highly-organised,
groups of the fishes, it is hardly likely that we are really
presented here with the first examples of this great class. On
the contrary, whether the so-called "Conodonts" should prove
to be the teeth of fishes or not, we are justified in expecting
that unequivocal remains of this group of animals will still be
found in the Lower Silurian. It is interesting, also, to note
that the first appearance of fishes--the lowest class of vertebrate
animals--so far as known to us at present, does not take place
until after all the great sub-kingdoms of invertebrates have
been long in existence; and there is no reason for thinking that
future discoveries will materially affect the _relative_ order
of succession thus indicated.
LITERATURE.
From the vast and daily-increasing mass of Silurian literature, it
is impossible to do more than select a small number of works which
have a classical and historical interest to the English-speaking
geologist, or which embody researches on special groups of Silurian
animals--anything like an enumeration of all the works and papers
on this subject being wholly out of the question. Apart, therefore,
from numerous and in many cases extremely important memoirs,
by various well-known observers, both at home and abroad, the
following are some of the more weighty works to which the student
may refer in investigating the physical characters and succession
of the Silurian strata and their fossil contents:--
(1) 'Siluria.' Sir Roderick Murchison.
(2) 'Geology of Russia in Europe.' Murchison (with M. de Verneuil
and Count von Keyserling).
(3) 'Bassin Silurien de Boheme Centrale.' Barrande.
(4) 'Introduction to the Catalogue of British Palaeozoic Fossils in
the Woodwardian Museum of Cambridge.' Sedgwick.
(5) 'Die Urwelt Russlands.' Eichwald.
(6) 'Report on the Geology of Londonderry, Tyrone,' &c. Portlock.
(7) "Geology of North Wales"--'Mem. Geol. Survey of Great Britain,'
vol. iii. Ramsay.
(8) 'Geology of Canada,' 1863. Sir W. E. Logan; and the 'Reports of
Progress of the Geological Survey' since 1863.
(9) 'Memoirs of the Geological Survey of Great Britain,'
(10) 'Reports of the Geological Surveys of the States of New York,
Illinois, Ohio, Iowa, Michigan, Vermont, Wisconsin, Minnesota,'
&c. By Emmons, Hall, Worthen, Meek, Newberry, Orton, Winchell,
Dale Owen, &c.
(11) 'Thesaurus Siluricus.' Bigsby.
(12) 'British Palaeozoic Fossils.' M'Coy.
(13) 'Synopsis of the Silurian Fossils of Ireland,' M'Coy.
(14) "Appendix to the Geology of North Wales"--'Mem. Geol. Survey,'
vol. iii. Salter.
(15) 'Catalogue of the Cambrian and Silurian Fossils in the
Woodwardian Museum of Cambridge.' Salter.
(16) 'Characteristic British Fossils.' Baily.
(17) 'Catalogue of British Fossils.' Morris.
(18) 'Palaeozoic Fossils of Canada.' Billings.
(19) 'Decades of the Geological Survey of Canada.' Billings,
Salter, Rupert Jones.
(20) 'Decades of the Geological Survey of Great Britain.' Salter,
Edward, Forbes.
(21) 'Palaeontology of New York,' vols. i.-iii. Hall.
(22) 'Palaeontology of Illinois.' Meek and Worthen.
(23) 'Palaeontology of Ohio.' Meek, Hall, Whitfield, Nicholson.
(24) 'Silurian Fauna of West Tennessee' (Silurische Fauna des
Westlichen Tennessee). Ferdinand Roemer.
(25) 'Reports on the State Cabinet of New York.' Hall.
(26) 'Lethaea Geognostica.' Bronn.
(27) 'Index Palaeontologicus.' Bronn.
(28) 'Lethaea Rossica.' Eichwald.
(29) 'Lethaea Suecica.' Hisinger.
(30) 'Palaeontologica Suecica.' Angelin.
(31) 'Petrefacta Germaniae.' Goldfuss.
(32) 'Versteinerungen der Grauwacken-Formation in Sachsen.' Geinitz.
(33) 'Organisation of Trilobites' (Ray Society). Burmeister.
(34) 'Monograph of the British Trilobites' (Palaeontographical
Society). Salter.
(35) 'Monograph of the British Merostomata' (Palaeontographical Society).
Henry Woodward.
(36) 'Monograph of British Brachiopoda' (Palaeontographical Society).
Thomas Davidson.
(37) 'Graptolites of the Quebec Group.' James Hall.
(38) 'Monograph of the British Graptolitidae.' Nicholson.
(39) 'Monographs on the Trilobites. Pteropods, Cephalopods,
Graptolites,' &c. Extracted from the 'Systeme Silurien du Centre
de la Boheme.' Barrande.
(40) 'Polypiers Fossiles des Terrains Paleozoiques,' and 'Monograph
of the British Corals' (Palaeontographical Society). Milne
Edwards and Jules Haime.
CHAPTER XI.
THE DEVONIAN AND OLD RED SANDSTONE PERIOD.
Between the summit of the Ludlow formation and the strata which
are universally admitted to belong to the Carboniferous series
is a great system of deposits, to which the name of "Old Red
Sandstone" was originally applied, to distinguish them from certain
arenaceous strata which lie above the coal ("New Red Sandstone").
The Old Red Sandstone, properly so called, was originally described
and investigated as occurring in Scotland and in South Wales and
its borders; and similar strata occur in the south of Ireland.
Subsequently it was discovered that sediments of a different mineral
nature, and containing different organic remains, intervened
between the Silurian and the Carboniferous rocks on the continent
of Europe, and strata with similar palaeontological characters to
these were found occupying a considerable area in Devonshire.
The name of "Devonian" was applied to these deposits; and this
title, by common usage, has come to be regarded as synonymous
with the name of "Old Red Sandstone." Lastly, a magnificent series
of deposits, containing marine fossils, and undoubtedly equivalent
to the true "Devonian" of Devonshire, Rhenish Prussia, Belgium,
and France, is found to intervene in North America between the
summit of the Silurian and the base of the Carboniferous rocks.
Much difficulty has been felt in correlating the true "Devonian
Rocks" with the typical "Old Red Sandstone"--this difficulty arising
from the fact that though both formations are fossiliferous, the
peculiar fossils of each have only been rarely and partially found
associated together. The characteristic crustaceans and many of the
characteristic fishes of the Old Red are wanting in the Devonian;
whilst the corals and marine shells of the latter do not occur in
the former. It is impossible here to enter into any discussion
as to the merits of the controversy to which this difficulty
has given origin. No one, however, can doubt the importance and
reality of the Devonian series as an independent system of rocks
to be intercalated in point of time between the Silurian and
the Carboniferous. The want of agreement, both lithologically
and palaeontologically, between the Devonian and the Old Red,
can be explained by supposing that these two formations, though
wholly or in great part _contemporaneous_, and therefore strict
equivalents, represent deposits in two different geographical
areas, laid down under different conditions. On this view, the
typical Devonian rocks of Europe, Britain, and North America are
the deep-sea deposits of the Devonian period, or, at any rate, are
genuine marine sediments formed far from land. On the other hand,
the "Old Red Sandstone" of Britain and the corresponding "Gaspe
Group" of Eastern Canada represent the shallow-water shore-deposits
of the same period. In fact, the former of these last-mentioned
deposits contains no fossils which can be asserted positively
to be _marine_ (unless the Eurypterids be considered so); and
it is even conceivable that it represents the sediments of an
inland sea. Accepting this explanation in the meanwhile, we may
very briefly consider the general succession of the deposits of
this period in Scotland, in Devonshire, and in North America.
In Scotland the "Old Red" forms a great series of arenaceous and
conglomeratic strata, attaining a thickness of many thousands of
feet, and divisible into three groups. Of these, the _Lower Old
Red Sandstone_ reposes with perfect conformity upon the highest
beds of the Upper Silurian, the two formations being almost
inseparably united by an intermediate series of "passage-beds."
In mineral nature this group consists principally of massive
conglomerates, sandstones, shales, and concretionary limestones;
and its fossils consist chiefly of large crustaceans belonging to
the family of the _Eurypterids_, fishes, and plants. The _Middle
Old Red Sandstone_ consists of flagstones, bituminous shales,
and conglomerates, sometimes with irregular calcareous bands;
and its fossils are principally fishes and plants. It may be
wholly wanting, when the _Upper Old Red_ seems to repose
unconformably upon the lower division of the series. The _Upper
Old Red Sandstone_ consists of conglomerates and grits, along
with a great series of red and yellow sandstones--the fossils,
as before, being fishes and remains of plants. The Upper Old
Red graduates upwards conformably into the Carboniferous series.
The Devonian rocks of Devonshire are likewise divisible into a
lower, middle, and upper division. The _Lower Devonian_ or _Lynton
Group_ consists of red and purple sandstones, with marine fossils,
corresponding to the "Spirifer Sandstein" of Germany, and to the
arenaceous deposits (Schoharie and Cauda-Galli Grits) at the base
of the American Devonian. The _Middle Devonian_ or _Ilfracombe
Group_ consists of sandstones and flags, with calcareous slates
and crystalline limestones, containing many corals. It corresponds
with the great "Eifel Limestone" of the Continent, and, in a
general way, with the Corniferous Limestone and Hamilton group
of North America. The _Upper Devonian_ or _Pilton Group_, lastly,
consists of sandstones and calcareous shales which correspond with
the "Clymenia Limestone" and "Cypridina Shales" of the Continent,
and with the Chemung and Portage groups of North America. It
seems quite possible, also, that the so-called "Carboniferous
Slates" of Ireland correspond with this group, and that the former
would be more properly regarded as forming the summit of the
Devonian than the base of the Carboniferous.
In no country in the world, probably, is there a finer or more
complete exposition of the strata intervening between the Silurian
and Carboniferous deposits than in the United States. The following
are the main subdivisions of the Devonian rocks in the State of
New York, where the series may be regarded as being typically
developed (fig. 67):--
(1) _Cauda-Galli Grit_ and _Schoharie Grit_.--Considering the
"Oriskany Sandstone" as the summit of the Upper Silurian, the
base of the Devonian is constituted by the arenaceous deposits
known by the above names, which rest quite conformably upon the
Silurian, and which represent the Lower Devonian of Devonshire. The
_Cauda-Galli Grit_ is so called from the abundance of a peculiar
spiral fossil (_Spirophyton cauda-Galli_), which is of common
occurrence in the Carboniferous rocks of Britain, and is supposed
to be the remains of a sea-weed.
(2) The _Corniferous_ or _Upper Helderberg Limestone_.--A series
of limestones usually charged with considerable quantities of
siliceous matter in the shape of hornstone or chert (Lat. _cornu_,
horn). The thickness of this group rarely exceeds 300 feet; but
it is replete with fossils, more especially with the remains
of corals. The Corniferous Limestone is the equivalent of the
coral-bearing limestones of the Middle Devonian of Devonshire
and the great "Eifel Limestone" of Germany.
(3) The _Hamilton Group_--consisting of shales at the base
("Marcellus shales"); flags, shales, and impure limestones ("Hamilton
beds") in the middle; and again a series of shales ("Genesee
Slates") at the top. The thickness of this group varies from
200 to 1200 feet, and it is richly charged with marine fossils.
(4) The _Portage Group_.--A great series of shales, flags, and
shaly sandstones, with few fossils.
(5) The _Chemung Group_.--Another great series of sandstones and
shales, but with many fossils. The Portage and Chemung groups
may be regarded as corresponding with the Upper Devonian of
Devonshire. The Chemung beds are succeeded by a great series
of red sandstones and shales--the "Catskill Group"--which pass
conformably upwards into the Carboniferous, and which may perhaps
be regarded as the equivalent of the great sandstones of the
Upper Old Red in Scotland.
Throughout the entire series of Devonian deposits in North America
no unconformability or physical break of any kind has hitherto been
detected; nor is there any marked interruption to the current of
life, though each subdivision of the series has its own fossils.
No completely natural line can thus be indicated, dividing the
Devonian in this region from the Silurian on the one hand, and
the Carboniferous on the other hand. At the same time, there is
the most ample evidence, both stratigraphical and palaeontological,
as to the complete independence of the American Devonian series
as a distinct life-system between the older Silurian and the
later Carboniferous. The subjoined section (fig. 76) shows
diagrammatically the general succession of the Devonian rocks
of North America.
[Illustration: Fig. 76. GENERALIZED SECTION OF THE DEVONIAN ROCKS
OF NORTH AMERICA.]
[Illustration: Fig. 77.--Restoration of _Psilophyton princeps_.
Devonian, Canada. (After Dawson.)]
As regards the _life_ of the Devonian period, we are now acquainted
with a large and abundant terrestrial _flora_--this being the
first time that we have met with a land vegetation capable of
reconstruction in any fulness. By the researches of Goeppert,
Unger, Dawson, Carruthers, and other botanists, a knowledge has
been acquired of a large number of Devonian plants, only a few
of which can be noticed here. As might have been anticipated,
the greater number of the vegetable remains of this period have
been obtained from such shallow-water deposits as the Old Red
Sandstone proper and the Gaspe series of North America, and few
traces of plant-life occur in the strictly marine sediments.
Apart from numerous remains, mostly of a problematical nature,
referred to the comprehensive group of the Sea-weeds, a large
number of Ferns have now been recognised, some being, of the
ordinary plant-like type (_Pecopteris, Neuropteris, Alethopteris,
Sphenopteris_, &c.), whilst others belong to the gigantic group
of the "Tree-ferns" (_Psaronius, Caulopteris_, &c.) Besides these
there is an abundant development of the singular extinct types of
the _Lepidodendroids_, the _Sigillarioids_, and the _Calamites_,
all of which attained their maximum in the Carboniferous. Of
these, the _Lepidodendra_ may be regarded as gigantic, tree-like
Club-mosses (_Lycopodiaceoe_); the _Calamites_ are equally gigantic
Horse-tails (_Equisetaceoe_); and the _Sigillarioids_, equally huge
in size, in some respects hold a position intermediate between
the Club-mosses and the Pines (Conifers). The Devonian rocks have
also yielded traces of many other plants (such as _Annularia,
Asterophyllites, Cardiocarpon_, &c.), which acquire a greater
pre-dominance in the Carboniferous period, and which will be
spoken of in discussing the structure of the plants of the
Coal-measures. Upon the whole, the one plant which may be considered
as specially characteristic of the Devonian (though not confined
to this series) is the _Psilophyton_ (fig. 77) of Dr Dawson.
These singular plants have slender branching stems, with sparse
needle-shaped leaves, the young stems being at first coiled up,
crosier-fashion, like the young fronds of ferns, whilst the old
branches carry numerous spore-cases. The stems and branches seem
to have attained a height of two or three feet; and they sprang
from prostrate "root-stocks" or creeping stems. Upon the whole,
Principal Dawson is disposed to regard _Psilophyton_ as a
"generalised type" of plants intermediate between the Ferns and
the Club-mosses. Lastly, the Devonian deposits have yielded the
remains of the first actual _trees_ with which we are as yet
acquainted. About the nature of some of these (_Ormoxylon_ and
_Dadoxylon_) no doubt can be entertained, since their trunks
not only show the concentric rings of growth characteristic of
exogenous trees in general, but their woody tissue exhibits under
the microscope the "discs" which are characteristic of the wood of
the Pines and Firs (see fig. 2). The singular genus _Prototaxites_,
however, which occurs in an older portion of the Devonian series
than the above, is not in an absolutely unchallenged position.
By Principal Dawson it is regarded as the trunk of an ancient
_Conifer_--the most ancient known; but Mr Carruthers regards it
as more probably the stem of a gigantic sea-weed. The trunks
of _Prototaxites_ (fig. 78, A) vary from one to three feet in
diameter, and exhibit concentric rings of growth; but its woody
fibres have not hitherto been clearly demonstrated to possess discs.
Before leaving the Devonian vegetation, it may be mentioned that
the hornstone or chert so abundant in the Corniferous limestone
of North America has been shown to contain the remains of various
microscopic plants (_Diatoms_ and _Desmids_). We find also in
the same siliceous material the singular spherical bodies, with
radiating spines, which occur so abundantly in the chalk flints,
and which are termed _Xanthidia_. These may be regarded as probably
the spore-cases of the minute plants known as _Desmidioe_.
[Illustration: Fig. 78.--A, Trunk of _Prototaxites Logani_, eighteen
inches in diameter, as seen in the cliff near L'Anse Brehaut,
Gaspe; B, Two wood-cells showing spiral fibres and obscure pores,
highly magnified. Lower Devonian, Canada. (After Dawson)]
The Devonian _Protozoans_ have still to be fully investigated.
True Sponges (such as _Astrtoeospongia, Sphoerospongia_, &c.)
are not unknown; but by far the commonest representatives of
this sub-kingdom in the Devonian strata are _Stromatopora_ and
its allies. These singular organisms (fig. 79) are not only very
abundant in some of the Devonian limestones--both in the Old World
and the New--but they often attain very large dimensions. However
much they may differ in minor details, the general structure of
these bodies is that of numerous, concentrically-arranged, thin,
calcareous laminae, separated by narrow interspaces, which in turn
are crossed by numerous delicate vertical pillars, giving the whole
mass a cellular structure, and dividing it into innumerable minute
quadrangular compartments. Many of the Devonian _Stromatoporoe_
also exhibit on their surface the rounded openings of canals,
which can hardly have served any other purpose than that of
permitting the sea-water to gain ready access to every part of
the organism.
[Illustration: Fig. 79.--a, Part of the under surface of
_Stromatopora tuberculata_, showing the wrinkled basement membrane
and the openings of water-canals, of the natural size; b, Portion
of the upper surface of the same, enlarged; c, Vertical section of
a fragment, magnified to show the internal structure. Corniferous
Limestone, Canada. (Original.)]
[Illustration: Fig. 80.--_Cystiphyllum vesiculosum_, showing a
succession of cups produces by budding from the original coral.
Of the natural size. Devonian, America and Europe. (Original.)]
[Illustration: Fig. 81--_Zaphrentis cornicula_, of the natural
size. Devonian, America. (Original.)]
[Illustration: Fig. 82--_Heliophyllum exiguum_, viewed from in
front and behind. Of the natural size. Devonian, Canada. (Original.)]
[Illustration: Fig. 83.--Portion of a mass of _Crepidophyllum
Archiaci_, of the natural size. Hamilton Formation, Canada. (After
Billings.)]
No true _Graptolites_ have ever been detected in strata of Devonian
age; and the whole of this group has become extinguished--unless we
refer here the still surviving _Dictyonemoe_. The _Coelenterates_,
however, are represented by a vast number of _Corals_, of beautiful
forms and very varied types. The marbles of Devonshire, the Devonian
limestones of the Eifel and of France, and the calcareous strata
of the Corniferous and Hamilton groups of America, are often
replete with the skeletons of these organisms--so much so as to
sometimes entitle the rock to be considered as representing an
ancient coral-reef. In some instances the Corals have preserved
their primitive calcareous composition; and if they are embedded
in soft shales, they may weather out of the rock in almost all
their original perfection. In other cases, as in the marbles
of Devonshire, the matrix is so compact and crystalline that
the included corals can only be satisfactorily studied by means
of polished sections. In other cases, again, the corals have
been more or less completely converted into flint, as in the
Corniferous limestone of North America. When this is the case,
they often come, by the action of the weather, to stand out from
the enclosing rock in the boldest relief, exhibiting to the observer
the most minute details of their organization. As before, the
principal representatives of the Corals are still referable to
the groups of the _Rugosa_ and _Tabulata_. Amongst the Rugose
group we find a vast number of simple "cup-corals," generally
known by the quarrymen as "horns," from their shape. Of the many
forms of these, the species of _Cyathophyllum, Heliophyllum_
(fig. 82), _Zaphrentis_ (fig. 81), and _Cystiphyllum_ (fig. 80),
are perhaps those most abundantly represented--none of these
genera, however, except _Heliophyllum_, being peculiar to the
Devonian period. There are also numerous compound Rugose corals,
such as species of _Eridophyllum, Diphyphyllum, Syringopora,
Phillipsastroea_, and some of the forms of _Cyathophyllum_ and
_Crepidophyllum_ (fig. 83). Some of these compound corals attain
a very large size, and form of themselves regular beds, which
have an analogy, at any rate, with existing coral-reefs, though
there are grounds for believing that these ancient types differed
from the modern reef-builders in being inhabitants of deep water.
The "Tabulate Corals" are hardly less abundant in the Devonian
rocks than the _Rugosa_; and being invariably compound, they
hardly yield to the latter in the dimensions of the aggregations
which they sometimes form.
[Illustration: Fig. 84.--Portion of a mass of _Favosites
Gothlandica_, of the natural size. Upper Silurian and Devonian
of Europe and America. (Original.) Billings.]
[Illustration: Fig. 85.--Fragment of _Favosites hemispherica_,
of the natural size. Upper Silurian and Devonian of America.
(After Billings.)]
The commonest, and at the same time the largest, of these are
the "honeycomb corals," forming the genus _Favosites_ (figs.
84, 85), which derive both their vernacular and their technical
names from their great likeness to masses of petrified honeycomb.
The most abundant species are _Favosites Gothlandica_ and _F.
Hemispherica_, both here figured, which form masses sometimes
not less than two or three feet in diameter. Whilst _Favosites_
has acquired a popular name by its honey-combed appearance, the
resemblance of _Michelinia_ to a fossilised wasp's nest with the
comb exposed is hardly less striking, and has earned for it a
similar recognition from the non-scientific public. In addition
to these, there are numerous branching or plant-like Tabulate
Corals, often of the most graceful form, which are distinctive
of the Devonian in all parts of the world.
The _Echinoderms_ of the Devonian period call for little special
notice. Many of the Devonian limestones are "crinoidal;" and
the _Crinoids_ are the most abundant and widely-distributed
representatives of their class in the deposits of this period.
The _Cystideans_, with doubtful exceptions, have not been recognised
in the Devonian; and their place is taken by the allied group of
the "Pentremites," which will be further spoken of as occurring
in the Carboniferous rocks. On the other hand, the Star-fishes,
Brittle-stars, and Sea-urchins are all continued by types more
or less closely allied to those of the preceding Upper Silurian.
Of the remains of Ringed-worms (_Annelides_), the most numerous
and the most interesting are the calcareous envelopes of some
small tube-inhabiting species. No one who has visited the seaside
can have failed to notice the little spiral tubes of the existing
_Spirorbis_ growing attached to shells, or covering the fronds
of the commoner Sea weeds (especially _Fucus serratus_). These
tubes are inhabited by a small Annelide, and structures of a
similar character occur not uncommonly from the Upper Silurian
upwards. In the Devonian rocks, _Spirorbis_ is an extremely common
fossil, growing in hundreds attached to the outer surface of
corals and shells, and appearing in many specific forms (figs.
86 and 87); but almost all the known examples are of small size,
and are liable to escape a cursory examination.
[Illustration: Fig. 87.--a, _Spirobois omphalodes_, natural size
and enlarged. Devonian, Europe and America; b, _Spirorbis
Arkonensis_, of the natural size and enlarged; c, The same,
with the tube twisted in the reverse direction. Devonian, America.
(Onginal.)]
[Illustration: Fig. 88. a b, _Spirorbis laxus_, enlarged, Upper
Silurian, America; c, _Spirorbis spinulifera_, of the natural
size and enlarged, Devonian, Canada. (After Hall and the Author.)]
[Illustration: Fig. 88.--Devonian Trilobites; a, _Phacops latifrons_,
Devonian of Britain, the Continent of Europe, and South America;
b, _Homalonotus armatus_, Europe; c, _Phacops (Trimerocephalus)
loevis_, Europe; d, Head-shield of _Phacops (Portlockia)
granulatus_, Europe. (After Salter and Burmeister.)]
The _Crustaceans_ of the Devonian are principally _Eurypterids_
and _Trilobites_. Some of the former attain gigantic dimensions,
and the quarrymen in the Scotch Old Red give them the name of
"seraphim" from their singular scale-like ornamentation. The
_Trilobites_, though still sufficiently abundant in some localites,
have undergone a yet further diminution since the close of the
Upper Silurian. In both America and Europe quite a number of
generic types have survived from the Silurian, but few or no
new ones make their appearance during this period in either the
Old World or the New. The _species_, however, are distinct; and
the principal forms belong to the genera _Phacops_ (fig. 88, a,
c, d), _Homalonotus_ (fig. 88, b), _Proetus_, and _Bronteus_.
The species figured above under the name of _Phacops latifrons_
(fig. 88, a), has an almost world-wide distribution, being found
in the Devonian of Britain, Belgium, France, Germany, Russia,
Spain, and South America; whilst its place is taken in North
America by the closely-allied _Phacops rana_. In addition to the
_Trilobites_, the Devonian deposits have yielded the remains of a
number of the minute _Ostracoda_, such as _Entomis_ ("_Cypridina_"),
_Leperditia_, &c., which sometimes occur in vast numbers, as
in the so-called "_Cypridina_ Slates" of the German Devonian.
There are also a few forms of _Phyllopods_ (_Estheria_). Taken
as a whole, the Crustacean fauna of the Devonian period presents
many alliances with that of the Upper Silurian, but has only
slight relationships with that of the Lower Carboniferous.
Besides _Crustaceans_, we meet here for the first time with the
remains of _air-breathing Articulates_, in the shape of _Insects_.
So far, these have only been obtained from the Devonian rocks of
North America, and they indicate the existence of at least four
generic types, all more or less allied to the existing May-flies
(_Ephemeridoe_). One of these interesting primitive insects, namely,
_Platephemera antiqua_ (fig. 89), appears to have measured five
inches in expanse of wing; and another (_Xelloneura antiquorum_) has
attached to its wing the remains of a "stridulating-organ" similar
to that possessed by the modern Grasshoppers--the instrument, as
Principal Dawson remarks, of "the first music of living things
that Geology as yet reveals to us."
[Illustration: Fig. 89.--Wing of _Platephemera antiqua_ Devonian,
America. (After Dawson.)]
Amongst the _Mollusca_, the Devonian rocks have yielded a great
number of the remains of Sea-mosses (_Polyzoa_). Some of these
belong to the ancient type _Ptilodictya_, which seems to disappear
here, or to the allied _Clathropora_ (fig. 90), with its fenestrated
and reticulated fronds. We meet also with the graceful and delicate
stems of _Ceriopora_ (fig. 91).
[Illustration: Fig. 90.--Fragment of _Clathropora intertexta_,
of the natural size and enlarged. Devonian, Canada. (Original.)]
[Illustration: Fig. 91.--Fragment of _Ceriopora Hamiltonensis_,
of the natural size and enlarged. Devonian, Canada. (Original.)]
[Illustration: Fig. 92.--Fragment of _Fenestella magnifica_, of
the natural size and enlarged. Devonian, Canada. (Original.)]
[Illustration: Fig. 93.--Fragment of _Retepora Phillipsi_, of
the natural size and enlarged. Devonian, Canada. (Original.)]
[Illustration: Fig. 94.--Fragment of _Fenestella cribrosa_, of
the natural size and enlarged. Dovonian, Canada. (Original.)]
The majority of the Devonian _Polyzoa_ belong, however, to the
great and important Palaeozoic group of the Lace-corals (_Fenestella_,
figs. 92 and 94, _Retepora_, fig. 93, _Polypora_, and their allies).
In all these forms there is a horny skeleton, of a fan-like or
funnel-shaped form, which grew attached by its base to some foreign
body. The frond consists of slightly-diverging or nearly parallel
branches, which are either united by delicate cross-bars, or which
bend alternately from side to side, and become directly united
with one another at short intervals--in either case giving origin
to numerous oval or oblong perforations, which communicate to the
whole plant-like colony a characteristic netted and lace-like
appearance. On one of its surfaces--sometimes the internal, sometimes
the external--the frond carries a number of minute chambers or
"cells," which are generally borne in rows on the branches, and
of which each originally contained a minute animal.
[Illustration: Fig. 95.--_Spirifera sculptilis_. Devonian, Canada.
(After Billings.)]
[Illustration: Fig. 96.--_Spirifera mucronata_. Devonian, America.
(After Billings.)]
[Illustration: Fig. 97.--_Atrypa reticularis_. Upper Silurian
and Devonian of Europe and America. (After Billings.)]
The _Brachiopods_ still continue to be represented in great force
through all the Devonian deposits, though not occurring in the
true Old Red Sandstone. Besides such old types as _Orthis,
Strophomena, Lingula, Athyris_, and _Rhynchonella_, we find some
entirely new ones; whilst various types which only commenced their
existence in the Upper Silurian, now undergo a great expansion
and development. This last is especially the case with the two
families of the _Spiriferidoe_ and the _Produclidoe_. The
_Spirifers_, in particular, are especially characteristic of
the Devonian, both in the Old and New Worlds--some of the most
typical forms, such as _Spirifera mucronata_ (fig. 96), having
the shell "winged," or with the lateral angles prolonged to such
an extent as to have earned for them the popular name of
"fossil-butterflies." The closely-allied _Spirifera disjunda_
occurs in Britain, France, Spain, Belgium, Germany, Russia, and
China. The family of the _Productidoe_ commenced to exist in the
Upper Silurian, in the genus _Chonetes_, and we shall hereafter
find it culminating in the Carboniferous in many forms of the great
genus _Producta_[17] itself. In the Devonian period, there is an
intermediate state of things, the genus _Chonetes_ being continued
in new and varied types, and the Carboniferous _Produdoe_ being
represented by many forms of the allied group _Productella_.
Amongst other well-known Devonian Brachiopods may be mentioned
the two long-lived and persistent types _Atrypa reticularis_
(fig. 97) and _Strophomena rhomboidalis_ (fig. 98). The former
of these commences in the Upper Silurian, but is more abundantly
developed in the Devonian, having a geographical range that is
nothing less than world-wide; whilst the latter commences in the
Lower Silurian, and, with an almost equally cosmopolitan range,
survives into the Carboniferous period.
[Footnote 17: The name of this genus is often written _Productus_,
just as _Spirifera_ is often given in the masculine gender as
_Spirifer_ (the name originally given to it). The masculine
termination to these names is, however, grammatically incorrect,
as the feminine noun _cochlea_ (shell) is in these cases
_understood_.]
[Illustration: Fig. 98.--_Strophomena rhomboidalis_. Lower Silurian,
Upper Silurian, and Devonian of Europe and America.]
[Illustration: Fig. 99.--Different views of _Platyceras dumosum_,
of the natural size. Devonian, Canada. (Original.)]
The Bivalves (_Lamellibranchiata_) of the Devonian call for no
special comment, the genera _Pterinea_ and _Megalodon_ being,
perhaps, the most noticeable. The Univalves (_Gasteropods_), also,
need not be discussed in detail, though many interesting forms
of this group are known. The type most abundantly represented,
especially in America, is _Platyceras_ (fig. 99), comprising thin,
wide-mouthed shells, probably most nearly allied to the existing
"Bonnet-limpets," and sometimes attaining very considerable
dimensions. We may also note the continuance of the genus
_Euomphalus_, with its discoidal spiral shell. Amongst the
_Heteropods_, the survival of _Bellerophon_ is to be recorded;
and in the "Winged-snails," or _Pteropods_, we find new forms
of the old genera _Tentaculites_ and _Conularia_ (fig. 100).
The latter, with its fragile, conical, and often beautifully
ornamented shell, is especially noticeable.
[Illustration: Fig. 100.--_Conularia ornata, of the natural size.
Devonian, Europe.]
[Illustration: Fig. 101.--_Clymenia Sedgwickii_. Devonian, Europe.]
The remains of _Cephalopoda_ are far from uncommon in the Devonian
deposits, all the known forms being still Tetrabranchiate. Besides
the ancient types _Orthoceras_ and _Cyrtoceras_, we have now
a predominance of the spirally-coiled chambered shells of
_Goniatites_ and _Clymenia_. In the former of these the shell is
shaped like that of the _Nautilus_; but the partitions between the
chambers ("septa") are more or less lobed, folded, or angulated,
and the "siphuncle" runs along the _back_ or convex side of the
shell--these being characters which approximate _Goniatites_ to
the true Ammonites of the later rocks. In _Clymenia_, on the
other hand, whilst the shell (fig. 101) is coiled into a flat
spiral, and the partitions or septa are simple or only slightly
lobed, there is still this difference, as compared with the
_Nautilus_, that the tube of the siphuncle is placed on the _inner_
or concave side of the shell. The species of _Clymenia_ are
exclusively Devonian in their range; and some of the limestones
of this period in Germany are so richly charged with fossils of
this genus as to have received the name of "Clymenien-kalk."
The sub-kingdom of the _Vertebrates_ is still represented by
_Fishes_ only; but these are so abundant, and belong to such
varied types, that the Devonian period has been appropriately
called the "Age of Fishes." Amongst the existing fishes there are
three great groups which are of special geological importance,
as being more or less extensively represented in past time. These
groups are: (1) The _Bony Fishes_ (_Teleostei_), comprising most
existing fishes, in which the skeleton is more or less completely
converted into bone; the tail is symmetrically lobed or divided
into equal moieties; and the scales are usually thin, horny,
flexible plates, which overlap one another to a greater or less
extent. (2) The _Ganoid Fishes_ (_Ganoidei_), comprising the modern
Gar-pikes, Sturgeons, &c., in which the skeleton usually more or
less completely retains its primitive soft and cartilaginous
condition; the tail is generally markedly unsymmetrical, being
divided into two unequal lobes; and the scales (when present)
have the form of plates of bone, usually covered by a layer of
shining enamel. These scales may overlap; or they may be rhomboidal
plates, placed edge to edge in oblique rows; or they have the form
of large-sized bony plates, which are commonly united in the region
of the head to form a regular buckler. (3) The _Placoid Fishes_,
or _Elasmobranchii_, comprising the Sharks, Rays, and _Chimoeroe_
of the present day, in which the skeleton is cartilaginous; the
tail is unsymmetrically lobed; and the scales have the form of
detached bony plates of variable size, scattered in the integument.
It is to the two last of these groups that the Devonian fishes
belong, and they are more specially referable to the _Ganoids_.
The order of the Ganoid fishes at the present day comprises but
some seven or eight genera, the species of which principally or
exclusively inhabit fresh waters, and all of which are confined
to the northern hemisphere. As compared, therefore, with the Bony
fishes, which constitute the great majority of existing forms,
the Ganoids form but an extremely small and limited group. It was
far otherwise, however, in Devonian times. At this period, the
bony fishes are not known to have come into existence at all, and
the Ganoids held almost undisputed possession of the waters. To
what extent the Devonian Ganoids were confined to fresh waters
remains yet to be proved; and that many of them lived in the sea
is certain. It was formerly supposed that the Old Red Sandstone
of Scotland and Ireland, with its abundant fish-remains, might
perhaps be a fresh-water deposit, since the habitat of its fishes
is uncertain, and it contains no indubitable marine fossils. It
has been now shown, however, that the marine Devonian strata
of Devonshire and the continent of Europe contain some of the
most characteristic of the Old Red Sandstone fishes of Scotland;
whilst the undoubted marine deposit of the Corniferous limestone
of North America contains numerous shark-like and Ganoid fishes,
including such a characteristic Old Red genus as _Coccosleus_.
There can be little doubt, therefore, but that the majority of
the Devonian fishes were truly marine in their habits, though
it is probable that many of them lived in shallow water, in the
immediate neighbourhood of the shore, or in estuaries.
[Illustration: Fig. 102.--Fishes of the Devonian rocks of America.
a, Diagram of the jaws and teeth of _Dinichthys Hertzeri_,
viewed from the front, and greatly reduced; b, Diagram of the
skull of _Macropetalichthys Sullivanti_, reduced in size; c,
A portion of the enamelled surface of the skull of the same,
magnified; d, One of the scales of _Onychodus sigmoides_, of
the natural size; e, One of the front teeth of the lower jaw of
the same, of the natural size: f, Fin-spine of _Machoeracanthus
major_, a shark-like fish, reduced in size. (After Newberry.)]
[Illustration: Fig. 103.--_Cephalaspis Lyellii_. Old Red Sandstone,
Scotland. (After Page.)]
[Illustration: Fig. 104.--_Pterichthys cornutus_. Old Red Sandstone,
Scotland. (After Agassiz.)]
The Devonian Galloids belong to a number of groups; and it is
only possible to notice a few of the most important forms here.
The modern group of the Sturgeons is represented, more or less
remotely, by a few Devonian fishes--such as _Asterosteus_; and
the great _Macropetalichthys_ of the Corniferous limestone of
North America is believed by Newberry to belong to this group. In
this fish (fig. 102, b) the skull was of large size, its outer
surface being covered with a tuberculated enamel; and, as in the
existing Sturgeons, the mouth seems to have been wholly destitute
of teeth. Somewhat allied, also, to the Sturgeons, is a singular
group of armoured fishes, which is highly characteristic of the
Devonian of Britain and Europe, and less so of that of America.
In these curious forms the head and front extremity of the body
were protected by a buckler composed of large enamelled plates,
more or less firmly united to one another; whilst the hinder end
of the body was naked, or was protected with small scales. Some
forms of this group--such as _Pteraspis_ and _Coccosteus_--date
from the Upper Silurian; but they attain their maximum in the
Devonian, and none of them are known to pass upwards into the
overlying Carboniferous rocks. Amongst the most characteristic
forms of this group may be mentioned _Cephalaspis_ (fig. 103) and
_Pterichthys_ (fig. 104). In the former of these the head-shield is
of a crescentic shape, having its hinder angles produced backwards
into long "horns," giving it the shape of a "saddler's knife."
No teeth have been discovered; but the body was covered with
small ganoid scales, and there was an unsymmetrical tail-fin.
In _Pterichthys_--which, like the preceding, was first brought
to light by the labours of Hugh Miller--the whole of the head
and the front part of the body were defended by a buckler of
firmly-united enamelled plates, whilst the rest of the body was
covered with small scales. The form of the "pectoral fins" was
quite unique--these having the shape of two long, curved spines,
somewhat like wings, covered by finely-tuberculated ganoid plates.
All the preceding forms of this group are of small size; but
few fishes, living or extinct, could rival the proportions of
the great _Dinichthys_, referred to this family by Newberry.
In this huge fish (fig. 102, a) the head alone is over three
feet in length, and the body is supposed to have been twenty-five
or thirty feet long. The head was protected by a massive cuirass
of bony plates firmly articulated together, but the hinder end
of the body seems to have been simply enveloped in a leathery
skin. The teeth are of the most formidable description, consisting
in both jaws of serrated dental plates behind, and in front of
enormous conical tusks (fig. 102, a). Though immensely larger,
the teeth of _Dinichthys_ present a curious resemblance to those
of the existing Mud-fishes (_Lepidosiren_).
In another great group of Devonian Ganoids, we meet with fishes
more or less closely allied to the living _Polypteri_ (fig. 105)
of the Nile and Senegal. In this group (fig. 106) the pectoral
fins consist of a central scaly lobe carrying the fin-rays on
both sides, the scales being sometimes rounded and overlapping
(fig. 106), or more commonly rhomboidal and placed edge to edge
(fig. 105, A). Numerous forms of these "Fringe-finned" Ganoids
occur in the Devonian strata, such as _Holoptychius, Glyotoloemus,
Osteolepis, Phaneropleuron_, &c. To this group is also to be
ascribed the huge _Onychodus_ (fig. 102, d and e), with its
large, rounded, overlapping scales, an inch in diameter, and its
powerful pointed teeth. It is to be remembered, however, that
some of these "Fringe-finned" Ganoids are probably referable
to the small but singular group of the "Mud-fishes" (_Dipnoi_),
represented at the present day by the singular _Lepidosiren_
of South America and Africa, and the _Ceratodus_ of the rivers
of Queensland.
[Illustration: Fig. 105.--A, _Polypterus_, a recent Ganoid
fish; B, _Osteolepis_, a Devonian Ganoid; a a, Pectoral fins,
showing the fin-rays arranged round a central lobe.]
[Illusration: Fig. 106.--_Holoptychius nobilissimus_, restored.
Old Red Sandstone, Scotland. A, Scale of the same.]
Leaving the Ganoid fishes, it still remains to be noticed that
the Devonian deposits have yielded the remains of a number of
fishes more or less closely allied to the existing Sharks, Rays,
and _Chimoeroe_ (the _Elasmobranchii_). The majority of the forms
here alluded to are allied not to the true Sharks and Dog-fishes,
but to the more peaceable "Port Jackson Sharks," with their blunt
teeth, adapted for crushing the shells of Molluscs. The collective
name of "Cestracionts" is applied to these; and we have evidence of
their past existence in the Devonian seas both by their teeth, and
by the defensive spines which were implanted in front of a greater
or less number of the fins. These are bony spines, often variously
grooved, serrated, or ornamented, with hollow bases, implanted
in the integument, and capable of being erected or depressed
at will. Many of these "fin-spines" have been preserved to us
in the fossil condition, and the Devonian rocks have yielded
examples belonging to many genera. As some of the true Sharks
and Dog-fishes, some of the Ganoids, and even some Bony Fishes,
possess similar defences, it is often a matter of some uncertainty
to what group a given spine is to be referred. One of these spines,
belonging to the genus _Machoeracanthus_, from the Devonian rocks
of America, has been figured in a previous illustration (fig.
102, f).
In conclusion, a very few words may be said as to the validity of
the Devonian series as an independent system of rocks, preserving
in its successive strata the record of an independent system
of life. Some high authorities have been inclined to the view
that the Devonian formation has in nature no actual existence,
but that it is made up partly of beds which should be referred
to the summit of the Upper Silurian, and partly of beds which
properly belong to the base of the Carboniferous. This view seems
to have been arrived at in consequence of a too exclusive study
of the Devonian series of the British Isles, where the physical
succession is not wholly clear, and where there is a striking
discrepancy between the organic remains of those two members
of the series which are known as the "Old Red Sandstone" and
the "Devonian" rocks proper. This discrepancy, however, is not
complete; and, as we have seen, can be readily explained on the
supposition that the one group of rocks presents us with the
shallow water and littoral deposits of the period, while in the
other we are introduced to the deep-sea accumulations of the
same period. Nor can the problem at issue be solved by an appeal
to the phenomena of the British area alone, be the testimony of
these what it may. As a matter of fact, there is at present no
sufficient ground for believing that there is any irreconcilable
discordance between the succession of rocks and of life in Britain
during the period which elapsed between the deposition of the
Upper Ludlow and the formation of the Carboniferous Limestone,
and the order of the same phenomena during the same period in
other regions. Some of the Devonian types of life, as is the
case with all great formations, have descended unchanged from
older types; others pass upwards unchanged to the succeeding
period: but the fauna and flora of the Devonian period are, as
a whole, quite distinct from those of the preceding Silurian or
the succeeding Carboniferous; and they correspond to an equally
distinct rock-system, which in point of time holds an intermediate
position between the two great groups just mentioned. As before
remarked, this conclusion may be regarded as sufficiently proved
even by the phenomena of the British area; but it maybe said to
be rendered a certainty by the study of the Devonian deposits of
the continent of Europe--or, still more, by the investigation of
the vast, for the most part uninterrupted and continuous series
of sediments which commenced to be laid down in North America
at the beginning of the Upper Silurian, and did not cease till,
at any rate, the close of the Carboniferous.
LITERATURE.
The following list comprises the more important works and memoirs
to which the student of Devonian rocks and fossils may refer:--
(1) 'Siluria.' Sir Roderick Murchison.
(2) 'Geology of Russia in Europe.' Murchison (together with De
Verneuil and Count von Keyserling).
(3) "Classification of the Older Rocks of Devon and Cornwall"--'Proc.
Geol. Soc.,' vol. iii., 1839. Sedgwick and Murchison.
(4) "On the Physical Structure of Devonshire;" and on the
"Classification of the Older Stratified Rocks of Devonshire
and Cornwall"--'Trans. Geol. Soc.,' vol. v., 1840. Sedgwick
and Murchison.
(5) "On the Distribution and Classification of the Older or Palaeozoic
Rocks of North Germany and Belgium"--'Geol. Trans.,' 2d ser.,
vol. vi., 1842. Sedgwick and Murchison.
(6) 'Report on the Geology of Cornwall, Devon, and West Somerset.'
De la Beche.
(7) 'Memoirs of the Geological Survey of Ireland and Scotland.'
Jukes and Geikie.
(8) "On the Carboniferous Slate (or Devonian Rocks) and the Old
Red Sandstone of South Ireland and North Devon"--'Quart.
Journ. Geol. Soc.,' vol. xxii. Jukes.
(9) "On the Physical Structure of West Somerset and North Devon;"
and on the "Palaeontological Value of Devonian Fossils"--'Quart.
Journ. Geol. Soc.,' vol. iii. Etheridge.
(10) "On the Connection of the Lower, Middle, and Upper Old Red
Sandstone of Scotland"--'Trans. Edin. Geol. Soc.,' vol. i.
part ii. Powrie.
(11) 'The Old Red Sandstone,' 'The Testimony of the Rocks,' and
'Footprints of the Creator.' Hugh Miller.
(12) "Report on the 4th Geological District"--'Geology of New York,'
vol. iv. James Hall.
(13) 'Geology of Canada,' 1863. Sir W. E. Logan.
(14) 'Acadian Geology.' Dawson.
(15) 'Manual of Geology.' Dana.
(16) 'Geological Survey of Ohio,' vol. i.
(17) 'Geological Survey of Illinois,' vol. i.
(18) 'Palaeozoic Fossils of Cornwall, Devon, and West Somerset.'
Phillips.
(19) 'Recherches sur les Poissons Fossiles.' Agassiz.
(20) 'Poissous de l'Old Red.' Agassiz.
(21) "On the Classification of Devonian Fishes"--' Mem. Geol. Survey
of Great Britain,' Decade X. Huxley.
(22) 'Monograph of the Fishes of the Old Red Sandstone of Britain'
(Palaeontographical Society). Powrie and Lankester.
(23) 'Fishes of the Devonian System, Palaeontology of Ohio.' Newberry.
(24) 'Monograph of British Trilobites' (Palaeontographical Society);
Salter.
(25) 'Monograph of British Merostomata' (Palaeontographical Society).
Henry Woodward.
(26) 'Monograph of British Brachiopoda' (Palaeontographical Society).
Davidson.
(27) 'Monograph of British Fossil Corals' (Palaeontographical Society).
Milne-Edwards and Haime.
(28) 'Polypiers Foss. des Terrains Paleozoiques.' Milne-Edwards
and Jules Haime.
(29) "Devonian Fossils of Canada West"--'Canadian Journal,' new ser.,
vols. iv.-vi. Billings.
(30) 'Palaeontology of New York,' vol. iv. James Hall.
(31) 'Thirteenth, Fifteenth, and Twenty-third Annual Reports on the
State Cabinet.' James Hall.
(32) 'Palaeozoic Fossils of Canada,' vol. ii. Billings.
(33) 'Reports on the Palaeontology of the Province of Ontario for 1874
and 1875.' Nicholson.
(34) "The Fossil Plants of the Devonian and Upper Silurian Formations
of Canada"--'Geol. Survey of Canada.' Dawson.
(35) 'Petrefacta Germaniae.' Goldfuss.
(36) 'Versteinerungen der Grauwacken-formation.' &c. Geinitz.
(37) 'Beitrag zur Palaeontologie des Thueringer-Waldes.' Richter and
Unger.
(38) 'Ueber die Placodermen der Devonischen System.' Pander.
(39) 'Die Gattungen der Fossilen Pflanzen.' Goeppert.
(40) 'Genera et Species Plantarum Fossilium.' Unger.
CHAPTER XII.
THE CARBONIFEROUS PERIOD.
Overlying the Devonian formation is the great and important series
of the _Carboniferous Rocks_, so called because workable beds
of coal are more commonly and more largely developed in this
formation than in any other. Workable coal-seams, however, occur
in various other formations (Jurassic, Cretaceous, Tertiary), so
that coal is not an exclusively Carboniferous product; whilst
even in the Coal-measures themselves the coal bears but a very
small proportion to the total thickness of strata, occurring
only in comparatively thin beds intercalated in a great series
of sandstones, shales, and other genuine aqueous sediments.
Stratigraphically, the Carboniferous rocks usually repose conformably
upon the highest Devonian beds, so that the line of demarcation
between the Carboniferous and Devonian formations is principally
a palaeontological one, founded on the observed differences in
the fossils of the two groups. On the other hand, the close of
the Carboniferous period seems to have been generally, though
not universally, signalised by movements of the crust of the
earth, so that the succeeding Permian beds often lie unconformably
upon the Carboniferous sediments.
Strata of Carboniferous age have been discovered in almost every
large land-area which has been sufficiently investigated; but
they are especially largely developed in Britain, in various
parts of the continent of Europe, and in North America. Their
general composition, however, is, comparatively speaking, so
uniform, that it will suffice to take a comprehensive view of
the formation without considering any one area in detail, though
in each region the subdivisions of the formation are known by
distinctive local names. Taking such a comprehensive view, it is
found that the Carboniferous series is generally divisible into a
_Lower_ and essentially calcareous group (the "Sub-Carboniferous" or
"Carboniferous Limestone"); a _Middle_ and principally arenaceous
group (the "Millstone Grit"); and an Upper group, of alternating
shales and sandstones, with workable seams of coal (the
"Coal-measures").
I. The _Carboniferous, Sub-Carboniferous_, or _Mountain Limestone
Series_ constitutes the general base of the Carboniferous system.
As typically developed in Britain, the Carboniferous Limestone
is essentially a calcareous formation, sometimes consisting of a
mass of nearly pure limestone from 1000 to 2000 feet in thickness,
or at other times of successive great beds of limestone with
subordinate sandstones and shales. In the north of England the
base of the series consists of pebbly conglomerates and coarse
sandstones; and in Scotland generally, the group is composed
of massive sandstones with a comparatively feeble development
of the calcareous element. In Ireland, again, the base of the
Carboniferous Limestone is usually considered to be formed by
a locally-developed group of grits and shales (the "Coomhola
Grits" and "Carboniferous Slate"), which attain the thickness
of about 5000 feet, and contain an intermixture of Devonian with
Carboniferous types of fossils. Seeing that the Devonian formation
is generally conformable to the Carboniferous, we need feel no
surprise at this intermixture of forms; nor does it appear to be
of great moment whether these strata be referred to the former
or to the latter series. Perhaps the most satisfactory course
is to regard the Coomhola Grits and Carboniferous Slates as
"passage-beds" between the Devonian and Carboniferous; but any
view that may be taken as to the position of these beds, really
leaves unaffected the integrity of the Devonian series as a distinct
life-system, which, on the whole, is more closely allied to the
Silurian than to the Carboniferous. In North America, lastly,
the Sub-Carboniferous series is never purely calcareous, though
in the interior of the continent it becomes mainly so. In other
regions, however, it consists principally of shales and sandstones,
with subordinate beds of limestone, and sometimes with this beds
of coal or deposits of clay-ironstone.
II. _The Millstone Grit_.--The highest beds of the Carboniferous
Limestone series are succeeded, generally with perfect conformity,
by a series of arenaceous beds, usually known as the _Millstone
Grit_. As typically developed in Britain, this group consists of
hard quartzose sandstones, often so large-grained and coarse in
texture as to properly constitute fine conglomerates. In other
cases there are regular conglomerates, sometimes with shales,
limestones, and thin beds of coal--the thickness of the whole
series, when well developed, varying from 1000 to 5000 feet. In
North America, the Millstone Grit rarely reaches 1000 feet in
thickness; and, like its British equivalent, consists of coarse
sandstones and grits, sometimes with regular conglomerates. Whilst
the Carboniferous Limestone was undoubtedly deposited in a tranquil
ocean of considerable depth, the coarse mechanical sediments
of the Millstone Grit indicate the progressive shallowing of
the Carboniferous seas, and the consequent supervention of
shore-conditions.
III. _The Coal-measures_.--The Coal-measures properly so called
rest conformably upon the Millstone Grit, and usually consist of
a vast series of sandstones, shales, grits, and coals, sometimes
with beds of limestone, attaining in some regions a total thickness
of from 7000 to nearly 14,000 feet. Beds of workable coal are
by no means unknown in some areas in the inferior group of the
Sub-Carboniferous; but the general statement is true, that coal is
mostly obtained from the true Coal-measures--the largest known, and
at present most productive coal-fields of the world being in Great
Britain, North America, and Belgium. Wherever they are found, with
limited exceptions, the Coal-measures present a singular _general_
uniformity of mineral composition. They consist, namely, of an
indefinite alternation of beds of sandstone, shale, and coal,
sometimes with bands of clay-ironstone or beds of limestone,
repeated in no constant order, but sometimes attaining the enormous
aggregate thickness of 14,000 feet, or little short of 3 miles.
The beds of coal differ in number and thickness in different
areas, but they seldom or never exceed one-fiftieth part of the
total bulk of the formation in thickness. The characters of the
coal itself, and the way in which the coal-beds were deposited,
will be briefly alluded to in speaking of the vegetable life
of the period. In Britain, and in the Old World generally, the
Coal-measures are composed partly of genuine terrestrial
deposits--such as the coal--and partly of sediments accumulated
in the fresh or brackish waters of vast lagoons, estuaries, and
marshes. The fossils of the Coal-measures in these regions are
therefore necessarily the remains either of terrestrial plants
and animals, or of such forms of life as inhabit fresh or brackish
waters, the occurrence of strata with marine fossils being quite
a local and occasional phenomenon. In various parts of North
America, on the other hand, the Coal-measures, in addition to
sandstones, shales, coal-seams, and bands of clay-ironstone,
commonly include beds of limestone, charged with marine remains,
and indicating marine conditions. The subjoined section (fig. 107)
gives, in a generalised form, the succession of the Carboniferous
strata in such a British area as the north of England, where
the series is developed in a typical form.
As regards the _life_ of the Carboniferous period, we naturally
find, as has been previously noticed, great differences in different
parts of the entire series, corresponding to the different mode of
origin of the beds. Speaking generally, the Lower Carboniferous
(or the Sub-Carboniferous) is characterised by the remains of
marine animals; whilst the Upper Carboniferous (or Coal-measures)
is characterised by the remains of plants and terrestrial animals.
In all those cases, however, in which marine beds are found in
the series of the Coal-measures, as is common in America, then
we find that the fossils agree in their general characters with
those of the older marine deposits of the period.
[Illustration: Fig. 107. GENERALIZED SECTION OF THE CARBONIFEROUS
STRATA OF THE NORTH OF ENGLAND.]
Owing to the fact that coal is simply compressed and otherwise
altered vegetable matter, and that it is of the highest economic
value to man, the Coal-measures have been more thoroughly explored
than any other group of strata of equivalent thickness in the
entire geological series. Hence we have already a very extensive
acquaintance with the _plants_ of the Carboniferous period; and
our knowledge on this subject is daily undergoing increase. It
is not to be supposed, however, that the remains of plants are
found solely in Coal-measures; for though most abundant towards
the summit, they are found in less numbers in all parts of the
series. Wherever found, they belong to the same great types of
vegetation; but, before reviewing these, a few words must be
said as to the origin and mode of formation of _coal_.
The coal-beds, as before mentioned, occur interstratified with
shales, sandstones, and sometimes limestones; and there may,
within the limits of a single coal-field, be as many as 80 or
100 of such beds, placed one above the other at different levels,
and varying in thickness from a few inches up to 20 or 30 feet.
As a general rule, each bed of coal rests upon a bed of shale or
clay, which is termed the "under-clay," and in which are found
numerous roots of plants; whilst the strata immediately on the
top of the coal may be shaly or sandy, but in either case are
generally charged with the leaves and stems of plants, and often
have upright trunks passing vertically through them. When we
add to this that the coal itself is, chemically, nearly wholly
composed of carbon, and that its microscopic structure shows it
to be composed almost entirely of fragments of stems, leaves,
bark, seeds, and vegetable _debris_ derived from _land-plants_,
we are readily enabled to understand how the coal was formed.
The "_under-clay_" immediately beneath the coal-bed represents
an old land-surface--sometimes, perhaps, the bottom of a swamp
or marsh, covered with a luxuriant vegetation; the _coal bed_
itself represents the slow accumulation, through long periods,
of the leaves, seeds, fruits, stems, and fallen trunks of this
vegetation, now hardened and compressed into a fraction of its
original bulk by the pressure of the superincumbent rocks; and
the strata of sand or shale above the coal-bed--the so-called
"roof" of the coal--represent sediments quietly deposited as the
land, after a long period of repose, commenced to sink beneath
the sea. On this view, the rank and long-continued vegetation
which gave rise to each coal-bed was ultimately terminated by
a slow depression of the surface on which the plants grew. The
land-surface then became covered by the water, and aqueous sediments
were accumulated to a greater or less thickness upon the dense
mass of decaying vegetation below, enveloping any trunks of trees
which might still be in an erect position, and preserving between
their layers the leaves and branches of plants brought down from
the neighbouring land by streams, or blown into the wafer by the
wind. Finally, there set in a slow movement of elevation,--the
old land again reappeared above the water; a new and equally
luxuriant vegetation flourished upon the new land-surface; and
another coal-bed was accumulated, to be preserved ultimately in
a similar fashion. Some few beds of coal may have been formed by
drifted vegetable matter brought down into the ocean by rivers, and
deposited directly on the bottom of the sea; but in the majority
of cases the coal is undeniably the result of the slow growth and
decay of plants _in situ_: and as the plants of the coal are
not _marine_ plants, it is necessary to adopt some such theory
as the above to account for the formation of coal-seams. By this
theory, as is obvious, we are compelled to suppose that the vast
alluvial and marshy flats upon which the coal-plants grew were
liable to constantly-recurring oscillations of level, the successive
land-surfaces represented by the successive coal-beds of any
coal-field being thus successively buried beneath accumulations
of mud or sand. We have no need, however, to suppose that these
oscillations affected large areas at the same time; and geology
teaches us that local elevations and depressions of the land
have been matters of constant occurrence throughout the whole
of past time.
All the varieties of coal (bituminous coal, anthracite; cannel-coal,
&c.) show a more or less distinct "lamination"--that is to say,
they are more or less obviously composed of successive thin layers,
differing slightly in colour and texture. All the varieties of coal,
also, consist chemically of _carbon_, with varying proportions of
certain gaseous constituents and a small amount of incombustible
mineral or "ash." By cutting thin and transparent slices of coal,
we are further enabled, by means of the microscope, to ascertain
precisely not only that the carbon of the coal is derived from
vegetables, but also, in many cases, what kinds of plants, and what
parts of these, enter into the formation of coal. When examined
in this way, all coals are found to consist more or less entirely
of vegetable matter; but there is considerable difference in
different coals as to the exact nature of this. By Professor
Huxley it has been shown that many of the English coals consist
largely of accumulations of rounded discoidal sacs or bags, which
are unquestionably the seed-vessels or "spore-cases" of certain
of the commoner coal-plants (such as the _Lepidodendra_). The
best bituminous coals seem to be most largely composed of these
spore-cases; whilst inferior kinds possess a progressively increasing
amount of the dull carbonaceous substance which is known as "mineral
charcoal," and which is undoubtedly composed of "the stems and
leaves of plants reduced to little more than their carbon." On
the other hand, Principal Dawson finds that the American coals
only occasionally exhibit spore-cases to any extent, but consist
principally of the cells, vessels, and fibres of the bark,
integumentary coverings, and woody portions of the Carboniferous
plants.
The number of plants already known to have existed during the
Carboniferous period is so great, that nothing more can be done
here than to notice briefly the typical and characteristic _groups_
of these--such as the Ferns, the Calamites, the Lepidodendroids,
the Sigillarioids, and the Conifers.
[Illustration: Fig. 108.--_Odontopteris Schlotheimii_. Carboniferous,
Europe and North America.]
[Illustration: Fig. 109.--_Calamites cannoeformis_. Carboniferous
Rocks, Europe and North America.]
In accordance with M. Brongniart's generalisation, that the Palaeozoic
period is, botanically speaking, the "Age of Acrogens," we find
the Carboniferous plants to be still mainly referable to the
Flowerless or "Cryptogamous" division of the vegetable kingdom.
The flowering or "Phanerogamous" plants, which form the bulk
of our existing vegetation, are hardly known, with certainty,
to have existed at all in the Carboniferous era, except as
represented by trees related to the existing Pines and Firs,
and possibly by the Cycads or "false palms."[18] Amongst the
"Cryptogams," there is no more striking or beautiful group of
Carboniferous plants than the _Ferns_. Remains of these are found
all through the Carboniferous, but in exceptional numbers in
the Coal-measures, and include both herbaceous forms like the
majority of existing species, and arborescent forms resembling
the living Tree-ferns of New Zealand. Amongst the latter, together
with some new types, are examples of the genera _Psaronius_ and
_Caulopteris_, both of which date from the Devonian. The simply
herbaceous ferns are extremely numerous, and belong to such
widely-distributed and largely-represented genera as _Neuropteris,
Odontopteris_ (fig. 108), _Alethopteris, Pecopteris, Sphenopteris,
Hymenophyllites_, &c.
[Footnote 18: Whilst the vegetation of the Coal-period was mainly
a terrestrial one, aquatic plants are not unknown. Sea-weeds
(such as the _Spirophyton cauda-Galli_) are common in some of
the marine strata; whilst coal, according to the researches of
the Abbe Castracane, is asserted commonly to contain the siliceous
envelopes of Diatoms.]
The fossils known as _Calamites_ (fig. 109) are very common in
the Carboniferous deposits, and have given occasion to an abundance
of research and speculation. They present themselves as prostrate
and flattened striated stems, or as similar uncompressed stems
growing in an erect position, and sometimes attaining a length
of twenty feet or more. Externally, the stems are longitudinally
ribbed, with transverse joints at regular intervals, these joints
giving origin to a whorl or branchlets, which mayor may not give
origin to similar whorls of smaller branchlets still. The stems,
further, were hollow, with transverse partitions at the joints,
and having neither true wood nor bark, but only a thin external
fibrous shell. There can be little doubt but that the _Calamites_
are properly regarded as colossal representatives of the little
Horse-tails (_Equisetaceoe_) of the present day. They agree with
these not only in the general details of their organisation, but
also in the fact that the fruit was a species of cone, bearing
"spore-cases" under scales. According to Principal Dawson, the
_Calamites_ "grew in dense brakes on the sandy and muddy flats,
subject to inundation, or perhaps even in water; and they had
the power of budding out from the base of the stem, so as to
form clumps of plants, and also of securing their foothold by
numerous cord-like roots proceeding from various heights on the
lower part of the stem."
[Illustration: Fig. 110.--_Lepidodendron Sternbergii_, Carboniferous,
Europe. The central figure represents a portion of the trunk with
its branches, much reduced in size. The right-hand figure is
a portion of a branch with the leaves partially attached to it;
and the left-hand figure represents the end of a branch bearing
a cone of fructification.]
The _Lepidodendroids_, represented mainly by the genus
_Lepidodendron_ itself (fig. 110), were large tree-like plants,
which attain their maximum in the Carboniferous period, but which
appear to commence in the Upper Silurian, are well represented in
the Devonian, and survive in a diminished form into the Permian.
The trunks of the larger species of _Lepidodendron_ at times
reach a length of fifty feet and upwards, giving off branches in
a regular bifurcating manner. The bark is marked with numerous
rhombic or oval scars, arranged in quincunx order, and indicating
the points where the long, needle-shaped leaves were formerly
attached. The fruit consisted of cones or spikes, carried at the
ends of the branches, and consisting of a central axis surrounded
by overlapping scales, each of which supports a "spore-case"
or seed-vessel. These cones have commonly been described under
the name of _Lepidostrobi_. In the structure of the trunk there
is nothing comparable to what is found in existing trees, there
being a thick bark surrounding a zone principally composed of
"scalariform" vessels, this in turn enclosing a large central
pith. In their general appearance the _Lepidodendra_ bring to mind
the existing Araucarian Pines; but they are true "Cryptogams,"
and are to be regarded as a gigantic extinct type of the modern
Club-mosses (_Lycopodiaceoe_). They are amongst the commonest
and most characteristic of the Carboniferous plants; and the
majority of the "spore-cases" so commonly found in the coal appear
to have been derived from the cones of Lepidodendroids.
The so-called _Sigillanoids_, represented mainly by _Sigillaria_
itself (fig. 111), were no less abundant and characteristic of
the Carboniferous forests than the _Lepidodendra_. They commence
their existence, so far as known, in the Devonian period, but
they attain their maximum in the Carboniferous; and--unlike the
Lepidodendroids--they are not known to occur in the Permian period.
They are comparatively gigantic in size, often attaining a height
of from thirty to fifty feet or more; but though abundant and
well preserved, great divergence of opinion prevails as to their
true affinities. The _name_ of Sigillarioids (Lat. _sigilla_,
little seals or images) is derived from the fact that the bark
is marked with seal-like impressions or leaf-scars (fig. 111).
[Illustration: Fig. 111.--Fragment of the external surface of
_Sigillaria Groeseri_, showing the ribs and leaf-scars. The left-hand
figure represents a small portion enlarged. Carboniferous, Europe.]
Externally, the trunks of _Sigillaria_ present strong longitudinal
ridges, with vertical alternating rows of oval leaf-scars indicating
the points where the leaves were originally attached. The trunk
was furnished with a large central pith, a thick outer bark,
and an intermediate woody zone,--composed, according to Dawson,
partly of the disc-bearing fibres so characteristic of Conifers;
but, according to Carruthers, entirely made up of the "scalariform"
vessels characteristic of Cryptogams. The size of the pith was
very great, and the bark seems to have been the most durable
portion of the trunk. Thus we have evidence that in many cases
the stumps and "stools" of _Sigillarioe_, standing upright in
the old Carboniferous swamps, were completely hollowed out by
internal decay, till nothing but an exterior shell of bark was
left. Often these hollow stumps became ultimately filled up with
sediment, sometimes enclosing the remains of galley-worms,
land-snails, or Amphibians, which formerly found in the cavity
of the trunk a congenial home; and from the sandstone or shale
now filling such trunks some of the most interesting fossils of
the Coal-period have been obtained. There is little certainty
as to either the leaves or fruits of _Sigillaria_, and there
is equally little certainty as to the true botanical position
of these plants. By Principal Dawson they are regarded as being
probably flowering plants allied to the existing "false palms"
or "_Cycads_," but the high authority of Mr Carruthers is to
be quoted in support of the belief that they are Cryptogamic,
and most nearly allied to the Club-mosses.
[Illustration: Fig. 112.--_Stigmaria ficoides_. Quarter natural
size. Carboniferous.]
Leaving the botanical position of _Sigillaria_ thus undecided, we
find that it is now almost universally conceded that the fossils
originally described under the name of _Stigmaria_ are the _roots_
of _Sigillaria_, the actual connection between the two having been
in numerous instances demonstrated in an unmistakable manner.
The _Stigmarioe_ (fig. 112) ordinarily present themselves in
the form of long, compressed or rounded fragments, the external
surface of which is covered with rounded pits or shallow tubercles,
each of which has a little pit or depression in its centre. From
each of these pits there proceeds, in perfect examples, a long
cylindrical rootlet; but in many cases these have altogether
disappeared. In their internal structure, _Stigmaria_ exhibits
a central pith surrounded by a sheath of scalariform vessels,
the whole enclosed in a cellular envelope. The _Stigmarioe_ are
generally found ramifying in the "under-clay," which forms the
floor of a bed of coal, and which represents the ancient soil
upon which the _Sigillarioe_ grew.
[Illustration: Fig. 113.--_Trigonocarpon ovatum_. Coal-measures,
Britain. (After Liudley and Hutton.)]
The _Lepidodendroids and Sigillaroids, though the first were
certainly, and the second possibly, Cryptogamic or flowerless
plants, must have constituted the main mass of the forests of
the Coal period; but we are not without evidence of the existence
at the same time of genuine "trees," in the technical sense of
this term--namely, flowering plants with large woody stems. So
far as is certainly known, all the true trees of the Carboniferous
formation were _Conifers_, allied to the existing Pines and Firs.
They are recognised by the great size and concentric woody rings
of their prostrate, rarely erect trunks, and by the presence
of disc-bearing fibres in their wood, as demonstrated by the
microscope; and the principal genera which have been recognised
are _Dadoxylon, Paloeoxylon, Araucarioxylon_, and _Pinites_.
Their fruit is not known with absolute certainty, unless it be
represented, as often conjectured, by _Trigonocarpon_ (fig. 113).
The fruits known under this name are nut-like, often of considerable
size, and commonly three- or six-angled. They probably originally
possessed a fleshy envelope; and if truly referable to the
_Conifers_, they would indicate that these ancient evergreens
produced berries instead of cones, and thus resembled the modern
Yews rather than Pines. It seems, further, that the great group
of the _Cycads_, which are nearly allied to the _Conifers_, and
which attained such a striking prominence in the Secondary period,
probably commenced its existence during the Coal period; but
these anticipatory forms are comparatively few in number, and
for the most part of somewhat dubious affinities.
CHAPTER XIII.
THE CARBONIFEROUS PERIOD--Continued.
ANIMAL LIFE OF THE CARBONIFEROUS.
We have seen that there exists a great difference as to the mode
of origin of the Carboniferous sediments, some being purely marine,
whilst others are terrestrial; and others, again, have been formed
in inland swamps and morasses, or in brackish-water lagoons,
creeks, or estuaries. A corresponding difference exists necessarily
in the animal remains of these deposits, and in many regions
this difference is extremely well marked and striking. The great
marine limestones which characterise the lower portion of the
Carboniferous series in Britain, Europe, and the eastern portion
of America, and the calcareous beds which are found high up in
the Carboniferous in the western States of America, may, and
do, often contain the remains of drifted plants; but they are
essentially characterised by marine fossils; and, moreover, they
can be demonstrated by the microscope to be almost wholly composed
of the remains of animals which formerly inhabited the ocean. On
the other hand, the animal remains of the beds accompanying the
coal are typically the remains of air-breathing, terrestrial,
amphibious, or aerial animals, together with those which inhabit
fresh or brackish waters. Marine fossils may be found in the
Coal-measures, but they are invariably confined to special horizons
in the strata, and they indicate temporary depressions of the
land beneath the sea. Whilst the distinction here mentioned is
one which cannot fail to strike the observer, it is convenient
to consider the animal life of the Carboniferous as a whole: and
it is simply necessary, in so doing, to remember that the marine
fossils are in general derived from the inferior portion of the
system; whilst the air-breathing, fresh-water, and brackish-water
forms are almost exclusively derived from the superior portion
of the same.
[Illustration: Fig. 114.--Transparent slice of Carboniferous
Limestone, from Spergen Hill, Indiana, U.S., showing numerous
shells of _Endothyra_ (_Rotalia_), _Baiteyi_ slightly enlarged.
(Original.)]
[Illustration: Fig. 115.--_Fusulina cylindrica_, Carboniferous
Limestone, Russia.]
The Carboniferous _Protozoans_ consist mainly of _Foraminifera_
and _Sponges_. The latter are still very insufficiently known,
but the former are very abundant, and belong to very varied types.
Thin slices of the limestones of the period, when examined by the
microscope, very commonly exhibit the shells of _Foraminifera_
in greater or less plenty. Some limestones, indeed, are made up of
little else than these minute and elegant shells, often belonging
to types, such as the Textularians and Rotalians, differing little
or not at all from those now in existence. This is the case, for
example, with the Carboniferous Limestone of Spergen Hill in
Indiana (fig. 114), which is almost wholly made up of the spiral
shells of a species of _Endothyra_. In the same way, though to a
less extent, the black Carboniferous marbles of Ireland, and
the similar marbles of Yorkshire, the limestones of the west
of England and of Derbyshire, and the great "Scar Limestones" of
the north of England, contain great numbers of Foraminiferous
shells; whilst similar organisms commonly occur in the shale-beds
associated with the limestones throughout the Lower Carboniferous
series. One of the most interesting of the British Carboniferous
forms is the _Saccammina_ of Mr Henry Brady, which is sometimes
present in considerable numbers in the limestones of Northumberland,
Cumberland, and the west of Scotland, and which is conspicuous
for the comparatively large size of its spheroidal or pear-shaped
shell (reaching from an eighth to a fifth of an inch in size).
More widely distributed are the generally spindle-shaped shells
of _Fusulina_ (fig. 115), which occur in vast numbers in the
Carboniferous Limestone of Russia, Armenia, the Southern Alps,
and Spain, similar forms occurring in equal profusion in the
higher limestones which are found in the Coal-measures of the
United States, in Ohio, Illinois, Indiana, Missouri, &c. Mr Henry
Brady, lastly, has shown that we have in the _Nummulina Pristina_
of the Carboniferous Limestone of Namur a genuine _Nummulite_,
precursor of the great and important family of the Tertiary
Nummulites.
[Illustration: Fig. 116--Corals of the Carboniferous Limestone.
a. _Cyathophyllum paracida_, showing young corallites budded
forth from the disc of the old one; a', One of the corallites
of the same, seen in cross-section; b, Fragment of a mass of
_Lithostrotion irregulare_; b', One of the corallites of the
same, divided transversely; c, Portion of the simple cylindrical
coral of _Amplexus coralloides_; c', Transverse section of the
same species; d, _Zaphrentis vermicularis_, showing the depression
or "fossula" on one side of the cup; e, Fragrent of a mass of
_Syringopora ramulosa_; f, Fragment of _Coetetes tumidus_; f',
Portion of the same of the same, enlarged. From the Carboniferous
Limestone of Britain and Belgium. (After Thomson, De Koninck,
Milne-Edwards and Haime, and the Author.)]
The sub-kingdom of the _Coelenterates_, so far as certainly known,
is represented only by _Corals_;[19] but the remains of these are
so abundant in many of the limestones of the Carboniferous formation
as to constitute a feature little or not at all less conspicuous
than that afforded by the Crinoids. As is the case in the preceding
period, the Corals belong, almost exclusively, to the groups of
the _Rugosa_ and _Tabulata_; and there is a general and striking
resemblance and relationship between the coral-fauna of the Devonian
as a whole, and that of the Carboniferous. Nevertheless, there
is an equally decided and striking amount of difference between
these successive faunas, due to the fact that the great majority
of the Carboniferous _species_ are new; whilst some of the most
characteristic Devonian _genera_ have nearly or quite disappeared,
and several new genera now make their appearance for the first
time. Thus, the characteristic Devonian types _Heliophyllum,
Pachyphyllum, Chonophyllum, Acervularia, Spongophyllum, Smithia,
Endophyllum_, and _Cystiphyllum_, have now disappeared; and the
great masses of _Favosites_ which are such a striking feature
in the Devonian limestones, are represented but by one or two
degenerate and puny successors. On the other hand, we meet in
the Carboniferous rocks not only with entirely new genera--such
as _Axophyllum, Lophophyllum_, and _Londsdaleia_--but we have an
enormous expansion of certain types which had just begun to exist
in the preceding period. This is especially well seen in the Case
of the genus _Lithostrotion_ (fig. 116, b), which more than any
other may be considered as the predominant Carboniferous group of
Corals. All the species of _Lithostrotion_ are compound, consisting
either of bundles of loosely-approximated cylindrical stems, or of
similar "coral-lites" closely aggregated together into astraeiform
colonies, and rendered polygonal by mutual pressure. This genus
has a historical interest, as having been noticed as early as in
the year 1699 by Edward Lhwyd; and it is geologically important
from its wide distribution in the Carboniferous rocks of both the
Old and New Worlds. Many species are known, and whole beds of
limestone are often found to be composed of little else than
the skeletons of these ancient corals, still standing upright
as they grew. Hardly less characteristic of the Carboniferous
than the above is the great group of simple "cup-corals," of
which _Clisiophyllum_ is the central type. Amongst types which
commenced in the Silurian and Devonian, but which are still well
represented here, may be mentioned _Syringopora_ (fig. 116, e),
with its colonies of delicate cylindrical tubes united at intervals
by cross-bars; _Zaphrentis_ (fig. 116, d), with its cup-shaped
skeleton and the well-marked depression (or "fossula") on one side
of the calice; _Amplexus_ (fig. 116, c), with its cylindrical,
often irregularly swollen coral and short septa; _Cyathophyllum_
(fig. 116, a), sometimes simple, sometimes forming great masses
of star-like corallites; and _Choetetes_, with its branched stems,
and its minute, "tabulate" tubes (fig. 116, f). The above,
together with other and hardly less characteristic forms, combine
to constitute a coral-fauna which is not only in itself perfectly
distinctive, but which is of especial interest, from the fact that
almost all the varied types of which it is composed disappeared
utterly before the close of the Carboniferous period. In the
first marine sediments of a calcareous nature which succeeded to
the Coal-measures (the magnesian limestones of the Permian), the
great group of the _Rugose corals_, which flourished so largely
throughout the Silurian, Devonian, and Carboniferous periods,
is found to have all but disappeared, and it is never again
represented save sporadically and by isolated forms.
[Footnote 19: A singular fossil has been described by Professor
Martin Duncan and Mr Jenkins from the Carboniferous rocks under
the name of _Paloeocoryne_, and has been referred to the Hydroid
Zoophytes (_Corynida_). Doubt, however, has been thrown by other
observers on the correctness of this reference.]
[Illustration: Fig. 117.--_Platycrinus tricontadactylus_, Lower
Carboniferous. The left-hand figure shows the calyx, arms, and
upper part of the stem; and the figure next this shows the surface
of one of the joints of the column. The right-hand figure shows
the proboscis. (After M'Coy.)]
[Illustration: Fig. 118.--A, _Pentremites pyriformis_, side-view
of the body ("calyx"); B, The same viewed from below, showing the
arrangement of the plates; C, Body of _Pentremites conoideus_,
viewed from above. Carboniferous.]
Amongst the _Echinoderms_, by far the most important forms are
the Sea-lilies and the Sea-urchins--the former from their great
abundance, and the latter from their singular structure; but the
little group of the "Pentremites" also requires to be noticed.
The Sea-lilies are so abundant in the Carboniferous rocks, that it
has been proposed to call the earlier portion of the period the
"Age of Crinoids." Vast masses of the limestones of the period
are "crinoidal," being more or less extensively composed of the
broken columns, and detached plates and joints of Sea-lilies,
whilst perfect "heads" may be exceedingly rare and difficult
to procure. In North America the remains of Crinoids are even
more abundant at this horizon than in Britain, and the specimens
found seem to be commonly more perfect. The commonest of the
Carboniferous Crinoids belong to the genera _Cyathocrinus,
Actinocrinus, Platycrinus_, (fig. 117), _Poteriocrinus, Zeacrinus_,
and _Forbesiocrinus_. Closely allied to the Crinoids, or forming
a kind of transition between these and the Cystideans, is the
little group of the "Pentremites," or _Blastoids_ (fig. 118).
This group is first known to have commenced its existence in
the Upper Silurian, and it increased considerably in numbers
in the Devonian; but it was in the seas of the Carboniferous
period that it attained its maximum, and no certain representative
of the family has been detected in any later deposits. The
"Pentremites" resemble the Crinoids in having a cup-shaped body
(fig. 118, A) enclosed by closely-fitting calcareous plates,
and supported on a short stem or "column," composed of numerous
calcareous pieces flexibly articulated together. They differ from
the Crinoids, however, in the fact that the upper surface of
the body does not support the crown of branched feathery "arms,"
which are so characteristic of the latter. On the contrary, the
summit of the cup is closed up in the fashion of a flower-bud,
whence the technical name of _Blastoidea_ applied to the group
(Gr. _blastos_, a bud; _eidos_, form). From the top of the cup
radiate five broad, transversely-striated areas (fig. 118, C),
each with a longitudinal groove down its middle; and along each
side of each of these grooves there seems to have been attached
a row of short jointed calcareous filaments or "pinnules."
[Illustration: Fig. 119.--_Paloechinus ellipticus_, one of the
Carboniferous Sea-urchins. The left-hand figure shows one of the
"ambulacral areas" enlarged, exhibiting the perforated plates.
The right-land figure exhibits a single plate from one of the
"inter-ambulacral areas." (After M'Coy.)]
A few Star-fishes and Brittle-stars are known to occur in the
Carboniferous rocks; but the only other Echinodemls of this period
which need be noticed are the Sea-urchins (_Echinoids_). Detached
plates and spines of these are far from rare in the Carboniferous
deposits; but anything like perfect specimens are exceedingly
scarce. The Carboniferous Sea-urchins agree with those of the
present day in having the body enclosed in a shell formed by
an enormous number of calcareous plates articulated together.
The shell may be regarded as, typically, nearly spherical in
shape, with the mouth in the centre of the base, and the excretory
opening or vent at its summit. In both the ancient forms and the
recent ones, the plates of the shell are arranged in ten zones
which generally radiate from the summit to the centre of the base.
In five of these zones--termed the "ambulacral areas"--the plates
are perforated by minute apertures or "pores," through which
the animal can protrude the little water-tubes ("tube-feet") by
which its locomotion is carried on. In the other five zones--the
so-called "inter-ambulacral areas"--the plates are of larger
size, and are not perforated by any apertures. In all the modern
Sea-urchins each of these ten zones, whether perforate or
imperforate, is composed of two rows of plates; and there are
thus twenty rows of plates in all. In the Palaeozoic Sea-urchins,
on the other hand, the "ambulacral areas" are often like those of
recent forms, in consisting of _two_ rows of perforated plates
(fig. 119); but the "inter-ambulacral areas" are always quite
peculiar in consisting each of three, four, five, or more rows
of large imperforate plates, whilst there are sometimes four
or ten rows of plates in the "ambulacral areas" also: so that
there are many more than twenty rows of plates in the entire
shell. Some of the Palaeozoic Sea-urchins, also, exhibit a very
peculiar singularity of structure which is only known to exist
in a very few recently-discovered modern forms (viz., _Calveria_
and _Phormosoma_). The plates of the inter-ambulacral areas,
namely, overlap one another in an imbricating manner, so as to
communicate a certain amount of flexibility to the shell; whereas
in the ordinary living forms these plates are firmly articulated
together by their edges, and the shell forms a rigid immovable
box. The Carboniferous Sea-urchins which exhibit this extraordinary
peculiarity belong to the genera _Lepidechinus_ and _Lepidesthes_,
and it seems tolerably certain that a similar flexibility of
the shell existed to a less degree in the much more abundant
genus _Archoeocidaris_. The Carboniferous Sea-urchins, like the
modern ones, possessed movable spines of greater or less length,
articulated to the exterior of the shell; and these structures
are of very common occurrence in a detached condition. The most
abundant genera are _Archoeocidaris_ and _Paloechinus_; but the
characteristic American forms belong principally to _Melonites,
Oligoporus_, and _Lepidechinus_.
[Illustration: Fig. 120.--_Spirorbis (Microconchus) Carbonarius_,
of the natural size, attached to a fossil plant, and magnified.
Carboniferous Britain and North America. (After Dawson.)]
Amongst the _Annelides_ it is only necessary to notice the little
spiral tubes of _Spirorbis Carbonarius_ (fig. 120), which are
commonly found attached to the leaves or stems of the Coal-plants.
This fact shows that though the modern species of _Spirorbis_
are inhabitants of the sea, these old representatives of the
genus must have been capable of living in the brackish waters
of lagoons and estuaries.
[Illustration: Fig. 121.--_Prestwichia rotundata_, a Limuloid
Crustacean. Coal-measures, Britain. (After Henry Woodward.)]
[Illustration: Fig. 122.--Crustaceans of the Carboniferous Rocks.
a, _Phillipsia seminifera_, of the natural size--Mountain Limestone,
Europe; b, One valve of the shell of _Estheria tenella_, of the
natural size and enlarged--Coal-measures, Europe; c, Bivalved
shell of _Entomoconchus Scouleri_, of the natural size--Mountain
Limestone, Europe; d, _Dithyrocaris Scouleri_, reduced in
size--Mountain Limestone, Ireland; e, _Paloeocaris typus_, slightly
enlarged--Coal-measures, North America; f, _Anthrapaloemon gracilis_,
of the natural size--Coal-measures, North America. (After De
Koninck, M'Coy, Rupert Jones, and Meek and Worthen.)]
The _Crustaceans_ of the Carboniferous rocks are numerous, and
belong partly to structural types with which we are already familiar,
and partly to higher groups which come into existence here for the
first time. The gigantic _Eurypterids_ of the Upper Silurian and
Devonian are but feebly represented, and make their final exit
here from the scene of life. Their place, however, is taken by
peculiar forms belonging to the allied group of the _Xiphosura_,
represented at the present day by the King-crabs or "Horse-shoe
Crabs" (_Limulus_). Characteristic forms of this group appear
in the Coal-measures both of Europe and America; and though
constituting three distinct genera (_Prestwichia, Belinurus_,
and _Euprooeps_), they are all nearly related to one another. The
best known of them, perhaps, is the _Prestwichia rotundala_ of
Coalbrookdale, here figured (fig. 121). The ancient and formerly
powerful order of the _Trilobites_ also undergoes its final
extinction here, not surviving the deposition of the Carboniferous
Limestone series in Europe, but extending its range in America
into the Coal-measures. All the known Carboniferous forms are
small in size and degraded in point of structure, and they are
referable to but three genera (_Phillipsia, Griffithides_, and
_Brachymetopus_), belonging to a single family. The _Phillipsia
seminifera_ here figured (fig. 122, a) is a characteristic species
in the Old World. The Water-fleas (_Ostracoaa_) are extremely
abundant in the Carboniferous rocks, whole strata being often
made up of little else than the little bivalved shells of these
Crustaceans. Many of them are extremely small, averaging about
the size of a millet-seed; but a few forms, such as _Entomoconchus
Scouleni_ (fig. 122, c), may attain a length of from one to
three quarters of an inch. The old group of the _Phyllopods_
is is likewise still represented in some abundance, partly by
tailed forms of a shrimp-like appearance, such as _Dithyrocaris_
(fig. 122, d), and partly by the curious striated _Estherioe_
and their allies, which present a curious resemblance to the
true Bivalve Molluscs (fig. 122, b). Lastly, we meet for the
first time in the Carboniferous rocks with the remains of the
highest of all the groups of _Crustaceans_--namely, the so-called
"Decapods," in which there are five pairs of walking-limbs, and
the hinder end of the body ("abdomen") is composed of separate
rings, whilst the anterior end is covered by a head-shield or
"carapace." All the Carboniferous Decapods hitherto discovered
resemble the existing Lobsters, Prawns, and Shrimps (the _Macrura_),
in having a long and well-developed abdomen terminated by an
expanded tail-fin. The _Paloeocaris typus_ (fig. 122, e) and the
_Anthrapaloemon gracilis_ (fig. 122, f), from the Coal-measures
of Illinois, are two of the best understood and most perfectly
preserved of the few known representatives of the "Long-tailed"
Decapods in the Carboniferous series. The group of the Crabs
or "Short-tailed" Decapods (_Brachyura_), in which the abdomen
is short, not terminated by a tail-fin, and tucked away out of
sight beneath the body, is at present not known to be represented
at all in the Carboniferous deposits.
[Illustration: Fig. 123.--_Cyclophthalmus senior_. A fossil Scorpion
from the Coal-measures of Bohemia.]
[Illustration: Fig. 124.--_Xylobius Sigillarioe_, a Carboniferous
Myriapod. a, A specimen, of the natural size; b, Anterior
portion of the same, enlarged; c, Posterior portion, enlarged.
From the Coal-measures of Nova Scotia. (After Dawson.)]
[Illustration: Fig. 125--_Haplophlebium Barnesi_, a Carboniferous
insect, from the Coal-meastures of Nova Scotia. (After Dawson.)]
In addition to the water-inhabiting group of the Crustaceans, we
find the articulate animals to be represented by members belonging
to the air-breathing classes of the _Arachnida, Myriapoda_, and
_Insecta_. The remains of these, as might have been expected, are
not known to occur in the marine limestones of the Carboniferous
series, but are exclusively found in beds associated with the Coal,
which have been deposited in lagoons, estuaries, or marshes, in
the immediate vicinity of the land, and which actually represent
an old land-surface. The _Arachnids_ are at present the oldest
known of their class, and are represented both by true Spiders
and Scorpions. Remains of the latter (fig. 123) have been found
both in the Old and New Worlds, and indicate the existence in
the Carboniferous period of Scorpions differing but very little
from existing forms. The group of the _Myriapoda_, including
the recent Centipedes and Galley-worms, is likewise represented
in the Carboniferous strata, but by forms in many respects very
unlike any that are known to exist at the present day. The most
interesting of these were obtained by Principal Dawson, along
with the bones of Amphibians and the shells of Land-snails, in
the sediment filling the hollow trunks of _Sigillaria_, and they
belong to the genera _Xylobius_ (fig. 124) and Archiulus. Lastly,
the true _insects_ are represented by various forms of Beetles
(_Coleoptera_), _Orthoptera_ (such as Cockroaches), and
_Neuropterous_ insects resembling those which we have seen to
have existed towards the close of the Devonian period. One of the
most remarkable of the latter is a huge May-fly (_Haplophlebium
Barnesi_, fig. 125), with netted wings attaining an expanse of
fully seven inches, and therefore much exceeding any existing
Ephemerid in point of size.
[Illustration: Fig. 126.--Carboniferous _Polyzoa_. a, Fragment
of _Polypora dendroides_, of the natural size, Ireland; a' Small
portion of the same, enlarged to show the cells; b, Glauconome
pulcherrima_, a fragment, of the natural size, Ireland; b',
Portion of the same, enlarged; c, The central screw-like axis
of _Archimedes Wortheni_, of the natural size--Carboniferous,
America; c', Portion of the exterior of the frond of the same,
enlarged; c'', Portion of the interior of the frond of the
same showing the mouths of the cells, enlarged. (After M'Coy and
Hall.)]
The lower groups of the _Mollusca_ are abundantly represented
in the marine strata of the Carboniferous series by _Polyzoans_
and _Brachiopods_. Amongst the former, although a variety of other
types are known, the majority still belong to the old group of
the "Lace-corals" (_Fenestellidoe_), some of the characteristic
forms of which are here figured (fig. 126). The graceful netted
fronds of _Fenestella, Retepora_, and _Polypora_ (fig. 126, a)
are highly characteristic, as are the slender toothed branches
of _Glauconome_ (fig. 126, b). A more singular form, however,
is the curious _Archimedes_ (fig. 126, c), which is so
characteristic of the Carboniferous formation of North America.
In this remarkable type, the colony consists of a succession of
funnel-shaped fronds, essentially similar to _Fenestella_ in
their structure, springing in a continuous spiral from a strong
screw-like vertical axis. The outside of the fronds is simply
striated; but the branches exhibit on the interior the mouths of
the little cells in which the semi-independent beings composing
the colony originally lived.
[Illustration: Fig. 127.--Carboniferous _Braciopoda. a, _Producta
semireticulata_, showing the slightly concave dorsal valve; a'
Side view of the same, showing the convex ventral valve; b,
_Producta longispina_; c, _Orthis resupinata_; d, _Terebratula
hastata_; e, _Athyris subtilita_; f, _Chonetes Hardrensis_; g,
_Rhynchonella pleurodon_; h, _Spirifera trigonalis_. Most of
these forms are widely distributed in the Carboniferous Limestone
of Britain, Europe, America, &c. All the figures are of the natural
size. (After Davidson, De Koninck, and Meek.)]
The _Brachiopods_ are extremely abundant, and for the most part
belong to types which are exclusively or principally Palaeozoic
in their range. The old genera _Strophomena, Orthis_ (fig. 127,
c), _Athyris_ (fig. 127, e), _Rhynchonella_ (fig. 127, g),
and _Spirifera_ (fig. 127, h), are still well represented--the
latter, in particular, existing under numerous specific forms,
conspicuous by their abundance and sometimes by their size. Along
with these ancient groups, we have representatives--for the first
time in any plenty--of the great genus _Terebratula_ (fig. 127,
d), which underwent a great expansion during later periods,
and still exists at the present day. The most characteristic
Carboniferous Brachiopods, however, belong to the family of the
_Productidoe_, of which the principal genus is _Producta_ itself.
This family commenced its existence in the Upper Silurian with
the genus _Chonetes_, distinguished by its spinose hinge-margin.
This genus lived through the Devonian, and flourished in the
Carboniferous (fig. 127, f). The genus _Producta_ itself,
represented in the Devonian by the nearly allied _Productella_,
appeared first in the Carboniferous, at any rate, in force, and
survived into the Permian; but no member of this extensive family
has yet been shown to have over-lived the Palaeozoic period. The
_Productoe_ of the Carboniferous are not only exceedingly abundant,
but they have in many instances a most extensive geographical range,
and some species attain what may fairly be considered-gigantic
dimensions. The shell (fig. 127, a and b) is generally more
or less semicircular, with a straight hinge-margin, and having
its lateral angles produced into larger or smaller ears (hence
its generic name--"_cochlea producta_"). One valve (the ventral)
is usually strongly convex, whilst the other (the dorsal) is flat
or concave, the surface of both being adorned with radiating
ribs, and with hollow tubular spines, often of great length.
The valves are not locked together by teeth, and there is no
sign in the fully-grown shell of an opening in or between the
valves for the emission of a muscular stalk for the attachment
of the shell to foreign objects. It is probable, therefore, that
the _Productoe_, unlike the ordinary Lamp-shells, lived an
independent existence, their long spines apparently serving to
anchor them firmly in the mud or ooze of the sea-bottom; but Mr
Robert Etheridge, jun.; has recently shown that in one species
the spines were actually employed as organs of adhesion, whereby
the shell was permanently attached to some extraneous object,
such as the stem of a Crinoid. The two species here figured are
interesting for their extraordinarily extensive geographical
range--_Producta semireticulata_ (fig. 127, a) being found
in the Carboniferous rocks of Britain, the continent of Europe,
Central Asia, China, India, Australia, Spitzbergen, and North
and South America; whilst _P. Longispina_ (fig. 127, b) has
a distribution little if at all less wide.
[Illustration: Fig. 128.--_Pupa (Dendropupa) vetusta_, a
Carboniferous Land-snail from the Coal-measures of Nova Scotia.
a, The shell, of the natural size; b, The same, magnified;
c, Apex of the shell, enlarged; d, Portion of the surface,
enlarged. (After Dawson.)]
The higher _Mollusca_ are abundantly represented in the Carboniferous
rocks by Bivalves (_Lamellibranchs_), Univalves (_Gasteropoda_),
Winged-snails (_Pteropoda_), and _Cephalopods_. Amongst the Bivalves
we may note the great abundance of Scallops (_Aviculopecten_ and
other allied forms), together with numerous other types--some of
ancient origin, others represented here for the first time. Amongst
the Gasteropods, we find the characteristically Palaeozoic genera
_Macrocheilus_ and _Loxonema_, the almost exclusively Palaeozoic
_Euomphalus_, and the persistent, genus _Pleurotomaria_; whilst
the free-swimming Univalves (_Heteropoda_)are represented by
_Bellerophon_ and _Porcellia_, and the _Pteropoda_ by the old
genus _Conularia_. With regard to the Carboniferous Univalves,
it is also of interest to note here the first appearance of true
air-breathing or terrestrial Molluscs, as discovered by Dawson
and Bradley in the Coal-measures of Nova Scotia and Illinois. Some
of these (_Conulus priscus_) are true Land-snails, resembling the
existing _Zonites_; whilst others (_Pupa vetusta_, fig. 128) appear
to be generically inseparable from the "Chrysalis-shells" (_Pupa_)
of the present day. All the known forms--three in number--are of
small size, and appear to have been local in their distribution
or in their preservation. More important, however, than any of
the preceding, are the _Cephalopoda_, represented, as before,
exclusively by the chambered shells of the Tetrabranchiates.
The older and simpler type of these, with simple plain septa,
and mostly a central siphuncle, is represented by the straight
conical shells of the ancient genus Orthoceras, and the bow-shaped
shells of the equally ancient _Cyrtoceras_--some of the former
attaining a great size. The spirally-curved discoidal shells
of the persistent genus _Nautilus_ are also not unknown, and
some of these likewise exhibit very considerable dimensions.
Lastly, the more complex family of the _Ammonitidoe_, with lobed
or angulated septa, and a dorsally-placed siphuncle (situated on
the convex side of the curved shells), now for the first time
commences to acquire a considerable prominence. The principal
representative of this group is the genus _Goniatites_ (fig.
129), which commenced its existence in the Upper Silurian, is well
represented in the Devonian, and attains its maximum here. In this
genus, the shell is spirally curved, the septa are strongly lobed
or angulated, though not elaborately frilled as in the Ammonites,
and the siphuncle is dorsal. In addition to _Goniatites_, the
shells of true _Ammonites_, so characteristic of the Secondary
period, have been described by Dr Waagen as occurring in the
Carboniferous rocks of India.
[Illustration: Fig. 129.--_Goniatites (Aganides) Fossoe_.
Carboniferous Limestone.]
[Illustration: Fig. 130.--_Amblypterus macropterus_. Carboniferous.]
Coming finally to the _Vertebrata_, we have in the first place
to very briefly consider the Carboniferous _fishes_. These are
numerous; but, with the exception of the still dubious "Conodonts,"
belong wholly to the groups of the _Ganoids_ and the _Placoids_
(including under the former head remains which perhaps are truly
referable to the group of the _Dipnoi_ or Mud-fishes). Amongst the
_Ganoids_, the singular buckler-headed fishes of the Upper Silurian
and Devonian (_Cephalaspidoe_) have apparently disappeared; and
the principal types of the Carboniferous belong to the groups
respectively represented at the present day by the Gar pike
(_Lepidosteus_) of the North American lakes, and the _Polypterus_
of the rivers of Africa. Of the former, the genera _Paloeoniscus_
and _Amblypterus_ (fig. 130), with their small rhomboidal and
enamelled scales, and their strongly unsymmetrical tails, are
perhaps the most abundant. Of the latter, the most important are
species belonging to the genera _Megalichthys_ and _Rhizodus_,
comprising large fishes, with rhomboidal scales, unsymmetrical
("heterocercal") tails, and powerful conical teeth. These fishes
are sometimes said to be "sauroid," from their presenting some
Reptilian features in their organisation, and they must have been
the scourges of the Carboniferous seas. The remains of _Placoid_
fishes in the Carboniferous strata are very numerous, but consist
wholly of teeth and fin-spines, referable to forms more or less
closely allied to our existing Port Jackson Sharks, Dog-fishes,
and Rays. The teeth are of very various shapes and sizes,--some
with sharp, cutting edges (_Petalodus, Cladodus_, &c.); others in
the form of broad crushing plates, adapted, like the teeth of the
existing Port Jackson Shark (_Cestracion Philippi_), for breaking
down the hard shells of Molluscs and Crustaceans. Amongst the many
kinds of these latter, the teeth of _Psammodus_ and _Cochliodus_
(fig. 131) may be mentioned as specially characteristic. The
fin-spines are mostly similar to those so common in the Devonian
deposits, consisting of hollow defensive spines implanted in
front of the pectoral or other fins, usually slightly curved,
often superficially ribbed or sculptured, and not uncommonly
serrated or toothed. The genera _Ctenacanthus, Gyracanthus,
Homacanthus_, &c., have been founded for the reception of these
defensive weapons, some of which indicate fishes of great size
and predaceous habits.
[Illustration: Fig. 131.--Teeth of _Cochliodus contortus_.
Carboniferous Limestone, Britain.]
[Illustration: Fig. 132.--a, Upper surface of the skull of
_Anthracosaurus Russelli_, one-sixth of the natural size: b,
Part of one of the teeth cut across, and highly magnified to
show the characteristic labyrinthine structure; c, One of the
integumentary shields or scales, one-half of the natural size.
Coal-measures, Northumberland. (After Atthey.)]
In the Devonian rocks we meet with no other remains of
Vertebrated animals save fishes only; but the Carboniferous
deposits have yielded remains of the higher group
of the _Amphibians_. This class, comprising our existing
Frogs, Toads, and Newts, stands to some extent in a position midway
between the class of the fishes and that of the true
reptiles, being distinguished from the latter by the fact
that its members invariably possess gills in their early
condition, if not throughout life; whilst they are separated from
the former by always possessing true lungs when adult, and
by the fact that the limbs (when present at all) are never in
the form of fins. The Amphibians, therefore, are all
water-breathers when young, and have respiratory organs adapted
for an aquatic mode of life; whereas, when grown up, they
develop lungs, and with these the capacity for breathing air
directly. Some of them, like the Frogs and Newts, lose their
gills altogether on attaining the adult condition; but others,
such as the living _Proteus_ and _Menobranchus_, retain
their gills even after acquiring their lungs, and are thus fitted
indifferently for an aquatic or terrestrial existence. The name of
"Amphibia," though applied to the whole class, is thus not
precisely appropriate except to these last-mentioned forms
(Gr. _amphi_, both; _bios_, life). The Amphibians also
differ amongst themselves according as to whether they keep
permanently the long tail which they all possess when young (as
do the Newts and Salamanders), or lose this appendage when
grown up (as do the Frogs and Toads). Most of them have
naked skins, but a few living and many extinct forms have
hard structures in the shape of scales developed in the integument.
All of them have well-ossified skeletons, though some
fossil types are partially deficient in this respect; and all of
them which possess limbs at all have these appendages supported
by bones essentially similar to those found in the limbs
of the higher Vertebrates. All the Carboniferous Amphibians
belong to a group which has now wholly passed away--namely,
that of the _Labyrinthodonts_. In the marine strata which
form the base of the Carboniferous series these creatures have only
been recognised by their curious hand-shaped footprints, similar
in character to those which occur in the Triassic rocks, and which
will be subsequently spoken of under the name of _Cheirotherium_.
In the Coal-measures of Britain, the continent of Europe, and
North America, however, many bones of these animals have
been found, and we are now tolerably well acquainted with a
considerable number of forms. All of them seem to have
belonged to the division of Amphibians in which the long tail
of the young is permanently retained; and there is evidence
that some of them kept the gills also throughout life. The skull
is of the characteristic Amphibian type (fig. 132, a), with
two occipital condyles, and having its surface singularly pitted
and sculptured; and the vertebrae are hollowed out at both
ends. The lower surface of the body was defended by an armour
of singular integumentary shields or scales (fig. 132, c);
and an extremely characteristic feature (from which the entire
group derives its name) is, that the walls of the teeth are deeply
folded, so as to give rise to an extraordinary "labyrinthine"
pattern when they are cut across (fig. 132, b). Many of the
Carboniferous Labyrinthodonts are of no great size, some of
them very small, but others attain comparatively gigantic
dimensions, though all fall short in this respect of the huge
examples of this group which occur in the Trias. One of the
largest, and at the same time most characteristic, forms of the
Carboniferous series, is the genus _Anthracosaurus_, the
skull of which is here figured.
No remains of true Reptiles, Birds, or Quadrupeds have as yet
been certainly detected in the Carboniferous deposits in any part
of the world. It should, however, be mentioned, that Professor
Marsh, one of the highest authorities on the subject, has described
from the Coal-formation of Nova Scotia certain vertebrae which
he believes to have belonged to a marine reptile (_Eosaurus
Acadianus_), allied to the great _Ichthyosauri_ of the Lias. Up to
this time no confirmation of this determination has been obtained
by the discovery of other and more unquestionable remains, and
it therefore remains doubtful whether these bones of _Eosaurus_
may not really belong to large Labyrinthodonts.
LITERATURE.
The following list contains some of the more important of the
original sources of information to which the student of Carboniferous
rocks and fossils may refer:--
(1) 'Geology of Yorkshire,' vol. ii.; 'The Mountain Limestone
District.' John Phillips.
(2) 'Siluria.' Sir Roderick Murchison.
(3) 'Memoirs of the Geological Survey of Great Britain and Ireland.'
(4) 'Geological Report on Londonderry,' &c. Portlock.
(5) 'Acadian Geology.' Dawson.
(6) 'Geology of Iowa,' vol. i. James Hall.
(7) 'Reports of the Geological Survey of Illinois' (Geology and
Palaeontology). Meek, Worthen, &c.
(8) 'Reports of the Geological Survey of Ohio' (Geology and
Palaeontology). Newberry, Cope, Meek, Hall, &c.
(9) 'Description des Animaux fossiles qui se trouvent dans le
Terrain Carbonifere de la Belgique,' 1843; with subsequent
monographs on the genera _Productus_ and _Chonetes_,
on _Crinoids_, on _Corals_, &c. De Koninck.
(10) 'Synopsis of the Carboniferous Fossils of Ireland.' M'Coy.
(11) 'British Palaeozoic Fossils.' M'Coy.
(12) 'Figures of Characteristic British Fossils.' Baily.
(13) 'Catalogue of British Fossils.' Morris.
(14) 'Monograph of the Carboniferous Brachiopoda of Britain'
(Palaeontographical Society). Davidson.
(15) 'Monograph of the British Carboniferous Corals'
(Palaeontographical Society). Milne-Edwards and Haime.
(16) 'Monograph of the Carboniferous Bivalve Entomostraca of
Britain' (Palaeontographical Society). Rupert Jones, Kirkby, and
George S. Brady.
(17) 'Monograph of the Carboniferous Foraminifera of Britain'
(Palaeontographical Society). H. B. Brady.
(18) "On the Carboniferous Fossils of the West of Scotland"--'Trans.
Geol. Soc.,' of Glasgow, vol. iii., Supplement. Young and
Armstrong.
(19) 'Poissons Fossiles.' Agassiz.
(20) "Report on the Labyrinthodonts of the Coal-measures"--'British
Association Report,' 1873. L. C. Miall.
(21) 'Introduction to the Study of Palaeontological Botany.' John
Hutton Balfour.
(22) 'Traite de Paleontologie Vegetale.' Schimper.
(23) 'Fossil Flora.' Lindley and Hutton.
(24) 'Histoire des Vegetaux Fossiles.' Brongniart.
(25) 'On Calamites and Calamodendron' (Monographs of the
Palaeontographical Society). Binney.
(26) 'On the Structure of Fossil Plants found in the Carboniferous
Strata' (Palaeontographical Society). Binney.
Also numerous memoirs by Huxley, Davidson, Martin Duncan, Professor
Young, John Young, R. Etheridge, jun., Baily, Carruthers, Dawson,
Binney, Williamson, Hooker, Jukes, Geikie, Rupert Jones, Salter,
and many other British and foreign observers.
CHAPTER XIV.
THE PERMIAN PERIOD.
The Permian formation closes the long series of the Palaeozoic
deposits, and may in some respects be considered as a kind of
appendix to the Carboniferous system, to which it cannot be compared
in importance, either as regards the actual bulk of its sediments
or the interest and variety of its life-record. Consisting, as
it does, largely of red rocks--sandstones and marls--for the
most part singularly destitute of organic remains, the Permian
rocks have been regarded as a lacustrine or fluviatile deposit;
but the presence of well-developed limestones with indubitable
marine remains entirely negatives this view. It is, however,
not improbable that we are presented in the Permian formation,
as known to us at present, with a series of sediments laid down
in inland seas of great extent, due to the subsidence over large
areas of the vast land-surfaces of the Coal-measures. This view,
at any rate, would explain some of the more puzzling physical
characters of the formation, and would not be definitely negatived
by any of its fossils.
A large portion of the Permian series, as already remarked, consists
of sandstones and marls, deeply reddened by peroxide of iron, and
often accompanied by beds of gypsum or deposits of salt. In strata
of this nature few or no fossils are found; but their shallow-water
origin is sufficiently proved by the presence of the footprints
of terrestrial animals, accompanied in some cases by well-defined
"ripple-marks." Along with these are occasionally found massive
breccias, holding larger or smaller blocks derived from the older
formations; and these have been supposed to represent an old
"boulder-clay," and thus to indicate the prevalence of an arctic
climate. Beds of this nature must also have been deposited in
shallow water. In all regions, however, where the Permian formation
is well developed, one of its most characteristic members is a
Magnesian limestone, often highly and fantastically concretionary,
but containing numerous remains of genuine marine animals, and
clearly indicating that it was deposited beneath a moderate depth
of salt water.
It is not necessary to consider here whether this formation can
be retained as a distinct division of the geological series. The
name of _Permian_ was given to it by Sir Roderick Murchison,
from the province of Perm in Russia, where rocks of this age are
extensively developed. Formerly these rocks were grouped with
the succeeding formation of the Trias under the common name of
"New Red Sandstone." This name was given them because they contain
a good deal of red sandstone, and because they are superior to the
Carboniferous rocks, while the Old Red Sandstone is inferior.
Nowadays, however, the term "New Red Sandstone" is rarely employed,
unless it be for red sandstones and associated rocks, which are
seen to overlie the Coal-measures, but which contain no fossils by
which their exact age may be made out. Under these circumstances,
it is sometimes convenient to employ the term "New Red Sandstone."
The New Red, however, of the older geologists, is now broken up
into the two formations of the Permian and Triassic rocks--the
former being usually considered as the top of the Palaeozoic series,
and the latter constituting the base of the Mesozoic.
In many instances, the Permian rocks are seen to repose unconformably
upon the underlying Carboniferous, from which they can in addition
be readily separated by their lithological characters. In other
instances, however, the Coal-measures terminate upwards in red
rocks, not distinguishable by their mineral characters from the
Permian; and in other cases no physical discordance between the
Carboniferous and Permian strata can be detected. As a general
rule, also, the Permian rocks appear to pass upwards conformably
into the Trias. The division, therefore, between the Permian
and Triassic rocks, and consequently between the Palaeozoic and
Mesozoic series, is not founded upon any conspicuous or universal
physical break, but upon the difference in life which is observed
in comparing the marine animals of the Carboniferous and Permian
with those of the Trias. It is to be observed, however, that
this difference can be solely due to the fact that the Magnesian
Limestone of the Permian series presents us with only a small,
and not a typical, portion of the marine deposits which must have
been accumulated in some area at present unknown to us during the
period which elapsed between the formation of the great marine
limestones of the Lower Carboniferous and the open-sea and likewise
calcareous sediments of the Middle Trias.
The Permian rocks exhibit their most typical features in Russia
and Germany, though they are very well developed in parts of
Britain, and they occur in North America. When well developed,
they exhibit three main divisions: a lower set of sandstones,
a middle group, generally calcareous, and an upper series of
sandstones, constituting respectively the Lower, Middle, and Upper
Permians.
In Russia, Germany, and Britain, the Permian rocks consist of
the following members:--
1. The _Lower Permians_, consisting mainly of a great series
of sandstones, of different colours, but usually red. The base
of this series is often constituted by massive breccias with
included fragments of the older rocks, upon which they may happen
to repose; and similar breccias sometimes occur in the upper
portion of the series as well. The thickness of this group varies
a good deal, but may amount to 3000 or 4000 feet.
2. The _Middle Permians_, consisting, in their typical development,
of laminated marls, or "marl-slate," surmounted by beds of magnesian
limestone (the "Zechstein" of the German geologists). Sometimes
the limestones are degenerate or wholly deficient, and the series
may consist of sandy shales and gypsiferous clays. The magnesian
limestone, however, of the Middle Permians is, as a rule, so well
marked a feature that it was long spoken of as _the_ Magnesian
Limestone.
3. The _Upper Permians_, consisting of a series of sandstones
and shales, or of red or mottled marls, often gypsiferous, and
sometimes including beds of limestone.
In North America, the Permian rocks appear to be confined to the
region west of the Mississippi, being especially well developed
in Kansas. Their exact limits have not as yet been made out,
and their total thickness is not more than a few hundred feet.
They consist of sandstones, conglomerates, limestones, marls,
and beds of gypsum.
The following diagrammatic section shows the general sequence of
the Permian deposits in the north of England, where the series
is extensively developed (fig. 133):--
[Illustration: Fig. 133. GENERALISED SECTION OF THE PERMIAN ROCKS
IN THE NORTH OF ENGLAND.]
The record of the _life_ of the Permian period is but a scanty
one, owing doubtless to the special peculiarities of such of the
deposits of this age with which we are as yet acquainted. Red rocks
are, as a general rule, more or less completely unfossiliferous, and
sediments of this nature are highly characteristic of the Permian.
Similarly, magnesian limestones are rarely as highly charged with
organic remains as is the case with normal calcareous deposits,
especially when they have been subjected to concretionary action,
as is observable to such a marked extent in the Permian limestones.
Nevertheless, much interest is attached to the organic remains,
as marking a kind of transition-period between the Palaeozoic
and Mesozoic epochs.
[Illustration: Fig. 134.--_Walchia piniformis_, from the Permian
of Saxony, a, Branch; b, Twig, (After Gutbier.)]
The _plants_ of the Permian period, as a whole, have a distinctly
Palaeozoic aspect, and are far more nearly allied to those of the
Coal-measures than they are to those of the earlier Secondary
rocks; though the Permian _species_ are mostly distinct from
the Carboniferous, and there are some new genera. Thus, we find
species of _Lepidodendron, Calamites, Equisetites, Asterophyllites,
Annularia_, and other highly characteristic Carboniferous genera.
On the other hand, the _Sigillariods_ of the Coal seem to have
finally disappeared at the close of the Carboniferous period. Ferns
are abundant in the Permian rocks, and belong for the most part to
the well-known Carboniferous genera _Alethopteris, Neuropteris,
Sphenopteris_, and _Pecopteris_. There are also Tree-ferns referable
to the ancient genus _Psaronius_. The _Conifers_ of the Permian
period are numerous, and belong in part to Carboniferous genera.
A characteristic genus, however, is _Walchia_ (fig. 134),
distinguished by its lax short leaves. This genus, though not
exclusively Permian, is mainly so, the best-known species being
the _W. Piniformis_. Here, also, we meet with Conifers which
produce true cones, and which differ, therefore, in an important
degree from the Taxoid Conifers of the Coal-measures. Besides
_Walchia_, a characteristic form of these is the _Ullmania
selaginoides_, which occurs in the Magnesian Limestone of Durham,
the Middle Permian of Westmorland, and the "Kupfer-schiefer" of
Germany. The group of the _Cycads_, which we shall subsequently
find to be so characteristic of the vegetation of the Secondary
period, is, on the other hand, only doubtfully represented in
the Permian deposits by the singular genus _Noeggerathia_.
The _Protozoans_ of the Permian rocks are few in number, and
for the most part imperfectly known. A few _Foraminifera_ have
been obtained from the Magnesian Limestone of England, and the
same formation has yielded some ill-understood Sponges. It does
not seem, however, altogether impossible that some of the singular
"concretions" of this formation may ultimately prove to have an
organic structure, though others would appear to be clearly of
purely inorganic origin. From the Permian of Saxony, Professor
Geinitz has described two species of _Spongillopsis_, which he
believes to be most nearly allied to the existing fresh-water
Sponges (_Spongilla_). This observation has an interest as bearing
upon the mode of deposition and origin of the Permian sediments.
The _Coelenterates_ are represented in the Permian by but a few
Corals. These belong partly to the _Tabulate_ and partly to the
_Rugose_ division; but the latter great group, so abundantly
represented in Silurian, Devonian, and Carboniferous seas, is
now extraordinarily reduced in numbers, the British strata of
this age yielding only species of the single genus _Polycoelia_.
So far, therefore, as at present known, all the characteristic
genera of the Rugose Corals of the Carboniferous had become extinct
before the deposition of the limestones of the Middle Permian.
The _Echinoderms_ are represented by a few _Crinoids_, and by a
Sea-urchin belonging to the genus _Eocidaris_. The latter genus
is nearly allied to the _Archoeocidaris_ of the Carboniferous, so
that this Permian form belongs to a characteristically Palaeozoic
type.
A few _Annelides_ (_Spirorbis, Vermilia_, &c.) have been described,
but are of no special importance. Amongst the _Crustaceans_,
however, we have to note the total absence of the great Palaeozoic
group of the _Trilobites_; whilst the little _Ostracoda_ and
_Phyllopods_ still continue to be represented. We have also to
note the first appearance here of the "Short-tailed" Decapods or
Crabs (_Brachyura_), the highest of all the groups of _Crustacea_,
in the person of _Hemitrochiscus paradoxus_, an extremely minute
Crab from the Permian of Germany.
[Illustration: Fig. 135.--Brachiopods of the Permian formation.
a, _Producta horrida_; b, _Lingula Credneri_; c, _Terebratula
elongata_; d and e, _Camarophoria globulina_. (After King.)]
Amongst the _Mollusca_, the remains of _Polyzoa_ may fairly be
said to be amongst the most abundant of all the fossils of the
Permian formation, The principal forms of these are the fronds
of the Lace-corals (_Fenestella, Retepora_, and _Synocladia_),
which are very abundant in the Magnesian Limestone of the north
of England, and belong to various highly characteristic species
(such as _Fenestella retiformis, Retepora Ehrenbergi_, and
_Synocladia virgulacea_). The _Brachiopoda_ are also represented
in moderate numbers in the Permian. Along with species of the
persistent genera _Discina, Crania_, and _Lingula_, we still
meet with representatives of the old groups _Spirifera, Athyris_,
and _Streptorhynchus_; and the Carboniferous _Productoe_ yet
survive under well-marked and characteristic types, though in
much-diminished numbers. The species of Brachiopods here figured
(fig. 135) are characteristic of the Magnesian Limestone in Britain
and of the corresponding strata on the Continent. Upon the whole,
the most characteristic Permian _Brachiopods_ belong to the genera
_Producta, Strophalosia_, and _Camarophoria_.
The _Bivalves_ (_Lamellibranchiata_) have a tolerably varied
development in the Permian rocks; but nearly all the old types,
except some of those which occur in the Carboniferous, have now
disappeared. The principal Permian Bivalves belong to the groups
of the Pearl Oysters (_Aviculidoe_) and the _Trigoniadoe_,
represented by genera such as _Bakewellia_ and _Schizodus_; the
true Mussels (_Mytilidoe_), represented by species which have
been referred to _Mytilus_ itself; and the Arks (_Arcadoe_),
represented by species of the genera _Arca_ (fig. 136) and
_Byssoarca_. The first and last of these three families have a
very ancient origin; but the family of the _Trigoniadoe_, though
feebly represented at the present day, is one which attained
its maximum development in the Mesozoic period.
[Illustration: Fig. 136.--_Arca antiqua_. Permian.]
The _Univalves_ (_Gasteropoda_) are rare, and do not demand special
notice. It may be observed, however, that the Palaeozoic genera
_Euomphalus, Murchisonia, Loxonema_, and _Macrocheilus_ are still
in existence, together with the persistent genus _Pleurotomaria_.
_Pteropods_ of the old genera _Theca_ and _Conularia_ have been
discovered; but the first of these characteristically Palaeozoic
types finally dies out here, and the second only survives but a
short time longer. Lastly, a few _Cephalopods_ have been found,
still wholly referable to the Tetrabranchiate group, and belonging
to the old genera _Orthoceras_ and _Cyrtoceras_ and the long-lived
_Nautilus_.
[Illustration: Fig. 137.--_Platysomus gibbosus_, a "heterocercal"
Ganoid, from the Middle Permian of Russia.]
Amongst _Vertebrates_, we meet in the Permian period not only
with the remains of Fishes and Amphibians, but also, for the
first time, with true Reptiles. The _Fishes_ are mainly _Ganoids_,
though there are also remains of a few Cestraciont Sharks. Not
only are the _Ganoids_ still the predominant group of Fishes, but
all the known forms possess the unsymmetrical ("heterocercal")
tail which is so characteristic of the Palaeozoic Ganoids. Most
of the remains of the Permian Fishes have been obtained from the
"Marl-slate" of Durham and the corresponding "Kupfer-schiefer" of
Germany, on the horizon of the Middle Permian; and the principal
genera of the Ganoids are _Paloeoniscus_ and _Platysomus_ (fig.
137).
The _Amphibians_ of the Permian period belong principally to the
order of the _Labyrinthodonts_, which commenced to be represented
in the Carboniferous, and has a large development in the Trias.
Under the name, however, of _Paloeosiren Beinerti_, Professor
Geinitz has described an Amphibian from the Lower Permian of
Germany, which he believes to be most nearly allied to the existing
"Mud-eel" (_Siren lacertina_) of North America, and therefore
to be related to the Newts and Salamanders (_Urodela_).
[Illustration: Fig. 138.--_Protorosaurus Speneri_, Middle Permian,
Thuringia, reduced in size. (After Von Meyer.) [Copied from Dana.]]
Finally, we meet in the Permian deposits with the first undoubted
remains of true _Reptiles_. These are distinguished, as a class,
from the _Amphibians_, by the fact that they are air-breathers
throughout the whole of their life, and therefore are at no time
provided with gills; whilst they are exempt from that metamorphosis
which all the _Amphibia_ undergo in early life, consequent upon
their transition from an aquatic to a more or less purely aerial
mode of respiration. Their skeleton is well ossified; they usually
have horny or bony plates, singly or in combination, developed
in the skin; and their limbs (when present) are never either
in the form of _fins_ or _wings_, though sometimes capable of
acting in either of these capacities, and liable to great
modifications of form and structure. Though there can be no doubt
whatever as to the occurrence of genuine Reptiles in deposits of
unquestionable Permian age, there is still uncertainty as to the
precise number of types which may have existed at this period.
This uncertainty arises partly from the difficulty of deciding
in all cases, whether a given bone be truely Labyrinthodont or
Reptilian, but more especially from the confusion which exists at
present between the Permian and the overlying Triassic deposits.
Thus there are various deposits in different regions which have
yielded the remains of Reptiles, and which cannot in the meanwhile
be definitely referred either to the Permian series or to the
Trias by clear stratigraphical or palaeontological evidence. All
that can be done in such cases is to be guided by the characters
of the Reptiles themselves, and to judge by their affinities to
remains from known Triassic or Permian rocks to which of these
formations the beds containing them should be referred; but it
is obvious that this method of procedure is seriously liable
to lead to error. In accordance, however, with this, the only
available mode of determination in some cases, the remains of
_Thecodontosaurus_ and _Palaeosaurus_ discovered in the dolomitic
conglomerates near Bristol will be considered as Triassic, thus
leaving _Protorosaurus_[20] as the principal and most important
representative of the Permian Reptiles.[21] The type-species of
the genus _Protorusaurus_ is the _P. Speneri_(fig. 138) of the
"Kupfer-schiefer" of Thuringia, but other allied species have
been detected in the Middle Permian of Germany and the north
of England. This Reptile attained a length of from three to four
feet; and it has been generally referred to the group of the
Lizards (_Lacertilia_), to which it is most nearly allied in
its general structure, at the same time that it differs from
all existing members of this group in the fact that its numerous
conical and pointed teeth were implanted in distinct sockets in
the jaws--this being a Crocodilian character. In other respects,
however, _Protorosaurus_ approximates closely to the living Monitors
(_Varanidoe_); and the fact that the bodies of the vertebrae are
slightly cupped or hollowed out at the ends would lead to the
belief that the animal was aquatic in its habits. At the same
time, the structure of the hind-limbs and their bony supports
proves clearly that it must have also possessed the power of
progression upon the land. Various other Reptilian bones have
been described from the Permian formation, of which some are
probably really referable to Labyrinthodonts, whilst others are
regarded by Professor Owen as referable to the order of the
"Theriodonts," in which the teeth are implanted in sockets, and
resemble those of carnivorous quadrupeds in consisting of three
groups in each jaw (namely, incisors, canines, and molars). Lastly,
in red sandstones of Permian age in Dumfriesshire have been
discovered the tracks of what would appear to have been _Chelonians_
(Tortoises and Turtles); but it would not be safe to accept this
conclusion as certain upon the evidence of footprints alone. The
_Chelichnus Duncani_, however, described by Sir William Jardine
in his magnificent work on the 'Ichnology of Annandale,' bears
a great resemblance to the track of a Turtle.
[Footnote 20: Though commonly spelt as above, it is probable
that the name of this Lizard was really intended to have been
_Proterosaurus_--from the Greek _proteros_, first; and _saura_,
lizard: and this spelling is followed by many writers.]
[Footnote 21: In an extremely able paper upon the subject (Quart.
Journ. Geol. Soc., vol. xxvi.), Mr Etheridge has shown that there
are good physical grounds for regarding the dolomitie conglomerate
of Bristol as of Triassic age, and as probably corresponding in
time with the Muschelkalk of the Continent.]
No remains of Birds or Quadrupeds have hitherto been detected
in deposits of Permian age.
LITERATURE.
The following works may be consulted by the student with regard
to the Permian formation and its fossils:--
(1) "On the Geological Relations and Internal Structure of the
Magnesian Limestone and the Lower Portions of the New Red
Sandstone Series, &c."--'Trans. Geol. Soc.,' ser. 2, vol. iii.
Sedgwick.
(2) 'The Geology of Russia in Europe.' Murchison, De Verneuil, and
Von Keyserling.
(3) 'Siluria,' Murchison.
(4) 'Permische System in Sachsen.' Geinitz and Gutbier.
(5) 'Die Versteinerungen des Deutschen Zechsteingebirges,' Geinitz.
(6) 'Die Animalischen Ueberreste der Dyas.' Geinitz.
(7) 'Monograph of the Permian Fossils of England' (Palaeontographical
Society). King.
(8) 'Monograph of the Permian Brachiopoda of Britain'
(Palaeontographical Society). Davidson.
(9) "On the Permian Rocks of the North-West of England and their
Extension into Scotland"--'Quart. Journ. Geol. Soc.,' vol. xx.
Murchison and Harkness.
(10) 'Catalogue of the Fossils of the Permian System of the Counties
of Northumberland and Durham.' Howse.
(11) 'Petrefacta Germaniae.' Goldfuss.
(12) 'Beitraege zur Petrefaktenkunde.' Munster.
(13) 'Ein Beitrag zur Palaeontologie des Deutschen Zechsteingebirges.'
Von Schauroth.
(14) 'Saurier aus dem Kupfer-schiefer der Zechstein-formation.' Von
Meyer.
(15) 'Manual of Palaeontology.' Owen.
(16) 'Recherches sur les Poissons Fossiles.' Agassiz.
(17) 'Ichnology of Annandale.' Sir William Jardine.
(18) 'Die Fossile Flora der Permischen Formation.' Goeppert.
(19) 'Genera et Species Plantarum Fossilium.' Unger.
(20) "On the Red Rocks of England of older Date than the Trias"
--'Quart. Journ. Geol. Soc.,' vol. xxvii. Ramsay.
CHAPTER XV.
THE TRIASSIC PERIOD.
We come now to the consideration of the great _Mesozoic_, or
Secondary series of formations, consisting, in ascending order,
of the Triassic, Jurassic, and Cretaceous systems. The Triassic
group forms the base of the Mesozoic series, and corresponds
with the higher portion of the New Red Sandstone of the older
geologists. Like the Permian rocks, and as implied by its name,
the _Trias_ admits of a subdivision into three groups--a Lower,
Middle, and Upper Trias. Of these sub-divisions the middle one
is wanting in Britain; and all have received German names, being
more largely and typically developed in Germany than in any other
country. Thus, the Lower Trias is known as the _Bunter Sandstein_;
the Middle Trias is called the _Muschelkalk_; and the Upper Trias
is known as the _Keuper_.
I. The lowest division of the Trias is known as the _Bunter
Sandstein_ (the _Gres bigarre_ of the French), from the generally
variegated colours of the beds which compose it (German, _bunt_,
variegated). The Bunter Sandstein of the continent of Europe
consists of red and white sandstones, with red clays, and thin
limestones, the whole attaining a thickness of about 1500 feet.
The term "marl" is very generally employed to designate the clays
of the Lower and Upper Trias; but the term is inappropriate, as
they may contain no lime, and are therefore not always genuine
marls. In Britain the Bunter Sandstein consists of red and mottled
sandstones, with unconsolidated conglomerates, or "pebble-beds,"
the whole having a thickness of 1000 to 2000 feet. The Bunter
Sandstein, as a rule, is very barren of fossils.
II. The Middle Trias is not developed in Britain, but it is largely
developed in Germany, where it constitutes what is known as the
_Muschelkalk_ (Germ. _Muschel_, mussel; _kalk_, limestone), from
the abundance of fossil shells which it contains. The Muschelkalk
(the _Calcaire coquillier_ of the French) consists of compact
grey or yellowish limestones, sometimes dolomitic, and including
occasional beds of gypsum and rock-salt.
III. The Upper Trias, or _Keuper_ (the _Marnes irisees_ of the
French), as it is generally called, occurs in England; but is
not so well developed as it is in Germany. In Britain, the Keuper
is 1000 feet or more in thickness, and consists of white and
brown sandstones, with red marls, the whole topped by red clays
with rock-salt and gypsum.
The Keuper in Britain is extremely unfossiliferous; but it passes
upwards with perfect conformity into a very remarkable group of
beds, at one time classed with the Lias, and now known under the
names of the Penarth beds (from Penarth, in Glamorganshire), the
Rhaetic beds (from the Rhaetic Alps), or the _Avicula contorta_ beds
(from the occurrence in them of great numbers of this peculiar
Bivalve). These singular beds have been variously regarded as the
highest beds of the Trias, or the lowest beds of the Lias, or as
an intermediate group. The phenomena observed on the Continent,
however, render it best to consider them as Triassic, as they
certainly agree with the so-called Upper St Cassian or Koessen
beds which form the top of the Trias in the Austrian Alps.
The Penarth beds occur in Glamorganshire, Gloucestershire,
Warwickshire, Staffordshire, and the north of Ireland; and they
generally consist of a small thickness of grey marls, white
limestones, and black shales, surmounted conformably by the lowest
beds of the Lias. The most characteristic fossils which they contain
are the three Bivalves _Cardium Rhoeticum, Avicula contorta_, and
_Pecten Valoniensis_; but they have yielded many other fossils,
amongst which the most important are the remains of Fishes and
small Mammals (_Microlestes_).
In the Austrian Alps the Trias terminates upwards in an extraordinary
series of fossiliferous beds, replete with marine fossils. Sir
Charles Lyell gives the following table of these remarkable
deposits:--
_Strata below the Lias in the Austrian Alps, in descending order._
/ Grey and black limestone, with calcareous
| marls having a thickness of about 50
| feet. Among the fossils, Brachiopoda
1. Koessen beds. | very numerous; some few species common
(Synonyms, Upper | to the genuine Lias; many peculiar.
St Cassian beds of < _Avicula contorta, Pecten Valoniensis_,
Escher and Merian.) | _Cardium Rhoeticum, Avicula_
| _inoequivalvis, Spirifer Muensteri_,
| Dav. Strata containing the above fossils
| alternate with the Dachstein beds, lying
\ next below.
/ White or greyish limestone, often in beds
| three or four feet thick. Total thickness
| of the formation above 2000 feet. Upper
| part fossiliferous, with some strata
2. Dachstein beds. < composed of corals (_Lithodendron_.)
| Lower portion without fossils. Among the
| characteristic shells are _Hemicardium_
| _Wulfeni, Megalodon triqueler_, and
\ other large bivalves.
/ Red, pink, or white marbles, from 800 to
| 1000 feet in thickness, containing more
| than 800 species of marine fossils, for
3. Hallstadt beds | the most part mollusca. Many species of
(or St Cassian). < _Orthoceras_. True _Ammonites_,
| besides _Ceratites_ and
| _Goniatites, Belemnites_ (rare),
| _Porcellia, Pleurotomania, Trochus_,
\ _Monotis salinaria_, &c.
/ A. Black and grey \ Among the fossils
4. A. Guttenstein beds. | limestone 150 feet | are _Ceratites_
B. Werfen beds, base | thick, alternating | _cassianus_,
of Upper Trias? | with the underlying | _Myacites_
Lower Trias of < Werfen beds. > _fassaensis_,
some geologists. | B. Red and green | _Naticella_
| shale and sandstone, | _costata_, &c.
\ with salt and gypsum./
In the United States, rocks of Triassic age occur in several
areas between the Appalachians and the Atlantic seaboard; but
they show no such triple division as in Germany, and their exact
place in the system is uncertain. The rocks of these areas consist
of red sandstones, sometimes shaly or conglomeratic, occasionally
with beds of impure limestone. Other more extensive areas where
Triassic rocks appear at the surface, are found west of the
Mississippi, on the slopes of the Rocky Mountains, where the
beds consist of sandstones and gypsiferous marls. The American
Trias is chiefly remarkable for having yielded the remains of a
small Marsupial (_Dromatherium_), and numerous footprints, which
have generally been referred to Birds (_Brontozoum_), along with
the tracks of undoubted Reptiles (_Otozoum, Anisopus_, &c.)
The subjoined section (fig. 139) expresses, in a diagrammatic
manner, the general sequence of the Triassic rocks when fully
developed, as, for example, in the Bavarian Alps:--
[Illustration: Fig. 139. GENERALIZED SECTION OF THE TRIASSIC ROCKS
OF CENTRAL EUROPE.]
With regard to the _life_ of the Triassic period, we have to
notice a difference as concerns the different members of the group
similar to that which has been already mentioned in connection
with the Permian formation. The arenaceous deposits of the series,
namely, resemble those of the Permian, not only in being commonly
red or variegated in their colour, but also in their conspicuous
paucity of organic remains. They for the most part are either
wholly unfossiliferous, or they contain the remains of plants or
the bones of reptiles, such as may easily have been drifted from
some neighbouring shore. The few fossils which may be considered
as properly belonging to these deposits are chiefly Crustaceans
(_Estheria_) or Fishes, which may well have lived in the waters
of estuaries or vast inland seas. We may therefore conclude,
with considerable probability, that the barren sandy and marly
accumulations of the Bunter Sandstein and Lower Keuper were not
laid down in an open sea, but are probably brackish-water deposits,
formed in estuaries or land-locked bodies of salt water. This at
any rate would appear to be the case as regards these members
of the series as developed in Britain and in their typical areas
on the continent of Europe; and the origin of most of the North
American Trias would appear to be much the same. Whether this view
be correct or not, it is certain that the beds in question were laid
down in _shallow_ water, and in the immediate vicinity of _land_,
as shown by the numerous drifted plants which they contain and
the common occurrence in them of the footprints of air-breathing
animals (Birds, Reptiles, and Amphibians). On the other hand, the
middle and highest members of the Trias are largely calcareous,
and are replete with the remains of undoubted marine animals. There
cannot, therefore, be the smallest doubt but that the Muschelkalk
and the Rhaetic or Koessen beds were slowly accumulated in an open
sea, of at least a moderate depth; and they have preserved for
us a very considerable selection from the marine fauna of the
Triassic period.
[Illustration: Fig. 140.--_Zamia spiralis_, a living Cycad.
Australia.]
The _plants_ of the Trias are, on the whole, as distinctively
Mesozoic in their aspect as those of the Permian are Palaeozoic.
In spite, therefore, of the great difficulty which is experienced
in effecting a satisfactory stratigraphical separation between
the Permian and the Trias, we have in this fact a proof that the
two formations were divided by an interval of time sufficient
to allow of enormous changes in the terrestrial vegetation of the
world. The _Lepidodendroids, Asterophyllites_, and _Annularioe_,
of the Coal and Permian formations, have now apparently wholly
disappeared: and the Triassic flora consists mainly of Ferns,
Cycads, and Conifers, of which only the two last need special
notice. The _Cycads_ (fig. 140) are true exogenous plants, which
in general form and habit of growth present considerable resemblance
to young Palms, but which in reality are most nearly related to
the Pines and Firs (_Coniferoe_). The trunk is unbranched, often
much shortened, and bears a crown of feathery pinnate fronds.
The leaves are usually "circinate"--they unroll in expanding,
like the fronds of ferns. The seeds are not protected by a
seed-vessel, but are borne upon the edge of altered leaves, or
are carried on the scales of a cone. All the living species of
Cycads are natives of warm countries, such as South America, the
West Indies, Japan, Australia, Southern Asia, and South Africa.
The remains of Cycads, as we have seen, are not known to occur
in the Coal formation, or only to a very limited extent towards
its close; nor are they known with certainty as occurring in
Permian deposits. In the Triassic period, however, the remains
of Cycads belonging to such genera as _Pterophyllum_ (fig. 141,
b), _Zamites_, and _Podozamites_ (fig. 141, c), are sufficiently
abundant to constitute quite a marked feature in the vegetation;
and they continue to be abundantly represented throughout the
whole Mesozoic series. The name "Age of Cycads," as applied to
the Secondary epoch, is therefore, from a botanical point of
view, an extremely appropriate one. The _Conifers_ of the Trias
are not uncommon, the principal form being _Veltzia_ (fig. 141,
a), which possesses some peculiar characters, but would appear
to be most nearly related to the recent Cypresses.
[Illustration: Fig. 141.--Triassic Conifers and Cycads. a, _Voltzia_
(_Schizoneura_) _heterophylla_, portion of a branch, Europe and
America; b, Part of the frond of _Pterophyllum Joegeri_, Europe;
c, Part of the frond of _Podozamites lanceolatus_, America.]
As regards the _Invertebrate animals_ of the Trias, our knowledge
is still principally derived from the calcareous beds which
constitute the centre of the system (the Muschelkalk) on the
continent of Europe, and from the St Cassain and Rhaetic beds
still higher in the series; whilst some of the Triassic strata
of California and Nevada have likewise yielded numerous remains
of marine Invertebrates. The _Protozoans_ are represented by
_Foraminifera_ and _Sponges_, and the _Coelenterates_ by a small
number of _Corals_; but these require no special notice. It may be
mentioned, however, that the great Palaeozoic group of the _Rugose_
corals has no known representative here, its place being taken
by corals of Secondary type (such as _Montlivaltia, Synastoea_,
&c.)
The _Echinoderms_ are represented principally by _Crinoids_,
the remains of which are extremely abundant in some of the
limestones. The best-known species is the famous "Lily-Encrinite"
(_Encrinus liliiformis_, fig. 142), which is characteristic of the
Muschelkalk. In this beautiful species, the flower-like head is
supported upon a rounded stem, the joints of which are elaborately
articulated with one another; and the fringed arms are composed
each of a double series of alternating calcareous pieces. The
Palaeozoic Urchins, with their supernumerary rows of plates, the
Cystideans, and the Pentremites have finally disappeared; but
both Star-fishes and Brittle-stars continue to be represented.
One of the latter--namely, the _Aspidura loricata_ of Goldfuss
(fig. 143)--is highly characteristic of the Muschelkalk.
[Illustration: Fig. 142.--Head and upper part of the column of
_Encrinus liliiformis_. The lower figure shows the articulating
surface of one of the joints of the column. Muschelkalk, Germany.]
[Illustration: Fig. 143.--_Aspidura loricata_, a Triassic Ophiuroid.
Muschelkalk, Germany.]
The remains of _Articulate Animals_ are not very abundant in the
Trias, if we except the bivalved cases of the little Water-fleas
(_Ostracoda_), which are occasionally very plentiful. There are
also many species of the horny, concentrically-striated valves
of the _Estherioe_ (see fig. 122, b), which might easily be
taken for small Bivalve Molluscs. The "Long-tailed" Decapods
of the type of the Lobster, are not without examples but they
become much more numerous in the succeeding Jurassic period.
Remains of insects have also been discovered.
Amongst the _Mollusca_ we have to note the disappearance, amongst
the lower groups, of many characteristic Palaeozoic types. Amongst
the _Polyzoans_, the characteristic "Lace-corals," _Fenestella,
Retepora_,[22] _Synocladia, Polypora_, &c., have become apparently
extinct. The same is true of many of the ancient types of
_Brachiopods_, and conspicuously so of the great family of the
_Productidoe_, which played such an important part in the seas
of the Carboniferous and Permian periods.
[Footnote 22: The genus _Retefora_ is really a recent one,
represented by living forms; and the so-called _Reteporoe_ of the
Palaeozoic rocks should properly receive another name (_Phyllopora_),
as being of a different nature. The name _Retepora_ has been here
retained for these old forms simply in accordance with general
usage.]
[Illustraton: Fig. 144. Triassic Lamellibranchs. a, _Daonella_
(_Halobia_) _Lommelli_; b, _Pecten Valoniensis_; c, _Myophoria
lineata_; d. _Cardium Rhoeticum_; e. _Avicula contorta_; f. _Avicula
socialis_.]
_Bivalves_ (_Lamellibranchiata_) and _Univalves_ (_Gasteropoda_)
are well represented in the marine beds of the Trias, and some of the
former are particularly characteristic either of the formation as a
whole or of minor subdivisions of it. A few of these characteristic
species are figured in the accompanying illustration (fig. 144).
Bivalve shells of the genera _Daonella_ (fig. 144, a) and _Halobia_
(_Monotis_) are very abundant, and are found in the Triassic
strata of almost all regions. These groups belong to the family
of the Pearl-oysters (_Aviculidoe_), and are singular from the
striking resemblance borne by some of their included forms to
the _Strophomenoe amongst the Lamp-shells, though, of course, no
real relation exists between the two. The little Pearl-oyster,
_Avicula socialis_ (fig. 144, f), is found throughout the greater
part of the Triassic series, and is especially abundant in the
Muschelkalk. The genus _Myophoria_ (fig. 144, c), belonging
to the _Trigoniadoe_, and related therefore to the Permian
_Schizodus_, is characteristically Triassic, many species of the
genus being known in deposits of this age. Lastly, the so-called
"Rhaetic" or "Koessen" beds are characterised by the occurrence
in them of the Scallop, _Pecten Valoniensis_ (fig. 144, b);
the small Cockle, _Cardium Rhoeticum_ (fig. 144, d); and the
curiously-twisted Pearl-oyster, _Avicula contorta_ (fig. 144,
e)--this last Bivalve being so abundant that the strata in
question are often spoken of as the "Avicula contorta beds."
[Illustration: Fig. 145.--_Ceratites nodosus_, viewed from the
side and from behind. Muschelkalk.]
Passing over the groups of the _Heteropods_ and _Pteropods_, we
have to notice the _Cephalopoda_, which are represented in the
Trias not only by the chambered shells of _Tetrabranchiates_, but
also, for the first time, by the internal skeletons of _Dibranchiate_
forms. The Trias, therefore, marks the first recognised appearance
of true Cuttle-fishes. All the known examples of these belong
to the great Mesozoic group of the _Belemnitidoe_; and as this
family is much more largely developed in the succeeding Jurassic
period, the consideration of its characters will be deferred till
that formation is treated of. Amongst the chambered _Cephalopods_
we find quite a number of the Palaeozoic _Orthoceratites_, some of
them of considerable size, along with the ancient _Cyrtoceras_
and _Goniatites_; and these old types, singularly enough, occur
in the higher portion of the Trias (St Cassian beds), but have,
for some unexplained reason, not yet been recognised in the lower
and equally fossiliferous formation of the Muschelkalk. Along
with these we meet for the first time with true _Ammonites_,
which fill such an extensive place in the Jurassic seas, and
which will be spoken of hereafter. The form, however, which is
most characteristic of the Trias is _Ceratites_ (fig. 145). In
this genus the shell is curved into a flat spiral, the volutions of
which are in contact; and it further agrees with both _Goniatites_
and _Ammonites_ in the fact that the septa or partitions between
the air-chambers are not simple and plain (as in the _Nautilus_
and its allies), but are folded and bent as they approach the
outer wall of the shell. In the _Goniatite_ these foldings of
the septa are of a simply lobed or angulated nature, and in the
_Ammonite_ they are extremely complex; whilst in the _Ceratite_
there is an intermediate state of things, the special feature
of which is, that those foldings which are turned towards the
mouth of the shell are merely rounded, whereas those which are
turned away from the mouth are characteristically toothed. The
genus _Ceratites_, though principally Triassic, has recently
been recognised in strata of Carboniferous age in India.
From the foregoing it will be gathered that one of the most important
points in connection with the Triassic _Mollusca_ is the remarkable
intermixture of Palaeozoic and Mesozoic types which they exhibit.
It is to be remembered, also, that this intermixture has hitherto
been recognised, not in the Middle Triassic limestones of the
Muschelkalk, in which--as the oldest Triassic beds with marine
fossils--we should naturally expect to find it, but in the St
Cassian beds, the age of which is considerably later than that
of the Muschelkalk. The intermingling of old and new types of
Shell-fish in the Upper Trias is well brought out in the annexed
table, given by Sir Charles Lyell in his 'Student's Elements of
Geology' (some of the less important forms in the table being
omitted here):--
GENERA OF FOSSIL MOLLUSCA IN THE ST CASSIAN AND HALLSTADT BEDS.
Common to | Characteristic of | Common to
Older Rocks. | Triassic Rocks | Newer Rocks.
| |
Orthoceras. | Ceratites. | Ammonites.
Bactrites. | Cochloceras. | Chemnitzia.
Macrocheilus. | Rhabdoceras. | Cerithium.
Loxonema. | Aulacoceras. | Monodonta.
Holopella. | Naticella. | Sphoera. |
