THE ANCIENT LIFE-HISTORY OF THE EARTH
A COMPREHENSIVE OUTLINE OF THE PRINCIPLES AND LEADING FACTS OF
PALÆONTOLOGICAL SCIENCE
BY H. ALLEYNE NICHOLSON
M.D., D.SC., M.A., PH. D. (GÖTT), F.R.S.E, F.L.S.
PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ST ANDREWS
PREFACE.
The study of Palæontology, 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 Palæontology 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 Palæontology 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 Palæontology, 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 Palæontology, 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 Palæontology, 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 Palæontology, must have recourse to other and more elaborate
treatises.
UNITED COLLEGE, ST ANDREWS.
October 2, 1876.
CONTENTS.
PART I.
PRINCIPLES OF PALÆONTOLOGY.
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 Palæontology--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 Palæontological evidence in stratigraphical
Geology--General sequence of the great formations.
CHAPTER IV.
The breaks in the palæontological 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 palæontological 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 PALÆONTOLOGY.
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 _Eozoön Canadense_--Comparison of _Eozoön_
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--Palæontological 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 Palæozoic 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 Palæontology 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 _Eozoön Canadense_.
23. Diagram illustrating the structure of _Eozoön_.
24. Microscopic section of _Eozoön 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 vertebræ 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 PALÆONTOLOGY.
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
Palæontology,[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, Palæontology 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 palæontologists,
with which we have mainly to do here; but before entering upon
this, there are some general questions, affecting Geology and
Palæontology 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 primæval
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.
Palæontology 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 (faunæ and floræ) 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 palæontology.
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
æons 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 PALÆONTOLOGY.
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 palæontology; and though it is
possible to proceed to a certain length in geology and zoology
without much palæontological 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 palæontology without
some acquaintance with both geology and natural history.
Palæontology, 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 palæontological 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 palæontologist, 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 palæontologist 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 palæontological
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 laminæ; 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 palæontologist, 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
_débris_ 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 palæontologist 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 palæontologist. 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 palæontological 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 palæontologist, 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 palæontology 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 Palæontology 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 palæontologist, 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 palæontological
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
palæontologist 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 Palæontology. 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 palæontological 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 palæontological 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 palæontological 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 palæontological evidence extant
or available as to the age of a given group of strata. In the
second place, palæontological 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 palæontologists
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, palæontologists 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 palæontologist 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 palæontology 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 palæontological 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 palæontology, 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 palæozoic 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 PALÆONTOLOGICAL 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 palæontological 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 palæontological 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 palæontological 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 palæontological 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° 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
palæontological 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 palæontological 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 Palæozoic 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 palæontologist 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 palæontology, 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 PALÆONTOLOGY