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[Illustration: LIFE IN THE SILURIAN AGE.]
THE CHAIN OF LIFE
IN
GEOLOGICAL TIME
_A SKETCH OF THE ORIGIN AND SUCCESSION OF
ANIMALS AND PLANTS_
BY
SIR J. WILLIAM DAWSON
C.M.G., LL.D., F.R.S., F.G.S., ETC.
AUTHOR OF
aEurooeACADIAN GEOLOGY,aEuroŁ aEurooeTHE STORY OF THE EARTH,aEuroŁ aEurooeEGYPT AND SYRIA; THEIR
PHYSICAL FEATURES IN RELATION TO BIBLE HISTORY,aEuroŁ ETC.
_THIRD AND REVISED EDITION_
WITH NUMEROUS ILLUSTRATIONS
THE RELIGIOUS TRACT SOCIETY,
56 PATERNOSTER ROW; 65 ST. PAULaEuro(TM)S CHURCHYARD;
AND 164 PICCADILLY
1888
RICHARD CLAY AND SONS.
LONDON AND BUNGAY.
PREFACE.
Questions as to the origin and history of life are not at the present
time answered by mere philosophical speculation and poetical imagining.
Such solutions of these questions as science can profess to have
obtained are based on vast accumulations of facts respecting the remains
of animals and plants preserved in the rocky beds of the earthaEuro(TM)s crust,
which have been successively accumulated in the course of its long
geological history. These facts undoubtedly afford the means of
attaining to very certain conclusions on many points relating to the
history of life on the earth. But, on the other hand, they have
furnished the material for hypotheses which, though confidently affirmed
to be indisputable, have no real foundation in nature, and are
indirectly subversive of some of the most sacred beliefs of mankind.
In these circumstances it is most desirable that those who are not
specialists in such matters should be in a position to judge for
themselves; and it does not appear impossible in the actual state of
knowledge, to present, in terms intelligible to the general reader, such
a view of the ascertained sequence of the forms of life as may serve at
once to give exalted and elevating views of the great plan of creation,
and to prevent the deceptions of pseudo-scientists from doing their evil
work. Difficulties, no doubt, attend the attempt. They arise from the
number and variety of the facts, from the uncertainties attending many
important points, from the new views constantly opening up in the
progress of discovery, and from the difficulty of presenting in an
intelligible form the preliminary data in biology and geology necessary
for the understanding of the questions in hand. In order, as far as
possible, to obviate these difficulties, the plan adopted in this work
has been to note the first known appearance of each leading type of
life, and to follow its progress down to the present time or until it
became extinct. This method is at least natural and historical, and has
commended itself to the writer as giving a very clear comprehension of
the actual state of our knowledge, and as presenting some aspects of the
subject which may be novel and suggestive even to those who have studied
it most deeply.
In selecting examples and illustrations, the writer has endeavoured to
avoid, as far as possible, those already familiar to the general reader.
He has carefully sought for the latest facts, while rejecting as
unproved many things that are confidently asserted; and has endeavoured
to avoid all that is irrelevant to the subject in hand, and to abstain
from all technical terms not absolutely essential. In a work at once so
wide in its scope, so popular in its character, and so limited in its
dimensions, a certain amount of hostile criticism on the part of
specialists is to be expected, some portion of it perhaps just, other
portions arising from narrow prejudices due to limited lines of study.
The writer is willing to receive such comments with attention and
gratitude, but he would deprecate the misuse of them in the interest of
those coteries which are at present engaged in the effort to torture
nature into a confession of belief in the doctrines of a materialistic
or agnostic philosophy.
The title of the work was suggested by that of GaudryaEuro(TM)s recent
attractive book, _Les EnchaA(R)nements du Monde animal_. It seemed well
fitted to express the connection and succession of forms of life,
without implying their derivation from one another, while it reminds us
that nature is not a fortuitously tangled skein, and that the links
which connect man himself with the lowest and oldest creatures bind him
also to the throne of the Eternal.
In the few years that have elapsed since the publication of the first
edition of this work, great additions have been made to our knowledge of
fossil animals and plants. Many new species have been described, and
many new facts have been discovered, respecting species previously
known. This rapid progress of discovery has, however, invalidated few of
the statements made in the first edition, and has certainly established
nothing against the general laws of the succession of life as stated in
this work.
Perhaps the most interesting phase of recent discovery is the tracing
back of certain forms of life to earlier periods of the earthaEuro(TM)s
geological history. Some of the most recent facts of this kind are the
finding, by M. Charles Brongniart, of a fossil insect, allied to the
_Blattae_ or cockroaches, in the Silurian of Spain, that of true
Scorpions in the Upper Silurian of Sweden by LindstrAÂm, and in the Upper
Silurian of Scotland by Peach, who has also described fossil Millipedes
from the Lower Devonian. The tendency of such discoveries is to carry
farther back the origin of highly specialised forms of life, and thus to
render less probable their origin by any process of gradual derivation.
Other discoveries serve to fill up blanks in our knowledge, and thus to
render the geological record less imperfect. Of this kind is the close
approximation now worked out in Western America between the end of the
reign of the great MesozoA-c reptiles and the beginning of that of the
mammals of the TertiaryaEuro"a great and abrupt revolution, effected
apparently by a _coup de main_. I have myself had opportunity to show
that a similarly sharp line separates that quaint old MesozoA-c flora of
pines, cycads and ferns, which extends upward into the Lower Cretaceous,
from the rich and luxuriant assemblage of broad-leaved trees of modern
aspect, which takes its place in the middle part of the same formation.
It is not too much to say that these and similar discoveries, while they
serve to bridge over gaps in the succession of organic beings, do not
favour the theory of slow modification of types. They rather point to a
law of rapid development of new forms under special conditions as yet
unknown to science, and this accompanied with the extinction of older
species. Recent discoveries also present many remarkable instances of
the early introduction of highly specialised types, of higher forms
preceding those that are lower in the same class, and of the persistence
of certain types throughout geological time without any important
change.
J. W. D.
MCGILL COLLEGE.
CONTENTS.
CHAP. PAGE
I. PRELIMINARY CONSIDERATIONS AS TO THE EXTENT AND
SOURCES OF OUR KNOWLEDGE 1
II. THE BEGINNING OF LIFE ON THE EARTH 21
III. THE AGE OF INVERTEBRATES OF THE SEA 45
IV. THE ORIGIN OF PLANT LIFE ON THE LAND 89
V. THE APPEARANCE OF VERTEBRATE ANIMALS 117
VI. THE FIRST AIR-BREATHERS 137
VII. THE EMPIRE OF THE GREAT REPTILES 165
VIII. THE FIRST FORESTS OF MODERN TYPE 185
IX. THE REIGN OF MAMMALS 207
X. THE ADVENT OF MAN 233
XI. REVIEW OF THE HISTORY OF LIFE 253
LIST OF ILLUSTRATIONS.
FRONTISPIECE.aEuro"Life in the Silurian Age _To face Title._
FIG. PAGE
1. Bank of stream or coast, showing stratification 4
2. Section at Niagara Falls 4
3. Section obtained by boring, near Goderich, Ontario 5
4. Inclined beds, holding fossil plants 6
5. Ideal section of the Apalachian Mountains 7
6. Generalised section across England from Menai Straits to
the Valley of the Thames 9
7. Generalised section from the Laurentian of Canada to the
coal-field of Michigan 9
8. Unconformable superposition of Devonian Conglomerate on
Silurian slates, at St. AbbaEuro(TM)s Head, Berwickshire 10
9. Section of Trenton limestone, Montreal 14
10. Diagram showing different state of fossilisation of a
cell of a Tabulate Coral 15
11. Cast of erect tree (_Sigillaria_) in Sandstone 16
12. _Protichnites septem-notatus_ 17
12_a_. Footprints of modern _Limulus_, or king-crab 18
13. Current markings on shale, resembling a fossil plant 18
_Frontispiece._ Magnified and restored section of a portion
of _Eozoon canadense_ 20
14. Ideal section, showing the relations of the Laurentian and
Huronian 24
15. Small weathered specimen of _Eozoon_ 28
16. Nature-printed specimen of _Eozoon_ slightly etched with acid 29
17. Magnified group of canals in supplemental skeleton of
_Eozoon_ 31
18. Portion of _Eozoon_ magnified 100 diameters 31
19. Magnified portion of shell of _Calcarina_ 32
20. _AmA"ba_, a fresh-water naked Rhizopod; and _Actinophrys_,
a fresh-water Protozoon 34
21. _Nonionina_, a modern marine Foraminifer 34
22. _Stromatopora concentrica_ 35
23. _Caunopora planulata_ 36
24. _ArchA|ocyathus minganensis_. A Primordial Protozoon 37
25. Receptaculites. Restored 38
26. Section of _Loftusia Persica_. An Eocene Foraminifer 39
27. Foraminiferal Rock Builders, in the Cretaceous and Eocene 41
_Frontispiece._ _Paradoxides Regina_ (Matthew) 44
28. Group of Cambrian Animals 46
29. Portion of skeleton of Hexactinellid Sponge (_CA"loptychium_) 49
30. _Protospongia fenestrata_ (Salter) 50
31. _Astylospongia prA|morsa_ (Roemer) 51
32. Spicules of Lithistid Sponge (_Trichospongia_, Billings) 51
33. _Oldhamia antiqua_ (Forbes) 52
34. _Dictyonema sociale._ Enlarged 52
35. _Dictyonema Websteri_ (Dn.) 53
36. Group of modern Hydroids allied to Graptolites 54
37. Silurian GraptolitidA| 55
38. Central portion of Graptolite, with membrane, or float
(_Dichograpsus octobrachiatus_, Hall) 55
39. _Ptilodictya acuta_ (Hall). Bryozoan 55
39_a_. _Fenestella Lyelli_ (Dn.). A Carboniferous Bryozoan 56
40. _Chaetetes fibrosa._ A Tabulate Coral with microscopic
cells 56
41. _a_, _Stenopora exilis_ (Dn.). _b_, _Chaetetes tumidus_
(Edwards and Haine) 57
42. Living Anthozoan Coral (_AstrA|a_) 58
43. Tabulate Corals (_Halisites_ and _Favosites_) 59
44. Rugose Coral (_Heliophyllum Halli_) 59
44_a_. _Zaphrentis prolifica_ (Billings) 60
45. Rugose Corals (_Zaphrentis Minas_, Dn., and _Cyathophyllum_
_Billingsi_, Dn.) 60
46. Modern Crinoid (_Rhizocrinus Lofotensis_) 61
47. _PalA|aster Niagarensis_ (Hall) 62
48. _PalA|chinus ellipticus_ (McCoy) 62
49. _Pleurocystites squamosus_ 63
50. _Heterocrinus simplex_ (Meek) 63
51. Body of _Glyptocrinus_ 63
52. _Extracrinus Briareus_ 64
53. _Pentacrinus caput-medusA|_ 64
54. _Lingula anatina_ 65
55. Cambrian and Silurian LingulA| 65
56. _Terebratula sacculus_ (Martin) 66
57. Brachiopods; genus _Orthis_ 66
58. _Rhynchonella increbrescens_ (Hall) 66
59. _Spirifer mucronatus_ (Conrad) 67
59_a_. _Athyris subtilita_ (Hall) 67
60. _Productus cora_ (DaEuro(TM)Orbigny) 68
61. Group of Older PalA|ozoic Lamellibranchs 69
62. _Conularia planicostata_ (Dn.). A Carboniferous Pteropod 70
63. Silurian Sea-snails 70
64. Squid (_Loligo_) 72
65. Pearly Nautilus (_Nautilus pompilius_) 72
66. _Orthoceras_ 73
67. _Gomphoceras_ 73
68. _Lituites_ 73
69. _Nautilus Avonensis_ (Dn.) 74
70. _Goniatites crenistria_ (Philips) 74
71. _Ceratites nodosus_ (Schloth) 75
72. _Ammonites Jason_ (Reinecke) 76
72_a_. Suture of _Ammonites componens_ (Meek) 76
73. Cretaceous AmmonitidA| 77
74. _Belemnite_ 78
74_a_. _Belemnoteuthis antiquus_ 78
75. Cambrian Trilobites 79
76. Transverse section of _Calymene_. A Silurian Trilobite 80
76_a_. Burrows of Trilobite and of modern King-crab 81
77. Silurian Trilobites 82
78. Devonian and Carboniferous Trilobites 83
79. PalA|ozoic Ostracod Crustaceans 83
80. _Pterygotus anglicus_ 84
81. _Amphipeltis paradoxus_ (Salter) 85
82. _AnthropalA|mon Hilliana_ (Dn.) 85
_Frontispiece._ _Cordaites_, of the group of _Dory-Cordaites_ 88
83. _Protannularia Harknessii_ (Nicholson) 91
84. American Lower Silurian Plants 92
86. Fragment of outer surface of _Glyptodendron_ of Claypole 93
87. _Psilophyton princeps_ (Dn.) 95
88. Trunk of a Devonian Tree-fern (_Caulopteris Lockwoodi_, Dn.) 97
89. Frond of _ArchA|opteris Jacksoni_ (Dn.) 98
90. Portion of a branch of _Leptophleum rhombicum_ (Dn.) 98
91. _Calamites radiatus_ (Brongniart) 99
92. A Devonian Taxine Conifer (_Dadoxylon ouangondianum_, Dn.) 100
93. Group of Devonian fruits, &c. 101
94. Structures of the oldest-known Angiospermous Exogen
(_Syringoxylon mirabile_, Dn.) 102
95. _Asterophyllites parvula_ (Dn.) and _Sphenophyllum
antiquum_ (Dn.) 103
96. _Calamites_ 104
97. Carboniferous Ferns 105
98. Carboniferous Tree-ferns 107
99. _Lepidodendron corrugatum_ (Dn.) 108
100. _SigillariA|_ of the Carboniferous 109
101. _Trigonocarpum Hookeri_ (Dn.) 111
_Frontispiece._ Pteraspis. Restored 116
102. Siluro-Cambrian Conodonts 118
103. Lower Carboniferous Conodont 119
104. _a_, Head-shield of an Upper Silurian Fish (_Cyathaspis_);
_b_, Spine of a Silurian Shark (_Onchus tenui-striatus_,
Agass.); _c_, _d_, Scales of _Thecodus_ 121
105. _Cephalaspis Dawsoni_ (Lankester) 122
106. Devonian Placoganoid Fishes (_Pterichthys cornutus_,
_Cephalaspis Lyelli_) 123
107. Devonian Lepidoganoid Fishes (_Diplacanthus_ and
_Osteolepis_) 124
108. Modern Dipnoi (_Ceratodus Fosteri_ and _Lepidosiren
annectus_) 124
109. Anterior part of the palate of _Dipterus_ 125
110. Dental plate of _Conchodus plicatus_ (Dn.) 126
111. Dental plate of _Ceratodus Barrandii_ 126
112. Dental plate of _Ceratodus serratus_ 127
113. Jaws of _Dinichthys Hertzeri_ (Newberry) 127
114. Lower Jaw of _Dinichthys Hertzeri_ 128
115. Jaws of _Lepidosiren_ 128
116. A small Carboniferous Ganoid (_PalA|oniscus_
(_Rhadinichthys_) _Modulus_, Dn.) 129
117. Teeth and Spines of Carboniferous Sharks 130
118. Teeth of Cretaceous Sharks (_Otodus_ and _Ptychodus_) 131
119. Tooth of a Tertiary Shark (_Carcharodon_) 132
120. A Liassic Ganoid (_Dapedius_) 132
121. Cretaceous Fishes of the modern or Teleostian type
(_Beryx Lewesiensis_ and _Portheus molossus_, Cope) 133
122. Modern Ganoids (_Polypterus_ and _Lepidosteus_) 134
_Frontispiece._ A Microsaurian of the Carboniferous Period
(_Hylonomus Lyelli_) 136
123. Wings of Devonian Insects 140
124. Land-snail (_Pupa vetusta_, Dn.) 143
125. Land-snail (_Zonites_ (_Conulus_) _priscus_, Carpenter) 143
126. Millipedes (_Xylobius sigillariA|_, Dn.; _Archiulus
Xylobioides_, Scudder; _X. farctus_, Scudder) 145
127. Wings of Cockroaches 146
128. Wing of May-fly (_Haplophlebium Barnesii_, Scudder) 147
129. A Jurassic Sphinx-moth (_Sphinx Snelleri_, Weyenburgh) 148
130. An Eocene Butterfly (_Prodryas persephone_, Scudder) 149
131. Abdominal part of a Carboniferous Scorpion 150
132. Carboniferous Scorpion (_Eoscorpius carbonarius_,
Meek and Worthen) 151
133. Footprints of one of the oldest known Batrachians,
probably a species of _Dendrerpeton_ 152
134. _Archegosaurus Decheni_ 154
135. _Ptyonius_ 154
136. A large Carboniferous Labyrinthodont (_Baphetes
planiceps_, Owen) 155
137. _Baphetes planiceps_ (Owen) 156
138. A lizard-like Amphibian (_Hylonomus aciedentatus_) 157
139. _Stelliosaurus longicostatus_ (Fritsch) 158
140. Section showing the position of an erect _Sigillaria_,
containing remains of land animals 160
140_a_. Section of base of erect _Sigillaria_, containing
remains of land animals 161
_Frontispiece._ Inhabitants of the English Seas in the Age of
Reptiles 164
141. Arm of _Proterosaurus Speneri_ 166
142. Skeleton of _Ichthyosaurus_ 167
142_a_. Head of _Pliosaurus_ 168
142_b_. Paddle of _Plesiosaurus Oxoniensis_ 168
143. Skeleton of _Clidastes_ 170
144. An Anomodont Reptile of the Trias (_Dicynodon
lacerticeps_, Owen) 170
145. A Theriodont Reptile of the Trias (_Lycosaurus_) 170
146. Skeleton of _Pterodochylus crassirostris_ 170
147. Restoration of _Rhamphorhyncus Bucklandi_ 171
148. A Jurassic bird (_ArchA|opteryx macroura_) 172
149. Jaw of a Cretaceous Toothed Bird (_Ichthyornis dispar_) 173
150. Jaw of _Bathygnathus borealis_ (Leidy) 174
151. _Hadrosaurus Foulkii_ (Cope) 175
152. Jaws of _Megalosaurus_ 176
153. Tooth of _Megalosaurus_ 177
154. _Compsognathus_ 179
_Frontispiece._ Lower Cretaceous Leaves 184
155. _Sassafras cretaceum_ (Newberry) 190
156. _Liriodendron primA|vum_ (Newberry) 191
157. _Onoclea sensibilis_ 191
158. _Davallia tenuifolia_ 192
159. Eocene Leaves 194
160. An Ancient Clover (_Trifolium palA|ogA|um_, Saporta) 195
161. An Eocene Maple (_Acer sextianus_, Saporta) 195
162. A European _Magnolia_ of the Eocene (_M. dianA|_, Saporta) 195
163. Flower and Leaf of _Bombax sepultiflorum_ 196
164. Branch and Fruit of _Sequoia CouttsiA|_ (Heer) 197
165. _Cinnamomum Scheuchzeri_ (Heer) 198
_Frontispiece._ _Sivatherium giganteum_ 206
166. Jaw of _Dromatherium sylvestre_ (Emmons) 209
167. _Myrmecobius fasciatus_ 209
168. Jaw and Molar of _Phascolotherium Bucklandi_ 210
169. Jaw and Pre-molar of _Plagiaulax Becklesii_ 210
170. Restoration of _PalA|otherium magnum_ 211
171. Skull of a Lower Eocene Perissodactyl (_Coryphodon
Hamatus_) 214
172. Fore-foot of _Coryphodon_ 215
173. Skull of Upper Eocene Perissodactyl (_Dinoceras
mirabilis_) 216
174. Fore-foot of _Dinoceras_ 217
175. Skull of Miocene Perissodactyl (_Brontotherium ingens_,
Marsh) 217
176. Series of Equine feet 218
177. Skull of generalised Miocene Ruminant (_Oreodon major_) 221
178. Lower Jaw of _Megatherium_ 222
179. Ungual Phalanx and Claw-core of _Megatherium_ 222
180. Tooth of Eocene Whale (_Zeuglodon cetioides_) 223
181. _Mastodon ohioticus_ 225
182. Head of _Dinotherium giganteum_ 226
183. Wing of Eocene Bat (_Vespertilio aquensis_) 226
184. Skull of a Cymetar-toothed Tiger (_Machairodus
cultridens_) 228
185. Lower Jaw of _Dryopithecus Fontani_ 229
_Frontispiece._ Contemporaries of Post-Glacial Man 232
186. _Elephas primigenius_ 241
187. Tooth of _Elasmotherium_ 242
188. Engis Skull 243
189. Outlines of Three Prehistoric European Skulls compared
with an American Skull 244
190. Flint Implement found in KentaEuro(TM)s Cavern, Torquay 245
191. Bone Harpoon (PalA|ocosmic) 246
192. Sketch of a Mammoth carved on a portion of a Tusk of the
same Animal 249
Tabular View of Geological Periods and of Life-Epochs.
+--------------------------------------+---------------+--------------+
| GEOLOGICAL PERIODS. | ANIMAL LIFE. | VEGETABLE |
| | | LIFE. |
+--------------------------------------+---------------+--------------+
| | | |
|C {_Post- {Modern. | Age of _Man_ | |
|A N {Tertiary_ or {Post-Glacial. | and _modern | |
|I E {_Quaternary_. | Mammals_. | Age of |
|N o O { | | _Angiosperms_|
|O r Z {_Tertiary_ {Pleistocene or |Age of _Extinct| and _Palms_. |
|Z O { {Glacial. | Mammals_. | |
|O I { {Pliocene. | (Earliest | |
|I C.{ {Miocene. | Placental | |
|C { {Eocene. | Mammals.) | |
| | | |
| ----------------------------+---------------+--------------+
| | | |
| { {Upper, | | |
| M {_Cretaceous_ {Lower, or | | |
| E { {Neocomian. | | |
| S { | Age of | |
| O {_Jurassic_ {Oolite. | _Reptiles_ and| (Earliest |
| Z { {Lias. | _Birds_. |Modern Trees.)|
| O { | | Age of |
| I { | | _Cycads_ and |
| C. { {Upper, | (Earliest | _Pines_. |
| { {Middle, or | Marsupial | |
| {_Triassic_ {Muschelkalk. | Mammals.) | |
| { {Lower | | |
| | | |
| --------------------------------+---------------+--------------+
| | | |
| {Upper, | | |
| {_Permian_ {Middle, or | | |
| { {Magnesian | (Earliest | |
| { { Limestone. | true | |
| { {Lower. | Reptiles.) | |
| { | | |
| { {Upper | | |
| { { Coal-Formation.| | |
| {_Carbon- {Coal-Formation | | |
| { iferous_ {Carboniferous | | |
| { { Limestone. | | |
| { {Lower | | |
| P { { Coal-Formation.| | |
| A { | | |
| L { {Upper. | Age of | Age of |
| Ać {_Devonian_ {Middle. | _Amphibians_ |_Acrogens_ and|
| O { {Lower. | and _Fishes_.|_Gymnosperms_.|
| Z { | | (Earliest |
| O {_Silurian_ {Upper. | |Land Plants.) |
| I { {Lower. | | Age of |
| C. { | | _AlgA|_. |
| {_Siluro-Cam- | Age of | |
| { brian_ or {Upper. | _Mollusks_, | |
| {_Ordovician_ {Lower. | _Corals_, and | |
| { | _Crustaceans_.| |
| { {Upper. | | |
| {_Cambrian_ {Middle. | | |
| {Lower | | |
| | | |
| --------------------------------+---------------+--------------+
| | | |
| {_Huronian_ {Upper. | | |
| { {Lower. | | |
| E { | Age of | |
| O { | _Protozoa_. | Indications |
| Z { | (First animal | Plants |
| O { {Upper. | remains.) | not |
| I {_Laurentian_ {Middle, | |determinable. |
| C. {Lower, or | | |
| {Bojian. | | |
| | | |
+--------------------------------------+---------------+--------------+
THE CHAIN OF LIFE.
CHAPTER I.
PRELIMINARY CONSIDERATIONS AS TO THE EXTENT AND SOURCES OF OUR
KNOWLEDGE.
It is of the nature of true science to take nothing on trust or on
authority. Every fact must be established by accurate observation,
experiment, or calculation. Every law and principle must rest on
inductive argument. The apostolic motto, aEurooeProve all things, hold fast
that which is good,aEuroŁ is thoroughly scientific. It is true that the mere
reader of popular science must often be content to take that on
testimony which he cannot personally verify; but it is desirable that
even the most cursory reader should fully comprehend the modes in which
facts are ascertained and the reasons on which conclusions are based.
Failing this, he loses all the benefit of his reading in so far as
training is concerned, and cannot have full assurance of that which he
believes. When, therefore, we speak of life-epochs, or of links in a
chain of living beings, the question is at once raisedaEuro"What evidence
have we of the succession of such epochs? This question, with some
accessory points, must engage our attention in the present chapter.
Geology as a practical science consists of three leading parts. The
first and most elementary of these is the study of the different kinds
of rocks which enter into the composition of those parts of the earth
which are accessible to us, and which we are in the habit of calling the
crust of the earth. This is the subject of _Lithology_, which is based
on the knowledge of minerals, and has recently become a much more
precise department of science than heretofore, owing to the successful
employment of the microscope in the investigation of the minute
structure and composition of rocks. The second is the study of the
arrangement of the materials of the earth on the large scale, as beds,
veins, and irregular masses; and inasmuch as the greater part of the
rocks known to us in the earthaEuro(TM)s crust are arranged in beds or strata,
this department may be named _Stratigraphy_. A more general name
sometimes employed is that of _Petrography_. The third division of
geology relates to the remains of animals and plants buried in the rocks
of the earth, and which have lived at the time when those rocks were in
process of formation. These fossil remains introduce us to the history
of life on the earth, and constitute the subject of _PalA|ontology_.
It is plain that in considering what may be learned as to epochs in the
history of life we are chiefly concerned with the last of these
divisions. The second may also be important as a means of determining
the relative ages of the fossils. With the first we have comparatively
little to do.
Previous to observation and inquiry, we might suppose that the kinds of
animals and plants which now inhabit the earth are those which have
always peopled it; but a very little study of fossils suffices to
convince us that vast numbers of creatures once inhabitants of this
world have become extinct, and can be known to us only by their remains
buried in the earth. When we place this in connection with
stratigraphical facts, we further find that these extinct species have
succeeded each other at different times, so as to constitute successive
dynasties of life. On the one hand, when we know the successive ages of
fossil forms, these become to us, like medals or coins to the historian,
evidences of periods in the earthaEuro(TM)s history. On the other hand, we are
obliged in the first instance to ascertain the ages of the medals
themselves by their position in the successive strata which have been
accumulated on the surface. The series of layers which explorers like
Schliemann find on the site of an ancient city, and which hold the works
of successive peoples who have inhabited the place, thus present on a
small scale a faithful picture of the succession of beds and of forms of
life on the great earth itself.
Our leading criterion for estimating the relative ages of rocks is the
superposition of their beds on each other. The beds of sandstone, shale,
limestone, and other rocks which constitute the earthaEuro(TM)s crust have
nearly all been deposited thereon by water, and originally in attitudes
approaching to horizontality. Hence the bed that is the lower is the
older of any two beds. Hence also, when any cutting or section reveals
to us the succession of several beds, we know that fossil remains
contained in the lower beds must be of older date.
We can scarcely walk by the side of a stream which has been cutting into
its banks, or at the foot of a sea-cliff, or through a road-cutting,
without observing illustrations of this. For instance, in the section
represented in Fig. 1, we see at the surface the vegetable soil, below
this layers of gravel and sand, below this a bed of clay, and below this
hard limestone. Of these beds _a_ is the newest, _d_ the oldest; and if,
for example, we should find some marine shells in _d_, some fresh-water
shells in _c_, bones of land animals and flint arrowheads in _b_, and
fragments of modern pottery in _a_, we should be able at once to assign
their relative ages to these fossils, and to form some idea of the
succession of conditions and of life which had occurred in the locality.
On a somewhat larger scale, we have in Fig. 2 a section of the beds cut
through by the great Fall of Niagara. All of these except that marked
_a_ are very ancient marine rocks, holding fossil shells and corals, but
now forming part of the interior of a continent, and cut through by a
fresh-water river.
[Illustration: FIG. 1.aEuro"Bank of stream or coast, showing stratification.
_a_, Vegetable soil.
_b_, Gravel and sand.
_c_, Clays.
_d_, Limestone rock, slightly inclined.]
[Illustration: FIG. 2.aEuro"Section at Niagara Falls, showing the strata cut
through by the action of the Fall. Thickness of beds about 250 feet.
_a_, Boulder clay and gravelaEuro"Post-pliocene.
_b_, Niagara limestone } Upper Silurian, with
_c_, Niagara shale } marine shells and
_d_, Clinton limestone } corals.
_e_, Medina sandstone }]
In deep mines and borings still more profound sections may be laid open,
as in Fig. 3, which represents the sequence of beds ascertained by
boring with the diamond drill in search of rock salt near Goderich in
Canada. Here we have a succession of 1,500 feet of beds, some of which
must have been formed under very peculiar and exceptional conditions.
The beds of rock salt and gypsum must have been formed by the drying up
of sea-water in limited basins. Those of Dolomite imply precipitation
of carbonate of lime and magnesia in the sea-bottom. The marls must have
been formed largely by the driftage of sand and clay, while some of the
limestone was produced by accumulation of corals and shells. Such
deposits must not only have been successive, but must have required a
long time for their formation.
[Illustration: FIG. 3.aEuro"Section obtained by boring with the diamond
drill, near Goderich, Ontario, Canada, in the Salina series of the Upper
Silurian. From a memoir by Dr. Hunt in the Report of the Geological
Survey of Canada for 1876-7.
No. 1, Clay, gravel, and bouldersaEuro"Post-pliocene.
Nos. 2, 4, 7, 9, 13, Dol omite or magnesian limestone, with layers
of marl, limestone, and gypsum.
No. 3, Limestone with coralsaEuro"_Favosites_, etc.
Nos. 5, 11, 15, 17, Marls with layers of Dolomite and anhydrous
gypsum.
Nos. 6, 8, 10, 12, 14, 16, Rock salt.]
[Illustration: FIG. 4.aEuro"Inclined beds, holding fossil plants.
Carboniferous. South Joggins, Nova Scotia.
1. Shale and sandstone. Plants with _Spirorbis_ attached;
rain marks (?).
2. Sandstone and shale, 8 feet. Erect
_Calamites_. } An erect coniferous (?) tree, rooted
3. Gray sandstone, 7 feet. } on the shale, passes up through 15
4. Gray shale, 4 feet. } feet of the sandstones and shale.
5. Gray sandstone, 4 feet.
6. Gray shale, 6 inches. Prostrate and erect trees, with rootlets,
leaves, _Naiadites_, and _Spirorbis_ on the plants.
7. Main coal-seam, 5 feet coal in two beds.
8. Underclay, with rootlets.]
In Fig. 4 we have a bed of coal and its accompaniments. The coal itself
was produced by the slow accumulation of vegetable matter on a
water-soaked soil, and this was buried under successive beds of sand and
clay, now hardened into sandstone and shale, some of the beds holding
trees and reed-like plants, which still stand on the soils on which
they grew, and which must have been buried in sediment deposited in
inundations or after subsidence of the land. In this section we may also
observe that the beds are somewhat inclined; and that this is not their
original position is shown by the posture of the stems of trees, once
erect, but now inclined with the beds. This leads to a consideration
very important with reference to our present subject; namely, that as
our continents are mostly made up of beds deposited under water and
afterwards elevated, these beds have in this process experienced such
disturbances that they rarely retain their horizontal position, but are
tilted at various angles. When we follow such inclined strata over large
areas, we find that they undulate in great waves or folds, forming what
are called anticlinal and synclinal lines, and that the irregularities
of the surface of the land depend to a great extent on these
undulations, along with the projection of hard beds whose edges protrude
at the surface. In point of fact, as shown in Fig. 5, mountain ranges
depend on these crumplings of the earthaEuro(TM)s crust; and the primary cause
of these is probably the shrinkage of the mass of the earth owing to
contraction in cooling. When the disturbances of beds are extreme, they
often cause intricacies of structure difficult to unravel; but when of
moderate extent they very much aid us in penetrating below the surface,
for we can often see a great thickness of beds rising one from beneath
another, and can thus know by mere superficial examination the structure
of the earth to a great depth. It thus happens that geologists reckon
the thickness of the stratified deposits of the crust of the earth at
more than 70,000 feet, though they cannot penetrate it perpendicularly
to more than a fraction of that depth. The two sections, Figs. 6 and 7,
showing the sequence of beds in England and in the northern part of
North America, will serve, if studied by the reader, to show how, by
merely travelling over the surface and measuring the upturned edges of
beds, many thousands of feet of deposits may be observed, and their
relative ages distinctly ascertained.
[Illustration: FIG. 5.aEuro"Ideal section of the Apalachian Mountains
showing folding of the earthaEuro(TM)s crust.
_a_, Anticlinal axes.
_b_, Overturned strata.
_c_, Synclinals.
_d_, Unconformable beds.]
In studying any extensive section of rock we find that its members may
more or less readily be separated into distinct groups. Sometimes these
are distinguished by what is termed unconformability, that is, the lower
series has been disturbed or inclined before the upper has been
deposited upon it. This is seen on a grand scale in the section Fig. 7,
in the case of the Laurentian and Cambrian formations, and on a smaller
scale in Fig. 8 in the unconformable superposition of Devonian
conglomerate on Silurian slates at St. AbbaEuro(TM)s Head. In the last section
it is quite evident that the beds of the lower series have been bent
into abrupt folds and worn away to a considerable extent before the
deposition of the overlying series. In such a case we know not merely
that the upper series is newer than the lower, but that some
considerable time must have elapsed after the deposition of the one
before the other was laid down; and we are not surprised to find that
the fossils in the groups thus unconformable to each other are very
different.
But even when the beds are conformable, they can usually be separated
into groups, depending upon differences of mineral character, or changes
which have occurred in the mode of deposition. One group of beds, for
example, may be largely composed of limestone, another of sandstone or
shale. One group may be distinguished by containing some special
mineral, as, for example, rock salt or coal, while others may be
destitute of such special minerals. One group may show by its fossils
that it was deposited in the sea, others may be estuarine or
lacustrine. Thus we obtain the means of dividing the rocks of the earth
into groups of different ages, known as aEurooeFormations,aEuroŁ and marking
particular periods of geological time. By tracing these formations from
one district or region to another, we learn the further truth that the
succession is not merely local, but that, though liable to variation in
detail, its larger subdivisions hold so extensively that they may be
regarded as world-wide in their distribution.
[Illustration: FIG. 6. Generalised section across England from Menai
Straits to the Valley of the Thames.aEuro"After Ramsay.
0 Huronian? or Laurentian?
1 Cambrian and Lower Silurian.
2 Upper Silurian.
3 Devonian.
6, 7, 8 Trias and lias.
9 and 10 Jurassic.
11 Cretaceous.
12 Eocene.]
[Illustration: FIG. 7.aEuro"Generalised section from the Laurentian of
Canada to the coal-field of Michigan.
0 Laurentian (the Huronian is absent in the line of this section).
1 Cambrian.
2 Lower Silurian.
3 Upper Silurian.
4 Devonian.
5 Carboniferous.]
[Illustration: FIG. 8.aEuro"Unconformable superposition of Devonian
conglomerate on Silurian slates, at St. AbbaEuro(TM)s Head, Berwickshire.aEuro"After
Lyell.]
Putting together the facts thus obtained, we can frame a tabular
arrangement of the earthaEuro(TM)s strata, as in the table prefixed to this
chapter; and when we add the further discovery, very early made by
geologists, that the successive formations differ from each other in
their fossil remains, we have the means of recognising any particular
formation by its fossils, even when the stratigraphical evidence may be
obscure or wanting. Thus our knowledge of Epochs of Life, and indeed of
the whole geological history of the earth, is based on the superposition
of beds in the earthaEuro(TM)s crust, and on the diversity of fossil remains in
the successive beds so superimposed on each other; and it is on these
grounds that we are enabled to construct a Table of Geological
Formations representing the whole series of beds as far as known, with
the characteristic groups of fossils of each period. Here I might close
these preliminary considerations, but there are a few accessory
questions, important to our clear comprehension of the subject, which
may profitably occupy our attention for a short time.
One of these relates to the absolute duration of the time represented by
the geological history of the earth. Such estimates as our present
knowledge enables us to form are very indefinite. Whether we seek for
astronomical or geological data, we find great uncertainty. To such an
extent is this the case, that current estimates of the time necessary to
bring the earth from a state of primitive incandescence to its present
condition have varied from fifteen millions of years to five hundred
millions. Of the various modes proposed, perhaps the most satisfactory
as well as instructive is that based on the rate of denudation of our
present continents, as indicated by the amount of sediment carried down
by great rivers. The Mississippi, draining a vast and varied area in
temperate latitudes, is washing away the American land at the rate of
one foot in 6,000 years. The Ganges, in a tropical climate and draining
many mountain valleys, works at the rate of one foot in 2,358 years. The
mean of these two great rivers would give one foot in 4,179 years, at
which rate our continents would be levelled with the waters in about six
millions of years. But the land has been in process of renewal as well
as of waste in geological time; and a better measure will be afforded by
the amount of beds actually deposited. The entire thickness of all the
stratified rocks of Great Britain has been calculated by Ramsay at
72,000 feet. Now, if we suppose the waste in all geological time to have
been on the average the same as at present, and that this material has
been deposited to the thickness of 72,000 feet on a belt of sea margin
100 miles in width, we shall have about 86 millions of years as the time
required.[1] This has the merit of approximating to Sir William
ThomsonaEuro(TM)s calculation, based on the rate of cooling of the earth, that a
minimum of 100 millions of years may represent the time since a solid
crust first began to form. As it is more likely that the rate of
denudation has on the average been greater in former geological periods
than at present, we may perhaps estimate fifty or sixty millions of
years as the time required for the accumulation of all our formations.
Some geologists object to this as too little, but in this some of them
are influenced by the exigencies of theories of evolution, and others
appear to have no adequate conception of the vast lapse of time
represented by such numbers, in its relation to the actual rates of
denudation and deposition.
It should be mentioned here, however, that, on certain theories now
somewhat generally accepted, respecting the nature and source of solar
heat, the absolute duration of geological time would be much reduced
below the estimate of Sir Wm. Thomson. Prof. Tait has based on such data
an estimate of fifteen millions of years. Prof. Simon Newcomb says that
aEurooeon the only hypothesis science will now allow us to make respecting the
source of the solar heataEuroŁ (the gravitation hypothesis of Helmholtz) aEurooethe
earth was, twenty millions of years ago, enveloped in the fiery
atmosphere of the sun.aEuroŁ Dr. Kirkwood has called attention to these
results in connection with the planetary hypothesis of La Place, in the
_Proceedings of the American Philosophical Society_.[2] Should such
views prove to be well-founded, geological calculations as to the time
required for the successive formations may have to be revised.
If now we attempt to divide this time among the formations known to us,
according to their relative thicknesses, we have, according to an
elaborate estimate of Professor Dana, the time ratios of 12, 3, and 1
for the PalA|ozoic, Mesozoic, and Cainozoic periods respectively. Taking
the whole time since the beginning of the Cambrian as forty-eight
millions of years, we should thus have for the PalA|ozoic thirty-six
millions, for the Mesozoic nine, and for the Tertiary three. Another
calculation, recently made by Professors Hull and Haughton, gives the
following ratios:aEuro"
Azoic 34A.3 per cent.
PalA|ozoic 42A.5 aEuroŁ
Mesozoic and Cainozoic 23A.2 aEuroŁ
This calculation is, however, based on the absolute thickness of the
several series as ascertained in Great Britain, without reference to the
nature of the beds, as indicating different rates of accumulation. Under
either estimate it will be seen that the PalA|ozoic time greatly exceeds
the Mesozoic and Cainozoic together, and consequently that changes of
life seem to have proceeded at an accelerated rate as time wore on.
Another inquiry of some importance relates to the manner of preservation
of fossils, and the extent to which they constitute the material of
rocks. This inquiry is doubly important, as it bears on the genuineness
of fossil remains, and on the means we have of understanding their
nature.
Some rocks are entirely made up of matter that once was alive, or formed
part of living organisms. This is the case with some limestones, which
consist of microscopic shells, or of larger shells, corals, and similar
calcareous organisms, either entire or broken into fragments and
cemented together with pasty or crystalline limestone filling their
interstices. This may be seen in Fig. 9, which represents a magnified
slice of a Silurian limestone. Coal in like manner consists of
carbonised vegetable matter, retaining more or less perfectly its
organic structure, and sometimes even the external forms of its
constituent parts. More frequently, fossils are dispersed more or less
sparsely through the substance of beds composed of earthy matter; and
they have usually been more or less affected by chemical changes, or by
mechanical pressure, or are mineralised by different substances which
have either filled their pores by infiltration or have more or less
completely replaced their substance. Of course, as a rule, the softer
and more putrescible organic matters have perished by decay, and it is
only the harder and more resisting parts that remain. Even these have
often yielded to the enormous pressure to which they have been
subjected, and if at all porous, have been changed by the slow action of
percolating water charged with various kinds of mineral matter in
solution.
[Illustration: FIG. 9.aEuro"Section of Trenton limestone, magnified, showing
that it is composed of fragments of corals, crinoids, and shells.
Montreal.]
[Illustration: FIG. 10.aEuro"Diagram showing different state of
fossilisation of a cell of a tabulate coral (DawsonaEuro(TM)s _Dawn of Life_).
_a_ Natural condition, wall calcite cell empty.
_b_ Wall calcite, cells filled with the same.
_c_ Walls calcite, cells filled with silica or a silicate.
_d_ Wall silicified, cells filled with calcite.
_e_ Wall silicified, cell filled with silica.]
It thus happens that many fossils are infiltrated with mineral matter.
Wood, for example, may have the cavities of its cells and vessels filled
with silica or silicates, with sulphide or carbonate of iron, or with
limestone, while the woody walls of the cells may remain either as coaly
matter or charcoal. I have often seen the microscopic cells of fossil
wood not only filled in this way, but presenting under a high power
successive coats of deposit, like the banded structure of an agate.
In some cases not only are the pores filled with mineral matter, but
the solid parts themselves have been replaced, and the whole mass has
actually become stone, while still retaining its original structure.
Thus silicified wood is often as hard and solid as agate, and under the
microscope we see that the wood has entirely perished, and is
represented by silica or flint, differing merely in colour from that
which fills the cavities. In this case we may imagine the wood to have
been acted on by water holding in solution silica, combined with soda or
potash, in the manner of what is termed soluble glass. The wood, in
decay, would be converted into carbon dioxide, and this as formed would
seize on the potash or soda, leaving the silica in an insoluble state,
to be deposited instead of the carbon. Thus each particle of the carbon
of the wood, as removed by decay, would be replaced by a particle of
silica, till the whole became stone. By similar chemical changes corals
and shells are often represented by silica, or by pyrite, which has
taken the place of the original calcareous matter; and still more
remarkable changes sometimes occur, as when the siliceous spicules of
sponges have been replaced by carbonate of lime. The organic matter
present in the fossils greatly promotes these changes, by the substances
produced in its decay, and thus it often happens that the shells,
corals, etc., contained in limestone have been replaced by flint, while
the inclosing limestone is unchanged. Fig. 10 shows the various
conditions which a coral may assume under these different modes of
treatment.
[Illustration: FIG. 11.aEuro"Cast of erect tree (_Sigillaria_) in sandstone,
standing on a small bed of coal, South Joggins, Nova Scotia (DawsonaEuro(TM)s
_Acadian Geology_).]
The substance of a fossil may be entirely removed by decay or solution,
leaving a mere mould representing its external form, and this may
subsequently be filled with mineral matter, so as to produce a natural
cast of the object. This is very common in the case of fossil plants;
and large trunks of trees may sometimes be found represented, as seen in
Fig. 11, by stony pillars retaining nothing of the original wood except
perhaps a portion of the bark in the state of coal. It sometimes happens
that the substance of fossils has been removed, leaving mere empty
cavities, sometimes containing stony cores representing the internal
chambers of the fossils. Again, calcareous fossils imbedded in hard
rocks are often removed by weathering, leaving very perfect impressions
of their forms. For this reason the fossil remains contained in some
hard resisting rocks can be best seen as impressed moulds on the
weathered surfaces.
[Illustration: FIG. 12.aEuro"_Protichnites septem-notatus._ A supposed
series of crustacean footprints made in sand, now hardened into
sandstone. Cambrian.aEuro"After Logan.]
Lastly, we sometimes have impressions or footprints representing the
locomotion of fossil animals, rather than the fossils themselves. In
this way some extinct creatures are known to us only by their footsteps
on sand or clay, once soft, but now hardened into stone; and in the
case of some of the lower animals the trails thus made are often not
easily interpreted (Figs. 12, 12_a_). It has been found that even
sea-weeds drifted by the tide make impressions of this kind, which, when
they occur in old rocks, are very mysterious. Even rain-drops are
capable of being permanently impressed on rocks, and constitute a kind
of fossils. Besides these we have many kinds of imitative markings which
simulate fossils, as those of concretions or nodules, which are often
very fantastic in shape, those of dendritic crystallisation giving
moss-like forms, and the complicated tracery produced on muddy shores
by the little rills of water which follow the receding tide (Fig. 13).
Such things are often mistaken by the ignorant for fossil remains, but
are easily distinguished by a practised eye.
[Illustration: FIG. 12_a_.aEuro"Footprints of modern _Limulus_, or
king-crab, in the sand, which enable us to interpret those in Fig. 12.]
The reader who has followed these, perhaps somewhat dry, details, will
be rewarded for his patience by having some conception of the conditions
in which we find fossil remains, and of the evidence by which we can
refer these to different periods in the history of the earth.
[Illustration: FIG. 13.aEuro"Current markings on shale, resembling a fossil
plant. Reduced from a photograph (DawsonaEuro(TM)s _Acadian Geology_).]
Carrying this knowledge with us, and at the same time glancing at the
table of successive formations prefixed to this chapter, we shall be
prepared, without any additional geological study, to understand the
statements to be made in the following chapters, and to appreciate the
actual nature of the succession of life in so far as it is at present
known.
[Illustration: MAGNIFIED AND RESTORED SECTION OF A PORTION OF EOZOON
CANADENSE.
The shaded portions show the animal matter of the Chambers, Tubuli,
Canals, and Pseudopodia; the unshaded portions the calcareous
skeleton.]