CLASS-BOOK OF GEOLOGY
CLASS-BOOK
OF
GEOLOGY
ARCHIBALD GEIKIE, LL.D., F.R.S.
n
DIRECTOR-GENERAL OF THE GEOLOGICAL SURVEY OF THE UNITED KINGDOM, AND
DIRECTOR OF THE MUSEUM OF PRACTICAL GEOLOGY, JERMYN STREET,
LONDON ; FORMERLY MURCHISON PROFESSOR OF GEOLOGY AND
MINERALOGY IN THE UNIVERSITY OF EDINBURGH
ILLUSTRATED WITH WOODCUTS,-,;" 1
. LONDON
MACMILLAN AND CO.
1886
[ The RigJit of Translation and Reproduction is reserved. J
fNiO OKFV,
PREFACE.
THE present volume completes a series of educational
works on Physical Geography and Geology, projected by
me many years ago. In the Primers, published in 1873,
the most elementary facts and principles were presented in
such a way as I thought most likely to attract the learner,
by stimulating at once his faculties of observation and
reflection. The continued sale of large editions of these
little books in this country and in America, and the trans-
lation of them into most European languages, leads me to
believe that the practical methods of instruction adopted
in them have been found useful. They were followed in
1877 by my Class-Book of Physical Geography, in which,
upon as far as possible the same line of treatment, the sub-
ject was developed with greater breadth and fulness. This
volume was meant to be immediately succeeded by a corre-
sponding one on Geology, but pressure of other engage-
ments has delayed till now the completion of this plan.
So many introductory works on Geology have been
written that some apology or explanation seems required
from an author who adds to their number. Experience
of the practical work of teaching science long ago con-
vinced me that what the young learner primarily needs is
a class-book which will awaken his curiosity and interest.
There should be enough of detail to enable him to under-
stand how conclusions are arrived at. All through its
VI PREFACE.
chapters he should see how observation, generalisation,
and induction go hand in hand in the progress of scientific
research. But it should not be overloaded with technical
details which, though of the highest importance, cannot
be adequately understood until considerable advance has
been made in the study. It ought to present a broad,
luminous picture of each branch of the subject, necessarily,
of course, incomplete, but perfectly correct and intelligible
as far as it goes. This picture should be 'amplified in
detail by a skilful teacher. It may, however, so arrest
the attention of the learner himself as to lead him to seek,
of his own accord, in larger treatises, fuller sources of infor-
mation. To this ideal standard of a class-book I have
striven in some measure to approach.
Originally, I purposed that this present volume should
be uniform in size with the Class-Book of Physical Geography.
But, as the illustrations were in progress, the advantage of
adopting a larger page became evident, and with this greater
scope and my own enthusiasm for the subject the book
has gradually grown into what it now is. With few excep-
tions, the woodcuts have been drawn and engraved expressly
for this volume. Mr. Sharman has kindly made for me
most of the drawings of the fossils. The landscape
sketches are chiefly from my own note-books. I have to
thank Messrs. J. D. Cooper and M. Lacour for the skill
with which they have given in wood-engraving the expres-
sion of the originals.
2&th December 1885.
CONTENTS.
CHAPTER I. PAGE
INTRODUCTORY I
PART I.
THE MATERIALS FOR THE HISTORY OF THE
EARTH.
CHAPTER II.
THE INFLUENCE OF THE ATMOSPHERE IN THE
CHANGES OF THE EARTH'S SURFACE . . 13
CHAPTER III.
THE INFLUENCE OF RUNNING WATER IN GEOLOGICAL
CHANGES, AND HOW IT IS RECORDED . . 3!
CHAPTER IV.
THE MEMORIALS LEFT BY LAKES . . . .56
CHAPTER V.
HOW SPRINGS LEAVE THEIR MARK IN GEOLOGICAL HIS-
TORY 66
via CONTENTS.
CHAPTER VI.
ICE-RECORDS . 82
CHAPTER VII.
THE MEMORIALS OF THE PRESENCE OF THE SEA . 94
CHAPTER VIII.
HOW PLANTS AND ANIMALS INSCRIBE THEIR RECORDS
IN GEOLOGICAL HISTORY . . . . IO8
CHAPTER IX.
THE RECORDS LEFT BY VOLCANOES AND EARTHQUAKES 124
PART II.
ROCKS, AND HOW THEY TELL THE HISTORY
OF THE EARTH.
CHAPTER X.
THE MORE IMPORTANT ELEMENTS AND MINERALS OF
THE EARTH'S CRUST 153
CHAPTER XI.
THE MORE IMPORTANT ROCKS AND ROCK-STRUCTURES
IN THE EARTH'S CRUST . . .185
CONTENTS. ix
PART III.
THE STRUCTURE OF THE CRUST OF THE EARTH.
CHAPTER XII. PAGE
SEDIMENTARY ROCKS THEIR ORIGINAL STRUCTURES 226
CHAPTER XIII.
SEDIMENTARY ROCKS STRUCTURES SUPERINDUCED IN
THEM AFTER THEIR FORMATION . . . 244
CHAPTER XIV.
ERUPTIVE ROCKS AND MINERAL VEINS IN THE ARCHI-
TECTURE OF THE EARTH'S CRUST . . .263
CHAPTER XV.
HOW FOSSILS HAVE BEEN ENTOMBED AND PRESERVED,
AND HOW THEY ARE USED IN INVESTIGATING
THE STRUCTURE OF THE EARTH'S CRUST, AND
IN STUDYING GEOLOGICAL HISTORY . . .279
PART IV.
THE GEOLOGICAL RECORD OF THE HISTORY OF
THE EARTH.
CHAPTER XVI.
THE EARLIEST CONDITIONS OF THE GLOBE THE
ARCHAEAN PERIODS . . .298
CONTENTS.
CHAPTER XVII.
PAGE
THE PALAEOZOIC PERIODS SILURIAN . . -312
CHAPTER XVIII.
DEVONIAN AND OLD RED SANDSTONE . . -337
CHAPTER XIX.
CARBONIFEROUS ....... 348
CHAPTER XX.
PERMIAN . , . . . . . . -370
CHAPTER XXI.
THE MESOZOIC PERIODS TRIASSIC . . . -379
CHAPTER XXII.
JURASSIC ... 389
CHAPTER XXIII.
CRETACEOUS 46
CHAPTER XXIV.
TERTIARY OR CAINOZOIC EOCENE OLIGOCENE . 423
CONTENTS. xi
CHAPTER XXV. PAGE
MIOCENE PLIOCENE ...... 439
CHAPTER XXVI.
POST-TERTIARY OR QUATERNARY PERIODS PLEIS-
TOCENE OR POST-PLIOCENE RECENT . . -455
APPENDIX . 479
INDEX 499
LIST OF ILLUSTRATIONS.
1. Weathering of rock, as shown by old masonry. (The "false-
bedding " and other original structures of the stone are revealed
by weathering) . . . . . . . . .16
2. Passage of sandstone upwards into soil ..... 20
3. Passage of granite upwards into soil ..... 20
4. Talus-slopes at the foot of a line of cliffs ..... 26
5. Section of rain-wash or brick-earth ...... 26
6. Sand-dunes .......... 28
7. Erosion of limestone by the solvent action of a peaty stream,
Durness, Sutherlandshire ....... 33
8. Pot-holes worn out by the gyration of stones in the bed of a stream 40
9. Grand Canon of the Colorado ...... 44
10. Gullies torn out of the side of a mountain by descending torrents,
with cones of detritus at their base ..... 47
11. Flat stones in a bank of river-shingle, showing the direction of
the current that transported and left them .... 49
12. Section of alluvium showing direction of currents ... 50
13. River-terraces . . . . . . . . .51
14. Alluvial terraces on the side of an emptied reservoir . . 58
15. Parallel roads of Glen Roy . . . . . -59
1 6. Stages in the filling up of a lake ...... 60
17. Piece of shell-marl containing shells of Limna'a peregra . . 62
18. View of Axmouth landslip as it appeared in April 1885 . . 69
19. Section of cavern with stalactites and stalagmite ... 72
20. Section showing successive layers of growth in a stalactite . . 75
21. Travertine with impressions of leaves ..... 79
22. Glaciers and moraines ........ 85
23. Perched blocks scattered over ice-worn surface of rock . . 86
24. Stone smoothed and striated by glacier-ice . . . .89
25. Ice-striation on the floor and side of a valley .... 90
26. Duller of Buchan a caldron-shaped cavity or blow-hole worn
out of granite by the sea on the coast of Aberdeenshire . . 9^
27. The Stacks of Duncansby, Caithness, a wave-beaten coast-line . 97
28. Section of submarine plain ....... 100
29. Storm-beach ponding back a stream and forming a lake ; west
coast of Sutherlandshire . . . . . . .103
xiv LIST OF ILLUSTRATIONS.
FIG. PAGE
30. Section of a peat-bog . . . . . . . .no
31. Diatom -earth from floor of Antarctic Ocean, magnified 300
diameters. . . . . . . . . .112
32. Recent limestone (cockle, etc. ) . . . . . .114
33. Globigerina ooze magnified . , . , . . -115
34. Section of a coral-reef . . . . . . . .116
35. Cellular lava with a few of the cells filled up with infiltrated
mineral matter (Amygdules) . . . . . .130
36. Section of a lava-current . . . . . . -131
37. Elongation of cells in direction of flow of a lava-stream . . 132
38. Volcanic block ejected during the deposition of strata in water . 137
39. Volcanoes on lines of fissure . . . . . . .139
40. Outline of a volcanic neck ....... 142
41. Ground-plan of the structure of the Neck shown in Fig. 40 . 142
42. Section through the same Neck as in Figs. 40 and 41 . 143
43. Volcanic dykes rising through the bedded tuff of a crater . . 145
44. Group of quartz-crystals (Rock-crystal) . . . . 157
45. Calcite (Iceland spar) showing its characteristic rhombohedral
cleavage . . . . . . . . . .166
46. Cube, octahedron, dodecahedron . . . . . .167
47. Tetragonal prism and pyramid . . . . . .167
48. Orthorhombic prism . . . . . . . .167
49. Hexagonal prism, rhombohedron, and scalenohedron . . 168
50. Monoclinic prism. Crystal of Augite ..... 168
51. Triclinic prism. Crystal of Albite felspar .... 168
52. Section of a pebble of chalcedony . . . . . .171
53. Piece of haematite, showing the nodular external form and the
internal crystalline structure . . . . . .172
54. Octahedral crystals of magnetite in chlorite schist . . .173
55. Dendritic markings due to arborescent deposit of earthy manganese
oxide .......... 174
56. Cavity in a lava, filled with zeolite which has crystallised in long
slender needles . . . . . . . . .176
57. Hornblende crystal . . . . . . . 177
58. Olivine crystal . . . . . . . . .178
59. Calcite in the form of nail-head spar . . . . .179
60. Calcite in the form of dog-tooth spar . . . . .180
61. Sphaerosiderite or Clay-ironstone concretion enclosing portion of
a fern . . . . . . . . . .181
62. Gypsum crystals . . . . . . . . .182
63. Group of fluor-spar crystals . . . . . . .183
64. Concretions . . . . . . . . . .187
65. Section of a septarian nodule, with coprolite of a fish as a nucleus 188
66. Piece of oolite 189
67. Piece of pisolite ......... 189
68. Cavities in quartz containing liquids (magnified) . . . 190
69. Crystallites 191
70. Porphyritic structure . . . . . . . .192
71. Spherulites and fluxion-structure . . . . . .192
LIST OF ILLUSTRATIONS. xv
FIG.
PAGE
72. Schistose structure . . . . . I 94
73. Brecciated structure volcanic breccia, a rock composed of
angular fragments of lava, in a paste of finer volcanic debris 197
74. Conglomerate . . . . . .198
75. Concretionary forms assumed by Dolomite, Magnesian Lime-
stone, Durham ........ 206
76. Weathered surface of crinoidal limestone . . . .210
77. Group of crystals of felspar, quartz, and mica, from a cavity in
the Mourne Mountain granite . . . . . .216
78. Columnar basalts of the Isle of Staffa, resting upon tuff (to the
right is Fingal's Cave) ....... 219
79. Section of stratified rocks ....... 227
80. Section showing alternation of beds ..... 230
8 1. False-bedded sandstone . . . . . . .231
82. Ripple-marked surface ....... 232
83. Cast of a sun-cracked surface preserved in the next succeeding
layer of sediment ........ 233
84. Rain-prints on fine mud ....... 234
85. Vertical trees (Sigillaria) in sandstone, Swansea (Logan) . 238
86. Hills formed out of horizontal sedimentary rocks . . . 239
87. Section of overlap ........ 240
88. Unconformability . . . . . . . .241
89. Joints in a stratified rock ....... 246
90. Dip and Strike ......... 248
91. Clinometer. ......... 248
92. Dip, Strike, and Outcrop ....... 249
93. Inclined strata shown to be parts of curves .... 250
94. Curved strata (anticlinal fold), near St. Abb's Head . .251
95. Curved strata (synclinal fold), near Banff .... 252
96. Anticlines and Synclines ....... 253
97. Section of the Grosse Windgalle (10,482 feet), Canton Uri,
Switzerland, showing crumpled and inverted strata (after Heim ) 254
98. Distortion of fossils by the shearing of rocks . . . -255
99. Curved and cleaved rocks. Coast of Wigtonshire . . .256
100. Examples of normal Faults . . . . . . .257
101. Sections to show the relations of Plications to reversed
Faults 258
102. Throw of a Fault ........ 259
103. Ordinary unaltered red sandstone, Keeshorn, Ross-shire . 260
104. Sheared red sandstone forming now a micaceous schist, Kee-
shorn, Ross-shire ........ 260
105. Outline and section of a Boss traversing stratified rocks . . 265
1 06. Ground-plan of Granite-boss with ring of Contact -Meta-
morphism ......... 267
107. Intrusive Sheet ......... 268
108. Interstratified or contemporaneous Sheets .... 269
109. Section to illustrate evidence of contemporaneous volcanic
action .......... 270
no. Map of Dykes near Muirkirk, Ayrshire . .... 273
xvi LIST OF ILLUSTRATIONS.
FIG. PAGE
in. Section of a volcanic neck . . . . . . 274
112. Section of a mineral vein ....... 276
113. Common Cockle (Card-turn edule] ..... 285
114. Fragment of crumpled Schist ...... 309
115. A, Fucoid-like impression (Eophyton Linneanuni) from Cam-
brian rocks (g). B, An Upper Silurian sea-weecl (Clwndrites
verisimilis], natural size . . . . . . .319
116. Oldhamia radiata (natural size), Ireland . . . .321
117. Graptolites . .....'.... 323
118. Hydrozoon from the Cambrian rocks ..... 324
119. Silurian Rugose Coral ........ 325
1 20. Silurian Alcyonarian Coral . . . . . . .325
121. Silurian Cystidean ........ 326
122. Silurian Star-fish . ........ 326
123. Filled-up Burrows or Trails left by a sea-worm on the bed of
the Silurian sea . . . . . . . .327
124. Trilobites (Primordial or Cambrian) ..... 328
125. Trilobites (Lower and Upper Silurian) ..... 329
126. Silurian Phyllopod Crustacean . . . . . 330
127. Silurian Brachiopods . . . . . . . -331
128. Silurian Lamellibranch . . . . . . .332
129. Silurian Gasteropod ........ 332
130. Silurian Cephalopods ........ 334
131. Plants of the Devonian period ...... 339
132. Overlapping scales of an Old Red Sandstone fish . . . 340
133. Scale-covered Old Red Sandstone fishes .... 341
134. Plate-covered Old Red Sandstone fishes . . . 341
135. Devonian Eurypterid Crustacean ...... 343
136. Devonian trilobites . . . . . . . . 344
137. Devonian corals ......... 345
138. Devonian Brachiopods ....... 346
139. Devonian Lamellibranch and Cephalopod .... 347
140. Section of part of the Cape Breton coalfield, showing a succes-
sion of buried trees and land-surfaces . . . 352
141. Carboniferous Ferns ........ 355
142. Carboniferous Lycopod . . . . . ... 356
143. Carboniferous Equisetaceous Plants ..... 357
144. Sigillaria with Stigmaria roots . . . . . -357
145. Cordaites alloidius . . . . . . . .358
146. Carboniferous Foraminifer . . . . . . .361
147. Carboniferous Rugose Corals . . . . . .361
148. Carboniferous Sea-Urchin ....... 362
149. Carboniferous Crinoid ....... 362
150. Carboniferous Blastoid ....... 362
151. Carboniferous Trilobite ....... 363
152. Carboniferous Polyzoon ....... 364
153. Carboniferous Brachiopods ....... 365
154. Carboniferous Lamellibranchs ...... 366
LIST OF ILLUSTRATIONS. xvil
FIG. P AGE
155. Carboniferous Gasteropods ....... 366
156. Carboniferous Pteropod ....... 367
157. Carboniferous Cephalopods ...... 367
158. Carboniferous Fishes ........ 368
159. Permian Plants . ........ 373
160. Permian Brachiopods ........ 374
1 6 1. Permian Lamellibranchs ....... 375
162. Permian Ganoid Fish ........ 375
163. Permian Labyrinthodont ....... 376
164. Triassic Plants 382
165. Triassic Crinoid . . . . . . . . -383
1 66. Triassic Lamellibranchs . . . . '. . -384
167. Triassic Cephalopods ........ 384
168. Triassic Lizard 385
169. Triassic Crocodile Scutes . . . . . . -385
170. Triassic Marsupial Teeth ....... 386
171. Jurassic Cycad ......... 390
172. Jurassic reef-building Coral ....... 391
173. Jurassic Crinoid ........ 392
174. Jurassic Sea-urchin ........ 393
175. Jurassic Lamellibranchs ....... 394
176. Jurassic Ammonites ........ 395
177. Jurassic Belemnite . . . . . . . 395
178. Jurassic Crustacean ........ 396
179. Jurassic Fish ......... 396
1 80. Jurassic Sea-lizard ........ 397
181. Jurassic Pterosaur, or flying reptile ..... 398
182. Jurassic Bird ......... 400
183. Jurassic Marsupial Teeth and Jaw ..... 400
184. Cretaceous Plants ........ 409
185. Cretaceous Foraminifera ....... 410
1 86. Cretaceous Sponge . . . . . . . .410
187. Cretaceous Sea-urchins . . . . . . .411
1 8 8. Cretaceous Lamellibranchs . . . . . . .412
189. Cretaceous Lamellibranchs . . . . . . .413
190. Cretaceous Cephalopods ....... 414
191. Cretaceous Fish ......... 415
192. Cretaceous Deinosaur . 416
193. Eocene Plant ......... 428
194. Eocene Molluscs ........ 429
195. Eocene Mammal . . . . . . . .431
196. Skull of Tinoceras ingens . 432
197. Oligocene Molluscs ........ 436
198. Miocene Plants ......... 441
199. Mastodon augustidens ....... 442
200. Skull of Deinotherium giganteum . . . . .442
20 1. Pliocene Plants ......... 449
202. Pliocene Marine Shells ....... 450
xvm LIST OF ILLUSTRATIONS.
FIG. PAGE
203. Helladotherium Duvernoyi a gigantic animal intermediate in
structure between the giraffe and the antelope, Pikermi,
Attica . . . . . . . . . . 453
204. Pleistocene or Glacial Shells . . . . . .462
205. Mammoth from the skeleton in the Muse"e Royal, Brussels . 463
206. Back view of skull of Musk-sheep, Brick-earth, Crayford, Kent 463
207. Palaeolithic Implements ....... 470
208. Antler of Reindeer found at Bilney Moor, East Dereham, Norfolk 473
209. Neolithic Implements 475
CHAPTER I.
INTRODUCTORY.
THE main features of the dry land on which we live seem
to remain unchanged from year to year. The valleys and
plains familiar to our forefathers are still familiar to us, bear-
ing the same meadows and woodlands, the same hamlets
and villages, though generation after generation of men has
meanwhile passed away. The hills and mountains now
rise along the sky-line as they did long centuries ago,
catching as of old the fresh rains of heaven and gathering
them into the brooks and rivers which, through unknown
ages, have never ceased to flow seawards. So steadfast do
these features appear to stand, and so strong a contrast do
they offer to the shortness and changeableness of human
life, that they have become typical in our minds of all that
is ancient and durable. We speak of the firm earth, of the
everlasting hills, of the imperishable mountains, as if, where
all else is fleeting and mutable, these forms at least remain
unchanged.
And yet attentive observation of what takes place from
day to day around us shows that the surface of a country is
not now exactly as it used to be. We notice various changes
B
2 , t c t c c t , 1 1 , INTRODUCTORY. [CHAP.
of its topography going on now, which have doubtless been
in progress for a long time, and the accumulated effect of
which may ultimately transform altogether the character of
landscapes. A strong gale, for instance, will level thousands
of trees in its pathway, turning a tract of forest or woodland
into a bare space, which becomes perhaps a quaking morass,
or may be changed into arable ground by the farmer. A
flooded river will in a few hours cut away large slices from
its banks, and spreading over fields and meadows, will bury
many acres of fertile land under a covering of barren sand
and shingle. A long-continued, heavy rain, by loosening
masses of earth or rock on steep slopes, causes destructive
landslips. A hard frost splinters the naked fronts of crags
and cliffs, and breaks up bare soil. In short, every shower
of rain and gust of wind, if we could only watch them
narrowly enough, would be found to have done something
towards modifying the surface of the land. Along the sea-
margin, too, how ceaseless is the progress of change ! In
most places, the waves are cutting away the land, sometimes
even at so fast a rate as two or three feet in a year. Here
and there, on the other hand, they cast sand and silt ashore
so as to increase the breadth of the dry land.
These are ordinary everyday causes of alteration, and
though singly insignificant enough, their united effect after
long centuries cannot but be great. From time to time,
however, other less frequent but more powerful influences
come into play. In most large regions of the globe, the
ground is often convulsed by earthquakes, many of which
leave permanent scars upon the surface of the land. Vol-
canoes, too, in many countries pour forth streams of molten
rock and showers of dust and cinders that bury the surround-
ing districts and greatly alter their appearance.
i.] GEOLOGICAL CHANGES WITNESSED BY MAN. 3
Turning to the pages of human history, we find there
the records of similar changes in bygone times. Lakes, on
which our rude forefathers paddled their canoes and built
their wattled island -dwellings, have wholly disappeared.
Bogs, over whose treacherous surface they could not follow
the chase of red deer or Irish elk, have become meadows
and fields. Forests, where they hunted the wild boar, have
been turned into grassy pastures. Cities have been entirely
destroyed by earthquakes or have been entombed under
the piles of ashes discharged from a burning mountain.
So great have been the inroads of the sea that, in some
instances, the sites of what a few hundred years ago were
farms and hamlets, now lie under the sea half a mile or
more from the modern shore. Elsewhere the land has
gained upon the sea, and the harbours of an earlier time
are now several miles distant from the coast-line.
But man has naturally kept note only of the more im-
pressive changes, in other words, of those which had most
influence upon his own doings. We may be certain, how-
ever, that there have been innumerable minor alterations of
the surface of the land within human history, of which no
chronicler has made mention, either because they seemed
too trivial, or because they took place so imperceptibly as
never to be noticed. Fortunately, in many cases, these
mutations of the land have written their own memorials,
which can be as satisfactorily interpreted as the ancient
manuscripts from which our early national history is com-
piled.
In illustration of the character of these natural chronicles,
let us for a moment consider the subsoil beneath cities that
have been inhabited for many centuries. In London, for
example, when excavations are made for drainage, building,
4 INTRODUCTORY. [CHAP.
or other purposes, there are sometimes found, many feet
below the level of the present streets, mosaic pavements and
foundations, together with earthen vessels, bronze implements,
ornaments, coins, and other relics of Roman time. Now, if
we knew nothing, from actual authentic history, of the exist-
ence of such a people as the Romans, or of their former
presence in England, these discoveries, deep beneath the
surface of modern London, would prove that long before the
present streets were built, the site of the city was occupied
by a civilised race which employed bronze and iron for the
useful purposes of life, had a metal coinage, and showed
not a little artistic skill in its pottery, glass, and sculpture.
But down beneath the rubbish wherein the Roman remains
are embedded, lie gravels and sands from which rudely-
fashioned human implements of flint have been obtained.
Whence we further learn that, before the civilised metal-using
people appeared, an earlier race had been there, which
employed weapons and instruments of roughly chipped flint.
That this was the order of appearance of the successive
peoples that have inhabited the site of London is, of course,
obvious. But let us ask ourselves why it is obvious. We
observe that there are, broadly speaking, three layers or
deposits from which the evidence is derived. The upper
layer is that which contains the foundations and rubbish of
modern London. Next comes that which encloses the
relics of the Roman occupation. At the bottom lies the
layer that preserves the scanty traces of the early flint-folk.
The upper deposit is necessarily the newest, for it could not
be laid down until after the accumulation of those below it,
which must, of course, be progressively older, as they are
traced deeper from the surface. By the mere fact that the
layers lie one above another, we are furnished with a simple
i.] GEOLOGICAL METHODS. 5
clue which enables us to determine their relative time of
formation. We may know nothing whatever as to how old
they are. But we can be absolutely certain of what is
termed their "order of superposition," and this order marks
their chronological sequence, that is, it shows that the bottom
layer came first and the top layer last.
This kind of observation and reasoning will enable us to
detect almost everywhere proofs that the surface of the land
has not always been what it is to-day. In some districts, for
example, when the dark layer of vegetable soil is turned up
which supports the plants that keep the land so green, there
may be found below it sand and gravel, full of smooth well-
rounded stones. Such materials are to be seen in the course
of formation where water keeps them moving to and fro, as
on the beds of rivers, the margins of lakes, or the shores of
the sea. Wherever smoothed rolled pebbles occur, they
point to the influence of moving water ; so that we conclude,
even though the site is now dry land, that the sand and
gravel underneath it prove it to have been formerly under
water. Again, below the soil in other regions, lie layers of
oysters and other sea-shells. These remains, spread out
like similar shells on the beach or bed of the sea at the
present day, enable us to infer that where they lie the sea
once rolled.
Pits, quarries, or other excavations that lay open still
deeper layers of material, bring before us interesting and
impressive testimony regarding the ancient mutations of the
land. Suppose, by way of further illustration, that under-
neath a bed of sand full of oyster-shells, there lies a dark
. brown band of peat. This substance, composed of mosses
and other water-loving plants, is formed in boggy places by
the growth of marshy vegetation. Below the peat, there
6 INTRODUCTORY. [CHAP.
might occur a layer of soft white marl full of lake-shells,
such as may be observed on the bottoms of many lakes at
the present time. These three layers oyster-bed, peat, and
marl would present a perfectly clear and intelligible record
of a curious series of changes in the site of the locality.
The bottom layer of white marl with its peculiar shells
would show that at one time the place was occupied by a
lake. The next layer of peat would indicate that, by the
growth of marshy vegetation, the lake was gradually changed
into a morass. The upper layer of oyster-shells would prove
that the ground was then submerged beneath the sea. The
present condition of the ground shows that subsequently the
sea retired and the locality passed into dry land as it is
to-day.
It is evident that by this method of examination in-
formation may be gathered regarding early conditions of
the earth's surface, long before the authentic dates of human
history. Such inquiries form the subject of Geology, which
is the science that investigates the history of the earth. The
records in which this history is chronicled are the soils and
rocks under our feet. It is the task of the geologist so to
arrange and interpret these records as to show through what
successive changes the globe has passed, and how the dry
land has come to wear the aspect which it presents at the
present time.
Just as the historian would be wholly unable to decipher
the inscriptions of an ancient race of people unless he had
first discovered a key to the language in which they are
written, so the geologist would find himself baffled in his
efforts to trace backward the history of the earth if he were
not provided with a clue to the interpretation of the records
in which that history is contained. Such a clue is furnished
T.] GEOLOGICAL METHODS. 7
to him by a study of the operations of nature now in pro-
gress upon the earth's surface. Only in so far as he makes
himself acquainted with these modern changes, can he hope
to follow intelligently and successfully the story of earlier
phases in the earth's progress. It will be seen that this
truth has already been illustrated in the instances above
given of the evidence that the surface of the land has not
been always as it is now. The beds of sand and gravel, of
oyster-shells, of peat and of marl, would have told us nothing
as to ancient geography had we not been able to ascertain
their origin and history by finding corresponding materials
now in course of accumulation. To one ignorant of the
peculiarities of fresh-water shells, the layer of marl would
have conveyed no intelligible meaning. But knowing and
recognising these peculiarities, we feel sure that the marl
marks the site of a former lake. Thus the study of the
present supplies a key that unlocks the secrets of the past.
In order, therefore, to trace back the history of the earth,
the geologist must begin by carefully watching the changes
that now take place upon the earth, and by observing how
nature elaborates the materials that preserve more or less
completely the record of these changes. In the following
pages, I propose to follow this method of inquiry, and, as
far as the subject will permit, to start with no assumptions
which the learner cannot easily verify for himself. We shall
begin with the familiar everyday operations of the air, rain,
frost, and other natural agents. As these have been fully
described in my Class-Book of Physical Geography, it will
not be needful here to consider them again in detail. We
shall rather pass on to inquire in what various ways they
are engaged in contributing to the formation of new mineral
accumulations, and in thereby providing fresh materials for
8 INTRODUCTORY. [CHAP.
the preservation of the facts on which geological history is
founded. Having thus traced how new rocks are formed,
we may then proceed to arrange the similar rocks of older
time, marking what are the peculiarities of each and how
they may best be classified.
If the labours of the geologist were concerned merely
with the former mutations of the earth's surface, how sea
and land have changed places, how rivers have altered their
courses, how lakes have been filled up, how valleys have
been excavated, how mountains, peaks, and precipices have
been carved, how plains have been spread out, and how the
story of these revolutions has been written in enduring
characters upon the very framework of the land, he would
feel the want of one of the great sources of interest in the
study of the present face of nature. We naturally connect
all modern changes of the earth's surface with the life of the
plants and animals that flourish there, and more especially
with their influence on the progress of man himself. If
there were no similar connection of the ancient changes
with once living things if the history of the earth were
merely one of dead inert matter it would lose much of its
interest for us. But happily that history includes the records
of successive generations of plants and animals which, from
early times, have peopled land and sea. The remains of
these organisms have been preserved in the deposits of
different ages, and can be compared and contrasted with
those of the modern world.
To realise how such preservation has been possible, and
how far the forms so retained afford an adequate picture of
the life of the time to which they belonged, we must turn
once more to watch how nature deals with this matter at
the present time. Of the millions of flowers, shrubs, and
i.] NATURAL CHRONICLES. 9
trees which year after year clothe the land with beauty, how
many relics are preserved ? Where are the successive gener-
ations of insect, bird, and beast which have appeared in this
country since man first set foot upon its soil ? They have
utterly vanished. If all their living descendants could
suddenly be swept away, how could we tell that such plants
and animals ever lived at all ? It must be confessed that
of the vast majority not a trace remains. Nevertheless we
should be able to recover relics of some of them by searching
in the comparatively few places where, at the present day, we
see that the remains of plants and animals are entombed
and preserved. From the alluvial terraces of rivers, from
the silt of lake-bottoms, from the depths of peat-mosses, from
the floors of subterranean caverns, from the incrustations left
by springs, we might recover traces of some at least of the
plants and animals. And from these fragmentary and in-
complete records we might conjecture what may have been
the general character of the life of the time. By searching
the similar records of earlier ages the geologist has brought
to light many profoundly interesting vestiges of vegetation
and of animal life belonging to types that have long since
passed away.
It must be evident, however, that if we confine our
inquiries merely to its surface we shall necessarily gain a
most imperfect view of the general history of the earth.
Beneath that surface, as volcanoes show, there lies a hot
interior, which must have profoundly influenced the changes
of the outer parts or crust of the planet. The study of vol-
canoes enables us to penetrate, as it were, a little way into
that interior, and to understand some of the processes in
progress there. But our knowledge of the inside of the
earth can obviously be based only to a very limited extent
10 INTRODUCTORY. [CHAP.
on direct observation, for man cannot penetrate far below
the surface. The deepest mines do not go deep enough
to reach materials differing in any essential respect from
those visible above ground. Nevertheless, by inference
from such observations as can be made, and by repeated
and varied experiments in laboratories, imitating as closely
as can be devised what may be supposed to be the conditions
that exist deep within the globe, some probable conclusions
can be drawn even as to the changes that take place in those
deeper recesses that lie for ever concealed from our eyes.
These conclusions will be stated and the rocks will be
described, on the origin of which they appear to throw light.
I have compared the soils and rocks with which geology
deals to the records out of which the historian writes the
chronicles of a nation. We might vary the simile by liken-
ing them to the materials employed in the construction of a
great building. It is of course interesting enough to know
what kinds of marble, granite, mortar, wood, brass, or iron,
have been chosen by an architect. But much more import-
ant is it to inquire how these various substances have been
grouped together so as to form such a building. In like
manner, besides the nature and mode of origin of the various
rocks of which the visible and accessible part of the earth
consists, we ought to know how these varied substances have
been arranged so as to build up what we can see of the
terrestrial crust. In short, we should try to trace the archi-
tecture of the globe, noting how each variety of rock occupies
its own characteristic place, and how they are all grouped
and braced together to form the solid framework of the land.
This then will be the next subject for consideration.
But in a great historical edifice, like one of the minsters
of Europe, for example, there are often several different
i.] PRINCIPLES FOR THE LEARNER S GUIDANCE, n
styles. A student of architecture can detect these distinc-
tions, and by their means can show that a cathedral has not
been completed in one age ; that it may even have been
partially destroyed and rebuilt during successive centuries,
only finally taking its present form after many political
vicissitudes and changes of architectural taste. Each edifice
has thus a separate history, which is recorded by the way
the materials have been shaped and put together in the
various parts of the masonry. So it is with the architecture
of the earth. We have evidence of many demolitions and
rebuildings, and the story of their general progress can still be
deciphered among the rocks. It is the business of geology
to trace out that story, to put all the scattered materials
together, and to make known by what a long succession of
changes the earth has reached its present state. An outline
of what geology has accomplished in this task will form the
last and concluding part of this volume.
In the following chapters I wish two principles to be kept
steadily in view. In the first place, looking upon geology
as the study of the earth's history, we need not at first
concern ourselves with any details, save those that may be
needed to enable us clearly to understand what the general
character and progress of this history have been. In a
science which embraces so vast a range as geology, the
multiplicity of facts to be examined and remembered may
seem at first to be almost overwhelming. But a selection
of the essential facts is sufficient to give the learner a clear
view of the general principles and conclusions of the science,
and to enable him to enter with intelligence and interest
into more detailed treatises. In the second place, geology
is essentially a science of observation. The facts with which
it deals should, as far as possible, be verified by our own
12 INTRODUCTORY. [CHAP. i.
personal examination. We should lose no opportunity of
seeing with our own eyes the actual progress of the changes
which it investigates, and the proofs which it adduces of
similar changes in the far past. To do this will lead us into
the fields and hills, to the banks of rivers and lakes, and to
the shores of the sea. We can hardly take any country walk,
indeed, in which with duly observant eye we may not detect
either some geological operation in actual progress, or the
evidence of one which has now been completed. Having
learnt what to look for and how to interpret it when seen,
we are as it were gifted with a new sense. Every land-
scape comes to possess a fresh interest and charm, for we
carry about with us everywhere an added power of enjoy-
ment, whether the scenery has long been familiar or presents
itself for the first time. I would therefore seek at the outset
to impress upon those who propose to read the following
pages, that one of the main objects with which this book is
written is to foster a habit of observation, and to serve as a
guide to what they are themselves to look for, rather than
merely to relate what has been seen and determined by
others. If they will so learn these lessons, I feel sure that
they will never regret the time and labour they may spend
over the task.
PART I.
THE MATERIALS FOR THE HISTORY OF
THE EARTH.
CHAPTER II.
THE INFLUENCE OF THE ATMOSPHERE IN THE CHANGES OF
THE EARTH'S SURFACE.
IN the history of mankind, no sharp line can be drawn
between the events that are happening now or have
happened within the last few generations, and those that
took place long ago, and which are sometimes, though
inaccurately, spoken of as historical. Every people is
enacting its history to-day just as fully as it did many cen-
turies ago. The historian recognises this continuity in
human progress. He knows that the feelings and aspira-
tions which guided mankind in old times were essentially
the same influences that impel them now, and therefore
that the wider his knowledge of his fellowmen of the present
day, the broader will be his grasp in dealing with the trans-
actions of former generations. So too is it with the history
of the earth. That history is in progress now as really as
it has ever been, and its events are being recorded in the
14 GEOLOGICAL WORK OF THE AIR. [CHAP.
same way and by the same agents as in the far past. Its
continuity has never been broken. Obviously, therefore,
if we would explore its records "in the dark backward and
abysm of time," we should first make ourselves familiar
with the manner in which these records are being written
from day to day before our eyes.
In this first Part, attention will accordingly be given to
the changes in progress upon the earth at the present time,
and to the various ways in which the passing of these changes
is chronicled in natural records. We shall watch the actual
transaction of geological history, and mark in what way its
incidents inscribe themselves on the page of the earth's
surface. 1 Every day and hour some geological event,
trifling and transient or stupendous and durable, comes and
goes, leaving sometimes only an imperceptible trace of its
passage, at other times graving itself almost imperishably
in the annals of the globe. Tracing the origin and develop-
ment of these geological annals of the present time, we
shall best qualify ourselves for deciphering the records of
the early revolutions of the planet. We are thus led to
study the various chronicles compiled respectively by the
air, rain, rivers, springs, glaciers, the sea, plants and animals,
volcanoes, and earthquakes in other words, all the deposits
left by the operations of these agents, the scars or other
features made by them upon the earth's surface, and all
other memorials of geological change. Having learnt how
modern deposits are produced, and how they preserve the
story of their origin, we shall then be able to group with
them the corresponding deposits of earlier times, and to
1 For descriptions of the ordinary operations of geological agents the
reader is referred to my Class- Book of Physical Geography. My object
now is to direct attention to what is most enduring in these operations,
and in what various ways they form permanent geological records.
ii.] UNIVERSAL DECAY OF EARTH S SURFACE. 15
embrace all the geological records, ancient as well as
modern, in one general scheme of classification. Such a
scheme will enable us to see the continuity of the materials of
geological history, and will fix definitely for us the character
and relative position of all the chief rocks out of which
the visible part of the globe is composed.
The gradual change that overtakes everything on the face
of the earth is expressed in all languages by familiar phrases
which imply that the mere passing of time is the cause of
the change. As Sir Thomas Browne quaintly said more than
two hundred years ago, " time antiquates antiquities, and
hath an art to make dust of all things." We speak of the
dust of antiquity and the gnawing tooth of time. We say
that things are time-eaten, worn with age, crumbling under
a weight of years. Nothing suggests such epithets so
strikingly as an old building. We know that the masonry
at first was smooth and fresh ; but now we describe it as
weather-beaten, decayed, corroded. So distinctive is this
appearance that it is always looked for in an ancient piece
of stone-work ; and if not seen, its absence at once suggests
a doubt whether the masonry can really be old. No matter
of what varieties of stone the edifice may have been built,
a few generations may be enough to give them this look
of venerable antiquity. The surface that was left smoothly
polished by the builders grows rough and uneven, with
scars and holes eaten into it. Portions of the original
polish that may here and there have escaped, serve as a
measure of how much has actually been removed from the
rest of the surface.
Now, if in the lapse of time, stone which has been
artificially dressed is wasted away, we may be quite certain
that the same stone in its natural position on the slope of
i6
GEOLOGICAL WORK OF THE AIR.
[CHAP.
a hill or valley, or by the edge of a river or of the sea, must
decay in a similar way. Indeed, an examination of any
crumbling building will show
that, in proportion as the
chiselled surface disappears,
the stone puts on the ordin-
ary look which it wears where
it has never been cut by
man, and where only the finger
of time has touched it. Could
we remove some of the de-
cayed stones from the build-
FIG. i. Weathering of rock, as, in g and insert them into a
shown by old masonry. (The natural crag or cliff of the
"false-bedding" and other ori- same kind of st their
gmal structures of the stone are
revealed by weathering.) peculiar time -worn aspect
would be found to be so ex-
actly that of the rest of the cliff that probably no one would
ever suspect that a mason's tools had once been upon them.
From this identity of surface between the time-worn
stones of an old building and the stone of a cliff we may
confidently infer that the decay so characteristic of ancient
masonry is as marked upon natural faces of rock. The
gradual disappearance of the artificial smoothness given by
the mason, and its replacement by the ordinary natural
rough surface of the*stone, shows that this natural surface
must also be the result of decay. And as the peculiar
crumbling character is universal, we may be sure that the
decay with which it is connected must be general over the
globe.
But the mere passing of time obviously cannot change
anything, and to say that it does is only a convenient
IT.] CAUSES OF WEATHERING. 17
figure of speech. It is not time, but the natural processes
which require time for their work, that produce the wide-
spread decay over the surface of the earth. Of these natural
processes, there are four that specially deserve consideration,
changes of temperature, saturation and desiccation, frost,
and rain.
(i.) Changes of Temperature. In countries where
the days are excessively hot, with nights correspondingly cool,
the surfaces of rocks heated sometimes, as in parts of Africa,
up to more than 130 Fahr. by a tropical sun, undergo con-
siderable expansion in consequence of this increase of
temperature. At night, on the other hand, the rapid radia-
tion quickly chills the stone and causes it to contract.
Hence the superficial parts, being in a perpetual state of
strain, gradually crack up or peel off. The face of a cliff is
thus worn slowly backward, and the prostrate blocks that
fall from it are reduced to smaller fragments and finally
to dust. Where, as in Europe and the settled parts of
North America, the contrasts of temperature are not so
marked, the same kind of waste takes place in a less striking
manner.
(2.) Saturation and Desiccation. Another cause of
the decay of the exposed surfaces of rocks is to be sought in
the alternate soaking of them with rain and drying of them
in sunshine, whereby the component particles of the stone
are loosened and fall to powder. Some kinds of stone
freshly quarried and left to this kind of action are rapidly
disintegrated. The rock called shale (see p. 202) is pecu-
liarly liable to decay from this cause. The cliffs into which
it sometimes rises show at their base long trails of rubbish
entirely derived from its waste.
(3.) Frost. Athird and familiar source of decay in stone
c
1 8 GEOLOGICAL WORK OF THE AIR. [CHAP.
exposed to the atmosphere is to be found in the action of
Frost. The water that falls from the air upon the surface
of the land soaks into the soil and into the pores of rocks.
When the temperature of the air falls below the freezing point,
the imprisoned moisture expands as it passes into ice, and
in expanding pushes aside the particles between which it is
entangled. Where this takes place in soil, the pebbles and
the grains of sand and earth are separated from each other
by the ice that shoots between them. They are all frozen
into a solid mass that rings like stone under our feet ; but,
as soon as a thaw sets in, the ice that formed the binding
cement passes into water which converts the soil into soft
earth or mud. This process, repeated winter after winter,
breaks up the materials of the soil and enables them to
be more easily made use of by plants and more readily
blown away by wind or washed off by rain. Where the
action of frost affects the surface of a rock, the particles
separated from each other are eventually blown or washed
away, or the rock peals off in thin crusts or breaks up
into angular pieces, which are gradually disintegrated and
removed.
(4.) Rain. One further cause of decay may be sought
in the remarkable power possessed by Rain of chemically
corroding stones. In falling through the atmosphere, rain
absorbs the gases of the air, and with their aid attacks sur-
faces of rock. With the oxygen thus acquired, it oxidises
those substances which can still take more of this gas, causing
them to rust. As a consequence of this alteration, the cohe-
sion of the particles is usually weakened, and the stone
crumbles down. With the carbon-dioxide or carbonic acid,
it dissolves and removes some of the more soluble ingre-
dients in the form of carbonates, thereby also usually loosen-
ir.] CAUSES OF WEATHERING. 19
ing the component particles of the stone. In general, the
influence of rain is to cause the exposed parts of rocks to
rot from the surface inward. Where the ground is protected
with vegetation, the decay is no doubt retarded ; but in the
absence of vegetation, the outer crust of the decayed layer
is apt to be washed off by rain, or when dried to powder
may be blown away and scattered by wind. As fast as it is
removed from the surface, however, it is renewed under-
neath by the continued soaking of rain into the stone.
Hence one of the first lessons to be learnt when from
the common evidence around us we seek to know what has
been the history of the ground on which we live is one of
ceaseless decay. All over the land, in all kinds of climates,
and from various causes, bare surfaces of soil and rock yield
to the influences of the atmosphere or weather. The decay
thus set in motion is commonly called " weathering." That
it may often be comparatively rapid is familiarly and in-
structively shown in buildings or open-air monuments of
which the dates are precisely known. Marble tombstones
in the graveyards of large towns, for example, hardly keep
their inscriptions legible for even so long as a century.
Before that time, the surface of the stone has crumbled
away into a kind of. sand. Everywhere the weather-eaten
surfaces, the crumbling crust of decayed stone, and the
scattered blocks and trains of rubbish, tell their tale of
universal waste.
It is well to take numerous opportunities of observing
the process of this decay in different situations and on
various kinds of materials. We can thus best realise the
important part which weathering must play in the changes
of the earth's surface, and we prepare ourselves for the con-
sideration of the next question that arises, What becomes
2O
GEOLOGICAL WORK OF THE AIR.
[CHAP.
of all the rotted material? a question to answer which leads
us into the very foundations of geological history.
. Openings from the soil down into the rock underneath
often afford instructive lessons re-
garding the decay of the surface of
the land. Fig. 2, for instance, is a
drawing of one of these sections,
in which a gradual passage may be
traced from solid sandstone (^under-
neath up into broken-up sandstone
(b\ and thence into the earthy layer
(c) that supports the vegetation of
the surface. Traced from below up-
wards, the rock is found to become
FIG. 2. -Passage of sand- more and more broken and crum .
stone upwards into soil.
blmg, with an increasing number of
rootlets that strike freely through it in all directions, until it
passes insensibly into the
uppermost dark layer of
vegetable soil or humus.
This dark layer owes its
characteristic brown or black
colour to the decaying re-
mains of vegetation diffused
through it. Again, granite
in its unweathered state is a
hard, compact, crystalline
rock that may be quarried
out in large solid blocks (a
FIG. 3. -Passage of granite upwards in Fig> ^ yt when traced
upward to within a few feet
from the surface it may be seen to have been split by in-
ii.] SOIL AND SUBSOIL. 21
numerable rents into fragments which are nevertheless still
lying in their original position. As these fragments are
attacked by percolating moisture, their surfaces decay, leaving
the still unweathered parts as rounded blocks (b\ which
might at first be mistaken for transported boulders. They
are, however, parts of the rock broken up in place, and not
fragments that have been carried from a distance. The
little quartz veins that traverse the solid granite can be
recognised running through the decayed and fresh parts
alike. But besides being broken into pieces, the granite
rots away and loses its cohesion. Some of the smaller
pieces can be crumbled down between the fingers, and
this decay increases towards the soil until the rock be-
comes a mere sand or sandy clay in which a few harder
kernels are still left. Into this soft layer roots may descend
from the surface and, like the sandstone, it passes upwards
into the overlying soil (c).
Soil and Subsoil. In such sections as the foregoing,
three distinct layers can be recognised which merge into
each other. At the bottom lies the rock, either undecayed
or at least still fresh enough to show its true nature. Next
comes the broken-up crumbling layer through which stray
roots descend. This is known as the subsoil. At the top
lies the dark band, crowded with rootlets and forming the
true soil. These three layers obviously represent successive
stages in the decay of the surface of the land. The soil is
the layer of most complete decay. The subsoil is an inter-
mediate band where the progress of decomposition has not
advanced so far, while the shattered rock underneath shows
the earlier stages of disintegration. Vegetation sends its
roots and rootlets through the rotted rock. As the plants
die, they are succeeded by others, and the rotted remains of
22 GEOLOGICAL WORK OF THE AIR. [CHAP.
their successive generations gradually darken the uppermost
decomposed layer. Worms, insects, and larger animals that
may die on the surface, likewise add their mouldering remains.
And thus from animals and plants there is furnished to the
soil that organic matter on which its fertility so much depends.
The very decay of the vegetation helps to promote that of
the underlying rock, for it supplies various organic acids
ready to be absorbed by percolating rain-water, the power
of which to decompose rocks is thereby increased (p. 32).
It is obvious, then, that in answer to the question, What
becomes of the rotted material produced by weathering? we
may confidently assert that, over surfaces of land protected
by a cover of vegetation, this material in large measure
accumulates where it is formed. Such accumulation will
naturally take place chiefly on flat or gently inclined ground.
Where the slope is steep, the decomposed layer will tend to
travel down-hill by mere gravitation, and to be further im-
pelled downward by descending rain-water.
If there is so intimate a connection between the soil at
the surface and the rock underneath, we can readily under-
stand that soils should vary from one district to another,
according to the nature of the underlying rocks. Clays will
produce clayey soil, sandstones, sandy soil, or, where these
two kinds of rock occur together, they may give rise to
sandy clay or loam. Hence, knowing what the underlying
rock is, we may usually infer what must be the character of
the overlying soil, or, from the nature of the soil, we may
form an opinion respecting the quality of the rock that lies
below.
But it will probably occur to the thoughtful observer
that when once a covering of soil and subsoil has been
formed over a level piece of ground, especially where there
ii.] SOIL AND SUBSOIL. . 23
is also an overlying carpet of verdure, the process of decay
should cease the very layer of rotted material coming event-
ually to protect the rock from further disintegration. Un-
doubtedly, under these circumstances, weathering is reduced
to its feeblest condition. But that it still continues will be
evident from some considerations, the force of which will be
better understood a few pages further on. If the process
were wholly arrested, then in course of time plants growing
on the surface would extract from the soil all the nutriment
they could get out of it, and with the increasing impoverish-
ment of the soil, they would dwindle away and finally die
out, until perhaps only the simpler forms of vegetation
would grow on the site. Something of this kind not im-
probably takes place where forests decay and are replaced
by scrub and grass. But the long-continued growth of the
same kind of plants upon a tract of land doubtless indicates
that in some way the process of weathering is not entirely
arrested, but that, as generation succeeds generation, the
plants are still able to draw nutriment from fresh portions
of decomposed rock. A cutting made through the soil and
subsoil shows that roots force their way downward into the
rock, which splits up and allows percolating water to soak
downwards through it. The subsoil thus gradually eats its
way into the solid rock below. Influences are at work also,
whereby there is an imperceptible removal of material from
the surface of the soil. Notable among these influences
are rain, wind, and earth-worms. Wherever soil is bare of
vegetation it is directly exposed to removal by rain and
wind. Ground is seldom so flat that rain may not flow a
little way along the surface before sinking underneath. In
its flow, it carries off the finer particles of the soil. These
may travel each time only a short way, but as the operation
24 GEOLOGICAL WORK OF THE AIR. [CHAI-.
is repeated, they are in the course of years gradually moved
down to lower ground or to some runnel or brook that
sweeps them away seaward. Both on gentle and on steep
slopes, this transporting power of rain is continually removing
the upper layer of bared soil. Where soil is exposed to the
sun, it is liable to be dried into mere dust, which is borne off
by wind. How readily this may happen is often strikingly
seen after dry weather in spring-time. The earth of ploughed
fields becomes loose and powdery, and clouds of its finer
particles are carried up into the air and transported to other
farms, as gusts of wind sweep across. " March dust," which
is a proverbial expression, may be remembered as an illus-
tration of one way in which the upper parts of the soil are
removed.
Even where a grassy covering protects the general sur-
face, bare places may always be found whence this covering
has been removed. Rabbits, moles, and other animals throw
out soil from their burrows. Mice sometimes lay it bare by
eating the pasture down to the roots. The common earth-
worms bring up to daylight in the course of a year an almost
incredible quantity of it in their castings. -Mr. Darwin
estimated that this quantity is in some places not less than
10 tons per annum over an acre of ground. Only the finest
particles of mould are swallowed by worms and conveyed
by them to the surface, and it is precisely these which are
most apt to be washed off by rain or to be dried and blown
away as dust by the wind. Where it remains on the
ground, the soil brought up by worms covers over stones
and other objects lying there, which consequently seem to
sink into the earth. The operation of these animals causes
the materials of the soil to be thoroughly mixed. In
tropical countries, the termite or " white ant " conveys a
ii.] TALUS-SLOPES. 25
prodigious amount of fine earth up into the open air. With
this material it builds hills sometimes 60 feet high and visible
for a distance of several miles ; likewise tunnels and chambers,
which it plasters all over the stems and branches of trees,
often so continuously that hardly any bark can be seen.
The fine soil thus exposed is liable to be blown away by
the wind or washed off by the fierce tropical rains.
Although, therefore, the layer of vegetable soil which
covers the land appears to be a permanent protection, it
does not really prevent a large amount of material from being
removed even from grassy ground. It forms the record of
the slow and almost imperceptible geological changes that
affect the regions where it accumulates, the quiet fall of rain,
the gradual rotting away of the upper part of the rock under-
neath, the growth and decay of a long succession of genera-
tions of plants, the ceaseless labours of the earth-worm, the
scarcely appreciable removal of material from the surface
by the action of rain and wind, and the equally insensible
descent of the crumbling subsoil farther and farther into
the solid stone below. Having learnt how all this is told
by the soil beneath our feet, we should be ready to recog-
nise in the soil of former ages a similar chronicle of quiet
atmospheric disintegration.
Talus. Besides soil and subsoil, there are other forms
in which decomposed rock accumulates on the surface of
the land. Where a large mass of bare rock rises up as a
steep bank or cliff, it is liable to constant degradation, and
the materials detached from its surface accumulate down the
slopes, forming what is known as a Talus (Fig. 4). In
mountainous or hilly regions, where rocky precipices rise
high into the air, there gather at their feet and down their
clefts long trails or screes of loose blocks split off from them
26
GEOLOGICAL WORK OF THE AIR.
[CHAP.
by the weather. Such slopes, especially where they are not
too steep, and where the rubbish that forms them is not too
FIG. 4. Talus-slopes at the foot of a line of cliffs.
coarse, may be more or less covered with vegetation, which
in some measure arrests the descent of the debris. But from
time to time, during heavy rains, deep gullies are torn out of
them by rapidly formed torrents, which
sweep down their materials to lower
levels (Fig. 10). The sections laid bare
in these gullies show that the rubbish
is arranged in more or less distinct
layers which lie generally parallel with
the surface of the slope ; in other
words, it is rudely stratified, and its
*- layers or strata are inclined at the angle
FIG. 5. -Section of rain- o f the declivity which seldom exceeds
wash or brick-earth. o
7. Vegetable soil. 6. 35 '
Brick-earth. 5. White Rain-wash, Brick-earth. On
sand. 4. Brick-earth, more gentle slopes, even where no bare
3. White sand. 2. rock pro j ects i nto the air the fall of rain
Brick-earth, i. Gravel
with seams of sand. gradually washes down the upper parts
of the soil to lower levels. Hence arise
thick accumulations of what is known as rain-wash soil
ii.] DUST AND SAND-DUNES. 27
mixed often with angular fragments of still undecomposed
rock, and not infrequently forming a kind of brick-earth
(Fig. 5). Deposits of this nature are still gathering now,
though their lower portions may be of great antiquity. In
the south-east of England, for instance, the brick-earths con-
tain the bones of animals that have long since passed away.
Dust. By the action of wind, above referred to, a vast
amount of fine dust and sand is carried up into the air and
strewn far and wide over the land. In dry countries, such as
large tracts of Central Asia, the air is often thick with a fine
yellow dust which may entirely obscure the sun at mid-day,
and which settles over everything. After many centuries,
a thick deposit is thus accumulated on the surface of the
land. Some of the ancient cities of the Old World, Nineveh
and Babylon for example, after being long abandoned by
man, have gradually been buried under the fine soil drifted
over them by the wind and intercepted and protected by the
weeds that grew up over the ruins. Even in regions where
there is a large annual rainfall, seasons of drought occur,
during which there may be a considerable drifting of the
finer particles of soil by the wind. We probably hardly
realise how much the soil is in some regions removed and
in others heightened from this cause.
Sand-dunes. Some of the most striking and familiar
examples of the accumulation of loose deposits by the wind
are those to which the name of Dunes is given. On sandy
shores, exposed to winds that blow landwards, the sand is
dried and then carried away from the beach, gathering into
long mounds or ridges which run parallel to the coast-line.
These ridges are often 50 or 60 feet, sometimes even more
than 250 feet high, with deep troughs and irregular circular
hollows between them, and they occasionally form a strip
28
GEOLOGICAL WORK OF THE AIR.
[CHAr.
several miles broad, bordering the sea. The particles
of sand are driven inland by the wind, and the dunes
gradually bury fields, roads, and villages, unless their progress
is arrested by the growth of vegetation over their shifting
surfaces. On many parts of the west coast of Europe, the
dunes are marching into the interior at the rate of 20 feet
in a year. Hence large tracts of land have within historic
FIG. 6. Sand-dunes.
times been entirely lost under them. In the north of Scot-
land, for example, an ancient and extensive barony, so noted
for its fertility that it was called "the granary of Moray,"
was devastated about the middle of the seventeenth century
by the moving sands, which now rise in barren ridges more
than 100 feet above the site of the buried land. In the
interior of continents also, where with great dryness of
climate there is a continual disintegration of the surface of
rocks, wide wastes of sand accumulate, as in the deserts of
Lybia and Arabia and in the heart of Australia.
ii.J DECAY OF THE SURFACE OF THE LAND. 29
There can be no doubt, however, that though in the
layer of vegetable soil, in the heaps of rubbish that gather
on slopes and at the base of rocky banks and precipices,
and in the widespread drifting of dust and sand over the
land by the action of the wind, we have evidence that much
of the material arising from the general decay of the surface
of the land accumulates under various forms upon that
surface, nevertheless its stay there is not permanent. Wind
and rain are continually removing it, sometimes in vast
quantities, into the sea. Every brook, made muddy by
heavy rain, is an example of this transport, for the mud that
discolours the water is simply the finer portions of the soil
washed off by rain. When we reflect upon the multitude
of streams, large and small, in all parts of the globe, and
consider that they are all busy carrying their freights of
mud to the sea, we can in some measure appreciate how
great must be the total annual amount of material so re-
moved. What becomes of this material will form the subject
of the succeeding chapters.
Summary. The first lesson to be learnt from an exam-
ination of the surface of the land is, that everywhere decay
is in progress upon it. Wherever the solid rock rises into
the air, it breaks up and crumbles away under the various
influences combined in the process of Weathering. The
wasted materials caused by this universal disintegration
partly accumulate where they are formed, and make soil.
But in large measure, also, they are blown away by wind and
washed off by rain. Even where they appear to be securely
protected by a covering of vegetation, the common earth-
worm brings the fine parts of them up to the surface, where
they come within reach of rain and wind, so that on tracts
permanently grassed over, there may be a continuous and
30 GEOLOGICAL WORK OF THE AIR. [CHAP. IT.
not inconsiderable removal of fine soil from the surface.
As the upper layers of soil are removed, roots and percolat-
ing water are enabled to reach down farther into the solid
rock which is broken up into subsoil, and thus the general
surface of the land is insensibly lowered.
Besides accumulating in situ as subsoil and soil, the
debris of decomposed rock forms talus-slopes and screes at
the foot of crags, and a layer of rain-wash or brick-earth
over gentler slopes. Where the action of wind comes
markedly into play, tracts of sand-dunes may be piled up
along the borders of the sea and of lakes, or in the arid in-
terior of continents ; and wide regions are in course of time
buried under the fine dust which is sometimes so thick in
the air as to obscure the noonday sun. But in none of these
forms can the accumulation of decomposed material be
regarded as permanent. So long as it is exposed to the
influences of the atmosphere, this material is liable to be
swept away from the surface of the land and borne outwards
into the sea.
CHAPTER III.
THE INFLUENCE OF RUNNING WATER IN GEOLOGICAL
CHANGES, AND HOW IT IS RECORDED.
IT appears, then, that from various causes all over the globe,
there is a continual decay of the surface of the land ; that
the decomposed material partly accumulates as soil, subsoil,
and sheets or heaps of loose earth or sand, but that much
of it is washed off the land by rain or blown into the rivers
or into the sea by wind. We have now to consider the
part taken by running water in this transport. From the
single rain-drop up to the mighty river, every portion of the
water that flows over the land is busy with its own share of
the work. When we reflect on the amount of rain that
falls annually over the land, and on the number of streams,
large and small, that are ceaselessly at work, we realise how
difficult it must be to form any fit notion of the entire
amount of change which, even in a single year, these agents
work upon the surface of the earth.
The influence of rain in the decay of the surface of the
land was briefly alluded to in the last chapter. As soon as
a drop of rain reaches the ground, it begins its appointed
geological task, dissolving what it can carry off in solution,
and pushing forward and downward whatever it has power
32 RECORDS OF RUNNING WATER. [CHAP.
to move. As the rain-drops gather into runnels, the same
duty, but on a larger scale, is performed by them ; and as
the runnels unite into large streams, and these into yet
mightier rivers, the operations, though becoming colossal in
magnitude, remain essentially the same in kind. In the
operations of the nearest brook, we see before us in minia-
ture a sample of the changes produced by the thousands of
rivers which, in all quarters of the globe, are flowing from
the mountains to the sea. Watching these operations from
day to day, we discover that they may all be classed under
two heads. In the first place, the brook hollows out the
channel in which it flows and thus aids in the general waste
of the surface of the land ; and in the second place, it carries
away fine silt and other material resulting from that waste.
Rivers are thus at once agents that themselves directly de-
grade the land, and that sweep the loosened detritus towards
the ocean. An acquaintance with each of these kinds of
work is needful to enable us to understand the nature of
the records which river-action leaves behind it.
Chemical Action of Running Water. We have
seen that rain in its descent from the clouds absorbs air, and
that with the oxygen and carbonic acid which it thus obtains
it proceeds to corrode the surfaces of rock on which it falls.
When it reaches the ground and absorbs the acids termed
"humous," which are supplied by the decomposing vegeta-
tion of the soil, it acquires increased power of eating into
the stones over which it flows. When it rolls along as a
runnel, brook, or river, it no doubt still attacks the rocks
of its channel, though its action in this respect is not so
easily detected. In some circumstances, however, the
solvent influence of river-water upon solid rocks is strikingly
displayed. Where the water contains a large proportion of
in.] CHEMICAL ACTION OF RUNNING WATER. 33
the acids of the soil, and flows over a kind of rock specially
liable to be eaten away by these acids, the most favourable
conditions are presented for observing the change. Thus,
a stream which issues from a peat-bog is usually dark brown
in colour, from the vegetable solutions which it extracts
from the moss. Among these solutions are some^of the
organic acids referred to, ready to eat into the surface of the
rocks or loose stones which the stream may encounter in
its descent. No kind of rock is more liable than limestone
FIG. 7. Erosion of limestone by the solvent action of a peaty stream.
Durness, Sutherlandshire.
to corrosion under such circumstances. Peaty water flowing
over it eats it away with comparative rapidity, while those
portions of the rock that rise above the stream escape solu-
tion, except in so far as they are attacked by rain. Hence
arise some curious features in the scenery of the water-
course. The walls of limestone above the water, being
attacked only by the atmosphere, are not eaten away so fast
as their base, over which the stream flows. They are con-
D
34 RECORDS OF RUNNING WATER. [CHAP.
sequently undermined, and are sometimes cut into dark
tunnels and passages (Fig. 7). Even where the solvent action
of the water of rivers is otherwise inappreciable, it can be
detected by means of chemical analysis. Thus rivers, partly
by the action of their water upon the loose stones and solid
rocks of their channels, and partly by the contributions they
receive from springs (which will be afterwards described),
convey a vast amount of dissolved material into the sea. The
substance thus invisibly transported consists of various mineral
salts. One of the most abundant of these carbonate of
lime is the substance that forms limestone, and furnishes
the mineral matter required for the hard parts of a large
proportion of the lower animals. It is a matter of some
interest to know that this substance, so indispensable for
the formation of the shells of so large a number of sea-
creatures, is constantly supplied to the. sea by the streams
that flow into it. The rivers of Western Europe, for instance,
have been ascertained to convey about i part of dissolved
mineral matter in every 5000 parts of water, and of this
mineral matter about a half consists of carbonate of lime.
It has been estimated that the Rhine bears enough carbonate
of lime into the sea every year to make three hundred and
thirty-two thousand millions of oysters of the usual size.
Another abundant ingredient of river-water is gypsum or
sulphate of lime, of which the Thames is computed to carry
annually past London not less than 180,000 tons. The total
quantity of carbonate of lime, removed from the limestones
of its basin by this river in a year, amounts, on an average,
to 140 tons from every square mile, which is estimated to
be equal to the lowering of the general surface to the extent
of T y- of an inch from each square mile in a century, or
one foot in 13,200 years.
in.] MECHANICAL ACTION OF RUNNING WATER. 35
Mechanical Action of Running Water Trans-
port. The dissolved material forms but a small proportion
of the total amount of mineral substances conveyed by rivers
from land to sea. A single shower of rain washes off fine
dust and soil from the surface of the ground into the nearest
brook which, though previously clear, now rolls along with a
discoloured current. An increase in the volume of the water
enables a stream to sweep along sand, gravel, and blocks of
stone lying in its channel, and to keep these materials moving
until, as the declivity lessens and the rain ceases, the current
becomes too feeble to do more than lazily carry onward the
fine silt that discolours it. Every stream, large or small, is
ceaselessly busy transporting mud, sand, or gravel. And
as the ultimate destination of all this sediment is the bottom
of the sea, it is evident that if there are no compensating
influences at work to repair its losses, the land must in the
end be all worn away.
Some of the most instructive lessons regarding the work
of running water on land are afforded by the beds of moun-
tain-torrents. Huge blocks, detached from the crags and
cliffs on either side, may there be seen cumbering the path-
way of the water, which seems quite powerless to move such
masses and can only sweep round them or find a passage
beneath them. But follow one of these torrents in its
descent, and you will find that by degrees the blocks,
losing their sharp edges, become rounded boulders, and
pass first into coarse shingle and then into fine gravel.
In the quieter reaches of the water, sheets of sand begin to
make their appearance, and at last when the stream reaches
the plains, no sediment of coarser grain than mere silt may
be seen in its channel. In the constant transport maintained
by watercourses, the transported materials, by being tossed
36 RECORDS OF RUNNING WATER. [CHAP.
along rocky channels and continually ground against each
other, diminish in size as they descend. A river flowing
from a range of mountains to the distant ocean may be
likened to a natural mill, into which large angular masses of
rock are cast at the upper end, and out of which only fine
sand and silt are discharged at the lower.
Partly, therefore, owing to the fine dust and soil swept
into them by wind and rain from the slowly decomposing
surface of the land, and partly to the friction of the detritus
which they sweep along their channels, rivers always con-
tain more or less mineral matter suspended in their water or
travelling with the current on the bottom. The amount of
material thus transported varies greatly in different rivers,
and at successive seasons even in the same river. In some
cases, the rain is spread so equably through the year that the
rivers flow onward with a quiet monotony, never rising much
above or sinking below their average level. In such circum-
stances, the amount of sediment they carry downward is
proportionately small. On the other hand, where either
from heavy periodical rains or from rapid melting of snow,
rivers are liable to floods, they acquire an enormously
increased power of transport, and their burden of sediment
is proportionately augmented. In a few days or weeks of
high water, they may convey to the sea a hundredfold the
amount of mineral matter which they could carry in a whole
year of their quieter mood.
Measurements have been made of the proportions of
sediment in the waters of different rivers at various seasons
of the year. The results as might be expected show great
variations. Thus the Garonne, rising among the higher
peaks of the Pyrenees, drains a large area of the south of
France, and is subject to floods by which an enormous
in.] TRANSPORT OF DETRITUS. 37
quantity of sediment is swept down from the mountains to
the plains. Its proportion of mud has been estimated to
be as much as i part in 100 parts of water. The Durance,
which takes its source high on the western flank of the
Cottian Alps, is one of the rapidest and muddiest rivers in
Europe. Its angle of slope varies from i in 208 to i in 467,
the declivity of the great rivers of the globe being probably
not more than i in 2600, while that of a navigable stream
ought not to exceed 10 inches per mile or i in 6336. The
Durance is, therefore, rather a torrent than a river. With
this rapidity of descent is conjoined an excessive capacity
for transporting sediment. In floods of exceptional severity,
the proportion of mud in the stream has been estimated at
one-tenth by weight of the water, while the average propor-
tion for nine years from 1867 to 1875 was about -gi . Prob-
ably the best general average is to be obtained from a river
which drains a wide region exhibiting considerable diversi-
ties of climate, topography, rocks, and soils. The Missis-
sippi presents a good illustration of these diversities, and
has accordingly been taken as a kind of typical river,
furnishing, so to speak, a standard by which the operations
of other rivers may be compared and which may perhaps
be assumed as a fair average for all the rivers of the globe.
Numerous measurements have been made of the proportion
of sediment carried into the Gulf of Mexico by this vast
river, with the result of showing that the average amount of
sediment is by weight i part in every 1500 parts of water,
or little more than one-third of the proportion in the water
of the Durance.
If now we assume that, all over the world, the general
average proportion of sediment floating in the water of
rivers is i part in every 1500 of water, we can readily
38' RECORDS OF RUNNING WATER. [CHAP.
understand how seriously in the course of time must the
land be lowered by the constant removal of so much de-
composed rock from its surface. Knowing the area of
the basin drained by a river, and also the proportion of
sediment in its water, we can easily calculate the general
loss from the surface of the basin. The ratio of the weight
or " specific gravity " of the silt to that of solid rock may
be taken to be as 19 is to 25. Accordingly the Mississippi
conveys annually from its drainage basin an amount of
sediment equivalent to the removal of g-^-Q- part of a
foot of rock from the general surface of the basin. At
this rate, one foot of rock will be worn away every 6000
years. If we take the general height of the land of the
whole globe to be 2120 feet, and suppose it to be continu-
ously wasted at the same rate at which the Mississippi basin
is now suffering, then the whole dry land would be carried
into the sea in 12,720,000 years. Or if we assume the
mean height of Europe to be 973 feet and that this continent
is degraded at the Mississippi rate of waste until the last
vestige of it disappears, the process of destruction would
be completed in rather less than 6,000,000 years. Such
estimates are not intended to be close approximations to
the truth. As the land is lowered, the rate of decay will
gradually diminish, so that the later stages of decay will be
enormously protracted. But by taking the rate of operation
now ascertained to be in progress in such a river basin as
the Mississippi, we obtain a valuable standard of com-
parison, and learn that the degradation of the land is much
greater and more rapid than might have been supposed.
Erosion of Watercourses. But rivers are not merely
carriers of the mud, sand, and gravel swept into their
channels by other agencies. By keeping these materials
in.] EROSION OF WATER-CHANNELS. 39
in motion, the currents reduce them in size, and at the same
time employ them to hollow out the channels wherein they
move. The mutual friction that grinds down large blocks
of rock into mere sand and mud, tells also upon the rocky
beds along which the material is driven. The most solid
rocks are worn down ; deep long gorges are dug out, and
the watercourses, when they have once chosen their sites,
remain on them and sink them gradually deeper and deeper
beneath the general level of the country. The surfaces of
stone exposed to this attrition assume the familiar smoothed
and rounded appearance which is known as water-worn.
The loose stones lying in the channel of a stream, and the
solid rocks as high up as floods can scour them, present this
characteristic aspect. Here and there, where a few stones
have been caught in an eddy of the current and are kept in
constant gyration, they reduce each other in dimensions, and
at the same time grind out a hollow in the underlying rock.
The sand and mud produced by the friction are swept off
by the current, and the stones when sufficiently reduced in
size are also carried away. But their places are eventually
taken by other blocks brought down by floods, so that the
supply of grinding material is kept up until the original
hollow is enlarged into a wide deep caldron, at the bottom
of which the stones can only be stirred by the heaviest floods.
Cavities of this kind, known as pot-holes, are of frequent
occurrence in rocky watercourses as well as on rocky shores,
in short, wherever eddies of water can keep shingle rotating
upon solid rock. As they often coalesce by the wearing
away of the intervening channel, they greatly aid in the
deepening of a watercourse. In most rocky gorges, a suc-
cession of old pot-holes may be traced far above the present
level of the stream (Fig. 8).
40 RECORDS OF RUNNING WATER. [CHAP.
That it is by means of the gravel and other detritus
pushed along the bottom by the current, rather than by the
mere friction of the water on its bed, that a river excavates
its channel, is most strikingly shown immediately below a
lake. In traversing a lake, the tributary streams are filtered.
.... - 1 ';."'..:. ,"~
FIG. 8. Pot-holes worn out by the gyration of stones in the bed of a
stream.
Depositing their sediment on the floor of the lake, they
unite in the clear transparent river which escapes at the
lower end. The Rhone, for instance, flows into the Lake
of Geneva as a turbid river ; it issues from that great reser-
voir at Geneva as a rushing current of the bluest, most
translucent water which, though it sweeps over ledges of
in.] MEANDERINGS OF STREAMS. 4 1
rock, has not yet been able to grind them down into a deep
gorge. The Niagara, also, filtered by Lake Erie, has not
acquired sediment enough to enable it to cut deeply into
the rocks over which it foams in its rapids before throwing
itself over the great Falls.
One of the most characteristic features of streams is the
singularly sinuous courses they follow. As a rule, too, the
flatter the ground over which they flow, the more do they
wind. Not uncommonly they form loops, the nearest bends
of which in the end unite, and as the current passes along
the now straightened channel, the old one is left to become
by degrees a lake or pond of stagnant water, then a marsh,
and lastly, dry ground. We might suppose that in flowing off
the land, water would take the shortest and most direct road
to the sea. But this is far from being the case. The
slightest inequalities of level have originally determined
sinuosities of the channels, and trifling differences in the
hardness of the banks, in the accumulation of sediment,
and in the direction of the currents and eddies have been
enough to turn a stream now to one side now to another,
until it has assumed its present meandering course. How
easily this may be done can be instructively observed on a
roadway or other bare surface of ground. Seen when quite
dry and smooth, hardly any depressions in which water
would flow might be detected on such a surface. But after
a heavy shower of rain, runnels of muddy water will be seen
coursing down the slope in serpentine channels that at once
recall the winding rivers of a great drainage-system. The
slightest differences of level have been enough to turn the
water from side to side. A mere pebble or projecting heap
of earth or tuft of grass has sufficed to cause a bend. The
water, though always descending, has only been able to reach
42 RECORDS OF RUNNING WATER. [CHAP.
the bottom by keeping the lowest levels, and turning from
right to left as these guided it.
When a river has once taken its course and has begun
to excavate its channel, only some great disturbance, such as
an earthquake or volcanic eruption, can turn it out of that
course. If its original pathway has been a winding one, it
goes on digging out its bed which, with all its bends, gradu-
ally sinks below the level of the surrounding country. The
deep and picturesque gorge in which the Moselle winds
from Treves to Coblenz has in this way been slowly eroded
out of the undulating tableland across which the river
originally flowed.
In another and most characteristic way, the shape of the
ground and the nature and arrangement of the rocks over
which they flow, materially influence rivers in the forms
into which they carve their channels. The Rhone and the
Niagara, for instance, though filtered by the lakes through
which they flow, do not run far before plunging into deep
ravines. Obviously such ravines cannot have been dug out
by the same process of mechanical attrition whereby river-
channels in general are eroded. Yet the frequency of gorges
in river scenery shows that they cannot be due to any ex-
ceptional operation. They may generally be accounted for
by some arrangement of rocks wherein a bed of harder
material is underlain by one more easily removable. Where
a stream, after flowing over the upper bed, encounters the
decomposable bed below, it eats away the latter more
rapidly. The overlying hard rock is thus undermined, and,
as its support is destroyed, slice after slice is cut away from
it. The waterfall which this kind of structure produces
continues to eat its way backward or up the course of the
stream, so long as the necessary conditions are maintained
in.] WATERFALLS AND GORGES. 43
of hard rocks lying upon soft. Any change of structure
which would bring the hard rocks down to the bed of the
channel, and remove the soft rocks from the action of the
current and the dash of the spray would gradually destroy
the waterfall. It is obvious that, by cutting its way back-
ward, a waterfall excavates a ravine.
The renowned Falls of Niagara supply a striking illustra-
tion of the process now described. The vast body of water
which issues from Lake Erie, after flowing through a level
country for a few miles, rushes down its rapids and then
plunges over a precipice of solid limestone. Beneath this
hard rock lies a band of comparatively easily eroded shale.
As the water loosens and removes the lower rock, large
portions of the face of the precipice behind the Falls are
from time to time precipitated into the boiling flood below.
The waterfall is thus slowly prolonging the ravine below the
Falls. The magnificent gorge in which the Niagara, after
its tumultuous descent, flows sullenly to Lake Ontario is not
less than 7 miles long, from 200 to 400 yards wide, and from
200 to 300 feet deep. There is no reason to doubt that
this chasm has been entirely dug out by the gradual reces-
sion of the Falls from the cliffs at Queenstown, over which
the river at first poured. We may form some conception
of the amount of rock thus removed from the estimate that
it would make a rampart about 12 feet high and 6 feet thick,
extending right round the whole globe at the equator. Still
more gigantic are the gorges or canons of the Colorado and
its tributaries in Western America. The Grand Canon of
the Colorado is 300 miles long, and in some places more
than 6000 feet deep (Fig. 9). The country traversed by it
is a network of profound ravines, at the bottom of which
the streams flow that have eroded them out of the table-land.
44
RECORDS OF RUNNING WATER. [CHAP.
in.] PERMANENT RECORDS OF RIVER-ACTION. 45
Permanent Records of River-Action. If, then, all
the streams on the surface of the globe are engaged in the
double task of digging out their channels and carrying away
the loose materials that arise from the decomposition of the
surface of the land, let us ask ourselves what memorials of
these operations they leave behind them. In what form do
the running waters of the land inscribe their annals in
geological history ? If these waters could suddenly be dried
up all over the earth, how could we tell what changes they
had once worked upon the surface of the land ? Can we
detect the traces of ancient rivers where there are no rivers
now?
From what has been said in this lesson it will be evident
that in answer to such questions as these, we may affirm that
one unmistakable evidence of the former presence of rivers
is to be found in the channels which they have eroded. The
gorges, rocky defiles, pot-holes, and water-worn rocks which
mark the pathway of a stream would long remain as striking
memorials of the work of running water. In districts, now
dry and barren, such as large regions in the Levant, there
are abundant channels (wadies) now seldom or never occu-
pied by a stream, but which were evidently at one time the
beds of active torrents.
But more universal testimony to the work of running
water is to be found in the deposits or alluvium which it has
accumulated. Spreading out on either side, sometimes far
beyond the limits of the ordinary or modern channels, these
deposits, even when worn into fragmentary patches, retain
their clear record of the operations of the river. Let us in
imagination follow the course of a river from the mountains
to the sea, and mark as we go the circumstances under which
the accumulation of sediment takes place.
46 RECORDS OF RUNNING WATER. [CHAP.
The power possessed by running water to carry forward
sediment depends mainly upon the velocity of the current.
The more rapidly a stream flows, the more sediment can it
transport, and the larger are the blocks which it can move.
The velocity is regulated chiefly by the angle of slope ; the
greater the declivity, the higher the velocity and the larger
the capacity of the stream to carry down debris. Any cause,
therefore, which lessens the velocity of a current diminishes
its carrying power, and if the water is bearing along gravel,
sand, or mud, some of these materials will begin to drop and
remain at rest on the bottom. In the course of every stream,
various conditions arise whereby the velocity of the current
is reduced. One of the most obvious of these is a diminu-
tion in the slope of the channel. Another is the union of a
rapid tributary with a more gently flowing stream. A third
is the junction of a stream with the still waters of a lake or
with the sea. In these circumstances, the flow of the water
being checked, the sediment at once begins to fall to the
bottom.
Tracing now the progress of a river, for illustrations of
this law of deposition, we find that among the mountains
where the river takes its rise, the torrents that rush down the
declivities have torn out of them such vast quantities of soil
and rock as to seam them with deep clefts and gullies.
Where each of these rapid streamlets reaches the valley
below, its rapidity of motion is at once lessened, and with
this slackening of speed and consequent loss of carrying
power, there is an accompanying deposit of detritus. Blocks
of rock, angular rubbish, rounded shingle, sand, and earth
are thrown down in the form of a cone of which the apex
starts from the bottom of the gully and the base spreads out
over the plain (Fig. 10). Such cones vary in dimensions
in.]
FANS OF ALLUVIUM.
47
according to the size of the torrent and the comparative ease
with which the rocks of the mountain-side can be loosened
and removed. Some of them, thrown down by the transient
runnels of the last sudden rain-storm, may not be more than
FIG. 10. Gullies torn out of the side of a mountain by descending
torrents, with cones of detritus at their base.
a few cubic yards in bulk. But on the skirts of moun-
tainous regions they may grow into masses hundreds of feet
thick and many miles in diameter. The valleys in a range
of mountains afford many striking examples of these alluvial
cones or fans, as they are called. Where the tributary
48 RECORDS OF RUNNING WATER. [CHAP.
torrents are numerous, a succession of such cones or fans,
nearly or quite touching each other, spreads over the
floor of a valley. From this cause, so large an amount
of detritus has within historic times been swept down into
some of the valleys of the Tyrol that churches and other
buildings are now half-buried in the accumulation.
Looking more closely at the materials brought down by
the torrents, we find them arranged in rude irregular layers,
sloping downwards into the plain, the coarsest and most
angular detritus lying nearest to the mountains, while more
rounded and water-worn shingle or sand extends to the outer
margin of the cone. This grouping of irregular layers of
angular and half-rounded detritus is most characteristic of
the action of torrents. Hence, where it occurs, even though
no water may run there at the present day, it may be re-
garded as indicating that at some former time a torrent swept
down detritus over that site.
Quitting the more abrupt declivities, and augmented by
numerous tributaries from either side, the stream whose
course we are tracing loses the character of a torrent and
assumes that of a river. It still flows with velocity enough
to carry along not only mud and sand but even somewhat
coarse gravel. The large angular blocks of the torrent
part of its course, however, are no longer to be seen, and all
the detritus becomes more and more rounded and smoothed
as we follow it towards the plains. At many places, deposits
of gravel or sand take place, more especially at the inner
side of the curves which the stream makes as it winds down
the valley. Sweeping with a more rapid flow round the
outer side of the curve, the current lingers in eddies on the
inner side and drops there a quantity of sediment. When
the water is low, these strips of sand and shingle on the
in.] ARRANGEMENT OF RIVER GRAVEL. 49
concave side of each curve of the river form a distinctive
feature in the scenery. It is interesting to walk along one
of these strips and to mark how the current has left its record
there. The stones are well smoothed and rounded, showing
that they have been rolled against each other along the
bottom of the channel for a sufficient distance to lose their
original sharp edges and to pass from the state of rough
angular detritus into that of thoroughly water-worn gravel.
Further, they will be found not to lie entirely at random,
as might at first sight be imagined. A little examination
will show that, where the stones are oblong, they are gener-
ally placed with their longer axis pointing across the stream.
This would naturally be the position which they would
assume where the current kept rolling them forward along
the channel. Those which are flat in shape will be observed
usually to slope up stream. That the sloping face must look
in the direction from which the current moves will be
evident from Fig. n, where a current, moving in the direc-
tion of the arrow
and gradually di-
minishing in force, FlG> x j.Flat stones in a bank of river-shingle,
would no longer be showing the direction of the current (indi-
able to overturn the cated ^ the arrow ) that trans P orted and left
them,
stones which it had
so placed as to offer the least obstacle to its passage. Had
the current flowed from the opposite quarter, it would have
found the upturned edges of the stones exposed to it, and
would have readily overturned them until they found a
position in which they again presented least resistance to
the water. In a section of gravel, it is thus often quite
possible to tell from what quarter the current flowed that
deposited the pebbles.
5
RECORDS OF RUNNING WATER.
[CHAP.
Yet another feature in the arrangement of the materials
deserves attention. It is well seen where a digging has been
made in one of the alluvial banks, but better still in a section
of one of the terraces to be immediately referred to. The
layers of gravel or sand in some bands may be observed to
be inclined at a steeper angle than in others, as shown in the
accompanying figure (Fig. 12). In such cases, it will be
noticed that the slope
of the more inclined
layers is down the stream,
and hence that their direc-
tion gives a clue to that
of the current which ar-
ranged them. We may
even catch similar layers
in the act of deposition
among shallow pools in
to which currents are
discharging sediment.
The gravel or sand may be observed moving along the
bottom, and then falling over the edge of a bank into the
bottom of the pool. As the sediment advances by succes-
sive additions to its steep slope in front, it gradually fills the
pool up. Its progress may be compared to that of a railway
embankment formed by the discharge of waggon -loads of
rubbish down its end. A section through such an embank-
ment would reveal a series of bands of variously coloured
materials inclined steeply towards the direction in which the
waggon-loads were thrown down. Yet the top of the em-
bankment may be kept quite level for the permanent way.
The nearly level bands (, c) in Fig 1 2 represent the general
bottom on which the sediment accumulated, while the steeper
FIG. 12. Section of alluvium showing
direction of currents.
in.] RIVER-TERRACES. 51
lines in the lower gravel (a) point to the existence and direc-
tion of the currents by which sediment was pushed forward
along that bottom. (Compare pp. 230, 231.)
As the river flows onward through a gradually expanding
valley, another characteristic feature becomes prominent.
Flanking each side of the flat land through which the stream
pursues its winding course, there runs a steep slope or bank
a few feet or yards in height, terminating above in a second
or higher plain, which again may be bordered with another
similar bank, above which there may lie a third plain.
These slopes and plains form a group of terraces, rising step
by step above and away from the river, sometimes to a
height of several hundred feet, and occasionally to the
number of 6 or 8 or even more (Fig. 13). Here and there,
FIG. 13. River-terraces.
by the narrowing of the intervening strip of plain, two
terraces merge into one, and at some places the river in
winding down the valley has cut away great slices from the
terraces, perhaps even entirely removing them and eating
back into the rock out of which the valley has been exca-
52 RECORDS OF RUNNING WATER. [CHAP.
vated. Sections are thus exposed showing a succession of
gravels, sands, and loams like those of the present river.
From the line of the uppermost terrace down to the spits of
shingle now forming in the channel, we have evidently a
series of river-deposits. But how could the river have flowed
at the level of these high gravels, so far above its present
limits? An examination of the behaviour of the stream
during floods will help towards an answer to this question.
When from heavy rains or melted snows the river over-
flows its banks, it spreads out over the level ground on either
side. The tract liable to be thus submerged during inun-
dations is called the flood-plain. As the river rises in flood,
it becomes more and more turbid from the quantity of mud
and silt poured into it by its tributaries on either side. Its
increase in volume likewise augments its velocity and con-
sequently its power of scouring its bed and of transporting
the coarser detritus resting there. Large quantities of
shingle may thus be swept out of the ordinary channel and
be strewn across the nearer parts of the flood-plain. As the
current spreads over this plain, its velocity and transporting
capacity diminish, and consequently sediment begins to be
thrown down. Grass, bushes, and trees, standing on the
flood-plain, filter some of the sediment out of the water.
Fine mud and sand, for instance, adhere to the leaves and
stems, whence they are eventually washed off by rain into the
soil underneath. In this way, the flood-plain is gradually
heightened by the river itself. At the same time, the bed of
the river is deepened by the scour of the current, until, in the
end, even the highest floods are no longer able to inundate the
flood-plain. The difference of level between that plain and
the surface of the river gradually increases ; by degrees the
river begins to cut away the edges of the terrace which it
in.] RIVER-TERRACES. 53
cannot now overflow and to form a new flood -plain at a
lower level. In this manner, it slowly lowers its bed, and
leaves on either side a set of alluvial terraces to mark suc-
cessive stages in the process of excavation. If during this
process the level of the land should be raised, the slope of
the rivers, and consequently their scour, would be aug-
mented, and they would thereby acquire greater capacity for
the formation of terraces. There is reason to believe that
this has taken place both in Europe and North America.
It is obvious that the highest terraces must be the oldest,
and that the series is progressively younger down to the
terrace that is being formed at the present time. Yet, in
the materials comprising any one terrace, those lying at the
top must be the youngest. This apparent contradiction
arises from the double action of the river in eroding its bed
and depositing its sediment. If there were no lowering of
the channel, then the deposits would follow the usual order
of sequence, the oldest being below and the youngest above.
This order is maintained in the constituents of each single
terrace, for the lowermost layers of gravel must evidently
have been accumulated before the deposit of those that
overlie them. But when the level of the water is lowered,
the next set of deposits must, though younger, lie at a
lower level than those that preceded them. In no case,
however, will the older beds be found really to overlie the
younger. They have been formed at different levels.
The gravel, sand, and loam laid down by a river are
marked by an arrangement in layers or beds lying one upon
another. This stratified disposition indeed is characteristic
of all sedimentary accumulations, and is best developed
where currents have been most active in transporting and
assorting the materials (p. 227). It is the feature that first
54 RECORDS OF RUNNING WATER. [CHAP.
catches the eye in any river -bank, where a section of the
older deposits or alluvium is exposed. Beds of coarser and
finer detritus alternate with each other, but the coarsest are
generally to be observed below and the finest above. The
deltas accumulated by rivers in lakes and in the sea will be
noticed in chapters iv. and vii.
But besides the inorganic detritus carried down by a
river, we have also to consider the fate of the remains of
plants and the carcases of animals that are swept down
the streams, especially during floods. Swollen by sudden
and heavy rains, a river will rise above its ordinary level and
uproot trees and shrubs. On such occasions, too, moles
and rabbits are drowned and buried in their burrows on the
alluvial flood-plain. Birds, insects, and even some of the
larger mammals are from time to time drowned and swept
away by floods and buried in the sediment, and their remains,
where of a durable kind or where sufficiently covered over,
may be preserved for an indefinite period. The shells and
fishes living in the river itself may also be killed during the
flood and may be entombed with the other organisms in the
sediment.
Summary. The material produced by the universal
decay of the surface of the land is washed off by rain and
swept seawards by brooks and rivers. The rate at which
the general level of the land is being lowered by the opera-
tion of running water may be approximately ascertained by
measuring or estimating the amount of mineral matter carried
seaward every year from a definite region, such as a river-
basin. Taking merely the matter in mechanical suspension,
and assuming that the proportion of it transported annually
in the water of the Mississippi may be regarded as an average
proportion for the rivers of Europe, we find that this con-
TIT.] SUMMARY. 55
tinent, at the Mississippi rate of degradation, might be re-
duced to the sea-level in rather less than 6,000,000 years.
In pursuing their course over the land, running waters
gradually deepen and widen the channels in which they
flow, partly by chemically dissolving the rocks and partly
by rubbing them down by the friction of the transported
sand, gravel, and stones. When they have once chosen
their channels, they usually keep to them, and the sinuous
windings, at first determined by trifling inequalities on the
surface of country across which the streams began to flow,
are gradually deepened into picturesque gorges. In the
excavation of such ravines, waterfalls play an important
part by gradually receding up stream. River -channels,
especially if cut deeply into the solid rock, remain as endur-
ing monuments of the work of running water.
But still more important as geological records, because
more frequent and covering a larger area, are the deposits
which rivers leave as their memorials. Whatever checks
the velocity of a current weakens its transporting power, and
causes it to drop some of its sediment to the bottom.
Accordingly, accumulations of sediment occur at the foot of
torrent slopes, along the lower and more level ground,
especially on the inner or concave side of the loops, over
the flood-plains, and finally in the deltas formed where rivers
enter lakes or the sea. In these various situations, thick
stratified beds of silt, sand, and gravel may be formed,
enclosing the remains of the plants and animals living on
the land at the time. As a river deepens its channel, it
leaves on either side alluvial terraces that mark successive
flood-plains over which it has flowed.
CHAPTER IV.
THE MEMORIALS LEFT BY LAKES.
ACCORDING to the law stated in last chapter, that when
water is checked in its flow, it must drop some of its sedi-
ment, lakes are pre-eminently places for the deposition and
accumulation of mineral matter. In their quiet depths, the
debris worn away from the surface of the land is filtered out
of the water and allowed to gather undisturbed upon the
bottom. The tributary streams may enter a large lake
swollen and muddy, but the escaping river is transparent.
It is evident, therefore, that lakes must be continually silt-
ing up, and that when this process is complete, the site of
a lake will be occupied by a series of deposits comprising
a record of how the water was made to disappear.
To those who know the aspect of lakes only in fine
weather, they may seem places where geological operations
are at their very minimum of activity. The placid surface
of the water ripples upon beaches of gravel or spits of sand ;
reeds and marshy plants grow out into the shallows; the
few streamlets that tumble down from the surrounding hills
furnish perhaps the only sounds that break the stillness, but
their music and motion are at once hushed when they lose
themselves in the lake. The scene might serve as a very
CHAP, iv.] FILLING UP OF LAKES. 57
emblem of perfectly undisturbed conditions of repose. But
come back to this same scene during an autumn storm,
when the mists have gathered all round the hills, and the
rain, after pouring down for hours, has turned every gully
into the track of a roaring torrent. Each tributary brook,
hardly visible perhaps in summer, now rushes foaming and
muddy from its dell and sweeps out into the lake. The
larger streams bear along on their swift brown currents trunks
of trees, leaves, twigs, with now and then the carcase of
some animal that has been drowned by the rising flood.
Hour after hour, from every side, these innumerable swollen
waters bear their freights of gravel, sand, and mud into the
lake. Hundreds or thousands of tons of sediment must thus
be swept down during a single storm. When we multiply
this result by the number of storms in a year and by the
number of years in an ordinary human life, we need not be
surprised to be told that even within the memory of the
present generation, and still more within historic times,
conspicuous changes have taken place in many lakes. In
the Lake of Lucerne, for example, the River Reuss, which
bears down the drainage of the huge mountains round the
St. Gothard, deposits about 7,000,000 cubic feet of sediment
every year. Since the year 1714 the Kander, which drains
the northern flanks of the centre of the Bernese Oberland,
is said to have thrown into the lower end of the Lake of
Thun such an amount of sediment as to form an area of 230
acres, now partly woodland, partly meadow and marsh. Since
the time of the Romans, the Rhone has filled up the upper
end of the Lake of Geneva to such an extent that a Roman
harbour, still called Port Valais, is now nearly 2 miles from
the edge of the lake, the intervening ground having been
converted first into marshes and then into meadows and farms.
58 GEOLOGICAL RECORDS LEFT BY LAKES. [CHAP.
It is at the mouths of streams pouring into a lake that
the process of filling up is most rapid and striking. But it
may be detected at many other places round the margin.
Instructive lessons on this subject may be learned at a
reservoir formed by damming back the waters of a steep-
sided valley, and liable to be sometimes nearly dry (Fig. 14).
In such a situation, when the water is low, it may be noticed
that a series of parallel lines runs all round the sides of
the reservoir, and that these lines consist of gravel, sand,
or earth. Each of them marks a former level of the water,
FIG. 14. Alluvial terraces on the side of an emptied reservoir.
and they show that the reservoir was not drained off at once
but intermittently, each pause in the diminution of level
being marked by a line of sediment. It is easy to watch
how these lines are formed along the present margin of the
water. The loose debris from the bare slope above, partly
by its own gravitation, partly by the wash of rain, slides
down into the water. But as soon as it gets there, its
further downward movement is arrested. By the ripple of
the water it is gently moved up and down, but keeps on the
whole just below the line to which the water reaches. So
long as it is concealed under the water, its position and
extent can hardly be realised. But as soon as the level of
IV.]
LAKE-TERRACES.
59
the reservoir sinks, the sediment is left as a marked shelf
or terrace. In natural lakes, the same process is going on,
though hardly recognisable, because concealed under the
water. But if by any means a lake could be rapidly emptied,
its former level would be marked by a shelf or alluvial terrace.
In some cases, the barrier of a lake has been removed, and
FIG. 15. Parallel roads of Glen Roy.
the sinking of the water has revealed the terrace. The
famous " parallel roads " of Glen Roy, in the west of Scot-
land, are notable examples (Fig. 15). The valleys in that
region were anciently dammed up by large glaciers. The
drainage accumulated behind the ice, filled up the valleys
and converted them into a series of lakes. The former levels
of these lakes and the successive stages of their diminution
6o
GEOLOGICAL RECORDS LEFT BY LAKES. [CHAP.
and disappearance are shown by the series of alluvial shelves
known as parallel roads. The highest of these is 1140 feet,
the middle terrace 1059 feet, and the lowest 847 feet above
the level of the sea.
Thus, partly by the washing of detritus down from the
adjoining slopes by rain, partly by the sediment carried into
them by streams, and partly by the growth of marshy vegeta-
tion along their margins, lakes are visibly diminishing in
size. In mountainous countries, every stage of this dis-
appearance may be observed (Fig. 16). Where the lakes
FIG. 1 6. Stages in the filling up of a lake. In A two streamlets are
represented as pouring their deltas into a lake. In B they have filled
the lake up, converting it into a meadow across which they wind on
their way down the valley.
are deep, the tongues of sediment or " deltas " which the
streams push in front of them have not yet been able to
advance far from the shore. In other cases, every tributary
has built up an alluvial plain which grows outwards and
along the coast, until it unites with those of its neighbours
to form a nearly continuous belt of flat meadow and marsh
round the lake. By degrees, as this belt increases in width,
the lake narrows, until the whole tract is finally converted
into an alluvial plain, through which the river and its tribu-
taries wind on their way to lower levels. The successive flat
meadow -like expansions of valleys among hills and moun-
iv.] DEPOSITS IN LAKES. 61
tains were probably in most cases originally lakes which have
in this manner been gradually filled up.
The bottoms of lakes must evidently contain many
interesting relics. Dispersed through the shingle, sand, and
mud that gather there, are the remains of plants and animals
that lived on the surrounding land. Leaves, fruits, twigs,
branches, and trunks embedded in the silt may preserve for
an indefinite period their record of the vegetation of the
time. The wings or wing-cases of insects, the shells of
land-snails, the bones of birds and mammals, carried down
into the depths of a lake and entombed in the silt there
will remain as a chronicle of the kind of animals that
haunted the surrounding hills and valleys.
The layers of gravel, sand, and silt laid down on the
floor of a lake do not essentially differ from these deposited
in the terraces of a river. But they ought generally to be
finer in grain, and the proportion of silt, mud, or clay among
them, especially away from the margin of the lake, must
usually be greater than in the alluvium of a river. They are,
no doubt, further distinguished by the greater abundance
of the remains of plants and animals preserved in them.
But lakes likewise serve as receptacles for a series of de-
posits which are peculiar to them, and which consequently
have much interest and importance as they furnish a ready
means of detecting the sites of lakes that have long disap-
peared. The molluscs of lacustrine waters are quite dis-
tinct from the snails of the adjoining shores. The shells of
these animals gather on the bottoms of some lakes in such
numbers as to form there a deposit of the white crumbling
marl, already referred to on p. 6. If nothing occurs to
interrupt this deposit, it may grow to be many feet or yards
in thickness. The shells in the upper parts are quite fresh,
62 GEOLOGICAL RECORDS LEFT BY LAKES. [CHAP.
some of the animals having only recently died ; but they
become more and more decayed below until, towards the
bottom of the deposit, the marl passes into a more compact
chalk-like substance in which few or no shells may be recog-
nisable (Fig. 17). On the sites of lakes that have been
FIG. 17. Piece of shell-marl containing shells of Limtteea peregra.
naturally filled up or artificially drained, such marl has been
extensively dug as a manure for land. Besides the shells
from the decay of which it is chiefly formed, it sometimes
yields the bones of deer, oxen, and other animals, whose
carcases must originally have sunk to the bottom of the lake
and been there gradually covered up in the growing mass
of marl. Many examples of these marl-deposits are to be
found among the drained lakes of Scotland and Ireland.
Yet another peculiar accumulation is met with on the
bottom of some lakes, particularly in Sweden. In the
neighbourhood of banks of reeds and on the sloping shallows
of the larger lakes, a deposit of hydrated peroxide of iron
takes place, in the form of concretions varying in size from
small grains like gunpowder up to cakes measuring 6
inches across. The iron is no doubt dissolved out of the
rocks of the neighbourhood by water containing organic
iv.] SALT-LAKES. ' 63
acids or carbonic acid. In this condition, it is liable to be
oxidised on exposure, and can then no longer be retained
in solution. It is accordingly precipitated to the bottom
where it collects in grains which by successive additions to
their surface become pellets, balls, or cakes. Possibly some
of the microscopic plants (diatoms) which abound on the
bottoms of the lakes may facilitate the accumulation of
the iron by abstracting this substance from the water and
depositing it inside their siliceous coverings. Beds of con-
cretionary brown ironstone are formed in Sweden from 10
to 200 yards long, 5 to 15 yards broad, and from 8 to
30 inches thick. During winter when the lakes are frozen
over, the iron is raked up from the bottom through holes
made for the purpose in the ice, and is largely used for the
manufacture of iron in the Swedish furnaces. When the
iron has been removed, it begins to form again, and instances
are known where, after the supply had been completely
exhausted, beds several inches in thickness were formed
again in twenty-six years.
The salt-lakes of desert regions present a wholly peculiar
series of deposits. These sheets of water have no outlet ;
yet there is reason to believe that most of them were at first
fresh, and discharged their outflow like ordinary lakes.
Owing to geological changes of level and of climate, they
have long ceased to overflow. The water that runs into
them, instead of escaping by a river, is evaporated back into
the air. But the various mineral salts carried by it in solu-
tion from rocks and soils are not evaporated also. They
remain behind in the lakes, which are consequently becoming
gradually salter. Among the salts thus introduced, common
salt (sodium-chloride) and gypsum (calcium-sulphate) are
two of the most important. These substances, as the water
64 GEOLOGICAL RECORDS LEFT BY LAKES. [CHAP.
evaporates in the shallows, bays, and pools, are precipitated to
the bottom where they form solid layers of salt and gypsum.
The latter substance begins to be thrown down when 37
per cent of the water containing it has been evaporated.
The sodium-chloride does not appear until 93 per cent of
the water has disappeared. In the order of deposit, there-
fore, gypsum comes before the salt. Some bitter lakes
contain sodium-carbonate, in others magnesium-chloride is
abundant. The Dead Sea, the Great Salt Lake of Utah,
and many other salt lakes and inland seas furnish interesting
evidence of the way in which they have gradually changed.
In their upper terraces, 1000 feet or more above the present
level of the water, fresh-water shells occur, showing that these
basins were at first fresh. But their valley-bottoms are now
crusted with gypsum and salt, and their waters are almost
wholly devoid of life. Such conditions as these help us to
understand how great deposits of gypsum and rock-salt were
formed in England, Germany, and many other regions where
the climate would not now permit of any such condensation
of the water (chapter xxi.)
Summary. The records inscribed by lakes in geologi-
cal history consist of layers of various kinds of sediment.
These deposits may form mere shelves or terraces along the
margin of the water which, if drained off, will leave them as
evidence of its former levels. By the long-continued opera-
tions of rain, brooks, and rivers, continually bringing down
sediment, lakes are gradually filled up with alluvium,
and finally become flat meadow-land with tributary streams
winding through it. The deposits that thus replace the
lacustrine water consist mainly of sand or gravel near shore,
while finer silt occupies the site of the deeper water. They
may also include beds of marl formed of fresh-water shells,
iv.] SUMMARY. 65
and sheets of brown iron ore. Throughout them all, re-
mains of the plants and animals of the surrounding land are
likely to be entombed and preserved.
Salt lakes leave, as their enduring memorial, beds of
rock-salt and gypsum, sometimes carbonate of soda and
other salts. Many of them were at first fresh, as is shown
by the presence of ordinary fresh-water shells in their upper
terraces. But by change of climate and long -continued
excess of evaporation over precipitation, the water has
gradually become more and more saline, and has sometimes
disappeared altogether, leaving behind it deposits of common
salt, gypsum, and other chemical precipitates.
CHAPTER V.
HOW SPRINGS LEAVE THEIR MARK IN GEOLOGICAL HISTORY.
THE changes made by running water upon the land are not
confined to that portion of the rainfall which courses along
the surface. Even when it sinks underground and seems
to have passed out of the general circulation, the subter-
ranean moisture does not remain inactive. After travelling
for a longer or shorter distance through the pores of rocks,
or along their joints and other divisional planes, it finds its
way once more to daylight and reappears in Springs. 1 In
this underground journey, it corrodes rocks, somewhat in
the same way as rain attacks those that are exposed to the
outer air, and it works some curious changes upon the face
of the land. Subterranean water thus leaves distinct and
characteristic memorials as its contributions to geological
history.
There are two aspects in which the work of underground
water may be considered here. In the first place, portions
of the substance of subterranean rocks are carried up above
ground; in the second place, some of these materials are
laid down again in a new form and take a conspicuous place
among the geological monuments of their time.
1 Physical Geography Class-Book, p. 222,
CHAP, v.] ORIGIN OF LANDSLIPS. 67
Abstraction of Material. In the removal of mineral
substance, water percolating through rocks acts in two distinct
ways, mechanical and chemical, each of which shows itself
in its own peculiar effects upon the surface. While slowly
filtering through porous materials, water tends to remove
loose particles and thus to lessen the support of overlying
rocks. But even where there is no transport, the water
itself, by saturating a porous layer that rests upon a more
or less impervious one, loosens the cohesion of that porous
layer. The overlying mass of rock is thus made to rest
upon a watery and weakened platform, and if from its posi-
tion it should have a tendency to gravitate in any given
direction, it may at last yield to this tendency and slide
downwards. Along the sides of sea-cliffs, on the precipitous
slopes of valleys or river-gorges, or on the declivities of hills
and mountains, the conditions are often extremely favour-
able for the descent of large masses of rock from higher to
lower levels.
Remarkable illustrations of such Landslips, as they are
called, have been observed along the south coast of England,
where certain porous sandy rocks underlying a thick sheet
of chalk rest upon more or less impervious clays, which,
by arresting the water in its descent, throw it out along
the base of the slopes. After much wet weather, the upper
surface of these clays becomes, as it were, lubricated by the
accumulation of water, and large slices of the overlying rocks,
having their support thereby weakened, break off from the
solid cliffs behind and slide down towards the sea. The
most memorable example occurred at Christmas time, in
the year 1839, on tne coast of Devonshire not far from
Axmouth. At that locality, the chalk-downs end off in a
line of broken cliff some 500 feet above the sea. From the
68 GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP.
edge of the downs flanked by this cliff a tract about 800
yards long, containing not less than 30 acres of arable land,
sank down with all its fields, hedgerows, and pathways.
This sunken mass, where it broke away from the upland,
left behind it a new cliff, showing along the crest the trun-
cated ends of the fields, of which the continuation was to
be found in a chasm more than 200 feet deep. While the
ground sank into this defile and was tilted steeply towards
the base of the cliff, it was torn up by a long rent running
on the whole in the line of the cliff, and by many parallel
and transverse fissures. Nearly half a century has passed
away since this landslip occurred. The cliff remains much
as it was at first, and the sunken fields with their bits of
hedgerow still slope steeply down to the bottom of the
declivity. But the lapse of time has allowed the influence
of the atmosphere to come into play. The outstanding
dislocated fragments with their vertical walls and flat tops,
showing segments of fields, have been gradually worn into
tower-like masses with sloping declivities of debris. The
long parallel rent has been widened by rain into a defile
with shelving sides. Everywhere the rawness of the original
fissures has been softened by the rich tapestry of verdure
which the genial climate of that southern coast fosters in
every sheltered nook. But the scars have not been healed,
and they will no doubt remain still visible for many a year
to come.
Along the south coast of England, many landslips, of
which there is no historical record, have produced some of
the most picturesque scenery of that region. Masses that
have slipped away from the main cliff have so grouped them-
selves down the slopes that hillocks and hollows succeed
each other in endless confusion, as in the well-known
v.]
ORIGIN OF LANDSLIPS.
6 9
70 GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP.
Undercliff of the Isle of Wight. Some of the tumbled
rocks are still fresh enough to show that they have fallen at
no very remote period, or even that the slipping still con-
tinues ; others, again, have yielded so much to the weather
that their date doubtless goes far back into the past, and
some of them are crowned with what are now venerable ruins.
The most stupendous landslips on record have occurred
in mountainous countries. Upwards of 150 destructive
examples have been chronicled in Switzerland. Of these,
one of the most memorable was that of the Rossberg, a
mountain lying behind the Rigi, and composed of thick
masses of hard red sandstone and conglomerate so arranged
as to slope down into the valley of Goldau. The summer
of the year 1806 having been particularly wet, so large an
amount of water had collected in the more porous layers of
rock as to weaken the support of the overlying mass ; conse-
quently a large part of the side of the mountain suddenly
gave way and rushed down into the valley, burying under
the debris about a square German mile of fertile land, four
villages containing 330 cottages and outhouses, and 457
inhabitants. To this day, huge angular blocks of sandstone
lying on the farther side of the valley bear witness to the
destruction caused by this landslip, and the scar on the
mountain-slope whence the fallen masses descended is still
fresh.
But it is by its chemical action on the rocks through
which it flows that subterranean water removes by far the
largest amount of mineral matter, and produces the greatest
geological change. Even pure water will dissolve a minute
quantity of the substance of many rocks. But rain is far
from being chemically pure water. In previous chapters
it has been described as taking oxygen and carbonic acid out
v.] SOLUTION BY SPRINGS. 71
of the air in its descent, and abstracting organic acids and
carbonic acid from the soil through which it sinks. By
help of these ingredients, it is enabled to attack even the
most durable rocks, and to carry some of their dissolved
substance up to the surface of the ground.
One of the substances most readily attacked and removed
even by pure water is the mineral known as carbonate of
lime. Among other impurities, natural waters generally
contain carbonic acid, which may be derived from the air
or from the soil; occasionally from some deeper subter-
ranean source. The presence of this acid gives the water
greatly increased solvent power, enabling it readily to attack
carbonate of lime, whether in the form of limestone, or
diffused through rocks composed mainly of other substances.
Even lime, which is not in the form of carbonate, but is
united with silica in various crystalline minerals (silicates,
p. 174), may by this means be decomposed and combined
with carbonic acid. It is then removed in solution as car-
bonate. So long as the water retains enough of free carbonic
acid, it can keep the carbonate of lime in solution and carry
it onward.
Limestone is a rock almost entirely composed of car-
bonate of lime. It occurs in most parts of the world, cover-
ing sometimes tracts of hundreds or thousands of square
miles, and often rising into groups of hills, or even into
ranges of mountains (see pp. 204, 209). The abundance
of this rock affords ample opportunity for the display of the
solvent action of subterranean water. Trickling down the
vertical joints and along the planes between the limestone
beds, the water dissolves and removes the stone, until in the
course of centuries these passages are gradually enlarged
into wide clefts, tunnels, and caverns, The ground be-
72 GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP.
comes honeycombed with openings into dark subterranean
chambers, and running streams fall into these openings and
continue their course underground.
Every country which possesses large limestone tracts
furnishes examples of the way in which such labyrinthine
tunnels and systems of caverns are excavated. In England,
for example, the Peak Cavern of Derbyshire is believed to
be 2300 feet long, and in some places 120 feet high. On a
much more magnificent scale are the caverns of Adelsberg
near Trieste, which have been explored to a distance of
FIG. 19. Section of cavern with stalactites and stalagmite.
between 4 and 5 miles, but are probably still more extensive.
The river Poik has broken into one part of the labyrinth
of chambers, through which it rushes before emerging again
to the light. Narrow tunnels expand into spacious halls,
beyond which egress is again afforded by low passages into
other lofty recesses. The most stupendous chamber
measures 669 feet in length, 630 feet in breadth, and in
feet in height. From the roofs hang pendent white stalac-
tites (p. 74), which, uniting with the floor, form pillars of
endless varieties of form and size. Still more gigantic is the
system of subterranean passages in the Mammoth Cave of
v.] DEPOSITS FROM SPRINGS. 73
Kentucky, the accessible parts of which are believed to have
a combined length of about 150 miles. The largest cavern
in this vast labyrinth has an area of two acres, and is covered
by a vault 125 feet high.
Of the mineral matter dissolved by permeating water out
of the rocks underground, by far the larger part is dis-
charged by springs into rivers and ultimately finds its way to
the sea. The total amount of material thus supplied to the
sea every year must be enormous. Much of it, indeed,
is abstracted from ocean-water by the numerous tribes of
marine plants and animals. In particular, the lime, silica,
and organic matter are readily seized upon to build up the
framework and furnish the food of these creatures. But,
probably, more mineral matter is supplied in solution than
is required by the organisms of the sea, in which case the
water of the sea must be gradually growing heavier and
salter.
Deposition of Material. But it is the smaller propor-
tion of the material not conveyed into the sea that specially
demands attention. Every spring, even the purest and most
transparent, contains mineral solutions in sufficient quantity
to be detected by chemical analysis. Hence all plants and
animals that drink the water of springs and rivers necessarily
imbibe these solutions which, indeed, supply some of the
mineral salts whereof the harder parts both" of plants and
animals are constructed. Many springs, however, contain so
large a proportion of mineral matter, that when they reach
the surface and begin to evaporate, they drop their solutions
as a precipitate, which settles down upon the bottom or on
objects within reach of the water. After years of undis-
turbed continuance, extensive sheets of mineral material may
in this manner be accumulated, which remain as enduring
74 GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP.
monuments of the work of underground water, even long
after the springs that formed them have ceased to flow.
Among the accumulations of this nature by far the most
frequent and important are those formed by what are called
Calcareous Springs. In regions abounding in limestone or
rocks containing much carbonate of lime, the subterranean
waters which, as we have seen, gradually erode such vast
systems of tunnels, clefts, and caverns, carry away the dis-
solved rock, and retain it in solution only so long as they
can keep their carbonic acid. As soon as they begin to
evaporate and to lose some of this acid, they lose also the
power of retaining so much carbonate of lime in solution.
This substance is accordingly dropped as a fine white powder
or precipitate, which gathers on the surfaces over which the
water trickles or flows.
The most familiar example of this process is to be seen
under the arches of bridges and vaults. Long pendent
white stalks or stalactites hang from between the joints of
the masonry, while wavy ribs of the same substance run
down the piers or walls, and even collect upon the ground
(stalagmite}. A few years may suffice to drape an archway
with a kind of fringe of these pencil-like icicles of stone.
Percolating from above through the joints between the
stones of the masonry, the rain-water, armed with its
minute proportion of carbonic acid, at once attacks the lime
of the mortar, forming carbonate of lime which is carried
downward in solution. Arriving at the surface of the arch,
the water gathers into a drop which remains hanging there
for a brief interval before it falls to the ground. That in-
terval suffices to allow some of the carbonic acid to escape,
and some of the water to evaporate. Consequently, round
the outer rim of the drop a slight precipitation of white
v.] STALACTITE AND STALAGMITE. 75
chalky carbonate of lime takes place. This circular pellicle,
after the drop falls, is increased by a similar deposit from the
next drop, and thus drop by drop the original rim or ring is
gradually lengthened into a tube which
may eventually be filled up inside and may
be thickened irregularly outside by the
trickle of calcareous water (Fig. 20). But
the deposition on the roof does not ex-
haust the stock of dissolved carbonate.
When the drops reach the ground the
same process of evaporation and precipita-
tion continues. Little mounds of the same
white chalky substance are built up on the
floor, and, if the place remain undisturbed,
may grow until they unite with the stalac-
tites from the roof, forming white pillars
that reach from floor to ceiling (Fig. 19,
and p. 204).
It is in limestone caverns that stalac-
titic growth is seen on the most colossal
scale. These quiet recesses having re-
mained undisturbed for many ages, the
process of solution and precipitation has FIG. 20. Section
advanced without interruption until, in showing successive
many cases, vast caverns have been trans- layer f of growth in
' a stalactite.
formed into grottoes of the most marvellous
beauty. White glistening fringes and curtains of carbonate
of lime, or spar, as it is popularly called, hang in endless
variety and beauty of form from the roof. Pillars of every
dimension, from slender wands up to thick-ribbed columns
like those of a cathedral, connect the roof and the pavement.
The walls, projecting in massive buttresses and retiring into
76 GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP.
alcoves, are everywhere festooned with a grotesque drapery
of stone. The floor is crowded with mounds and bosses
of strangely imitative forms which recall some of the oddest
shapes above ground. Wandering through such a scene,
the visitor somehow feels himself to be in another world,
where much of the architecture and ornament belongs to
styles utterly unlike those which can be seen anywhere else.
The material composing stalactite and stalagmite is at
first, as already stated, a fine white chalky pulp-like substance
which dries into a white powder. But as the deposition
continues, the older layers, being impregnated with calcareous
water, receive a precipitation of carbonate of lime between
their minute pores and crevices, and assume a crystalline
structure. Solidifying and hardening by degrees they end
by becoming a compact crystalline stone (spar) which rings
under the hammer.
The numerous caverns of limestone districts have offered
ready shelter to various kinds of wild animals and to man
himself. Some of them have been hyaena-dens, and from
under their hard floor of stalagmite, the bones of hyaenas
and of the creatures they fed upon are disinterred in abund-
ance. Rude human implements have likewise been obtained
from the same deposits, showing that man was contemporary
with animals which have long been extinct. The solvent
action of underground water has thus been of the utmost
service in geological history, first, in forming caverns that
could be used as retreats, and then in providing a hard in-
crustation which should effectually seal up and preserve the
relics of the denizens left upon the cavern-floors.
Calcareous springs, issuing from limestone or other rock
abounding in lime, deposit carbonate of lime as a white pre-
cipitate. So large is the proportion of mineral contained by
v.] CALCAREOUS DEPOSITS. 77
some waters that thick and extensive accumulations of it
have been formed. The substance thus deposited is known
by the names of Calcareous Tufa, Calc-sinter^ or Travertine.
It varies in texture, some kinds being loose and crumbling,
others hard and crystalline. In many places it is composed
of thin layers or laminae, of which sixty may be counted in
the thickness of an inch, but bound together into a solid
stone. These laminae mark the successive layers of deposit.
They are formed parallel to the surface over which the water
flows or trickles, hence they may be observed not only on
the flat bottoms of the pools, but irregularly enveloping the
walls of the channel as far up as the dash of water or spray
can reach. Rounded bosses may thus be formed above the
level of the stream, and the recesses may be hung with
stalactites.
The calcareous springs of Northern and Central Italy
have long been noted for the large amount of their dissolved
lime, the rapidity with which it is deposited, and the extensive
masses in which it has accumulated. Thus at San Filippo
in Tuscany, it is deposited at the rate of one foot in four
months, and it has been piled up to a depth of at least 250
feet, forming a hill a mile and a quarter long, and a third
of a mile broad. So compact are many of the Italian tra-
vertines that they have from time immemorial been exten-
sively used as a building stone, which can be dressed and is
remarkably durable. Many of the finest buildings of ancient
and modern Rome have been constructed of travertine.
A familiar feature of many calcareous springs deserves
notice here. The precipitation of calc-sinter is not always
due merely to evaporation. In many cases, where the pro-
portion of carbonate of lime in solution is so small that
under ordinary circumstances no precipitation of it would
78 GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP.
take place, large masses of it have been deposited in a
peculiar fibrous form. On examination, this precipitation
will be found to be caused by the action of plants, particu-
larly bog-mosses which, decomposing the carbonic acid in
the water, cause the lime-carbonate to be deposited along
their stems and leaflets. The plants are thus incrusted with
sinter which, preserving their forms, looks as if it were com-
posed of heaps of moss turned into stone. Hence the name
of petrifying springs often given to waters where this process
is to be seen. There is, however, no true petrifaction or
conversion of the actual substance of the plants into stone.
The fibres are merely incrusted with travertine, inside of
which they eventually die and decay. But as the plants
continue to grow outward, they increase the sinter by fresh
layers, while the inner and dead parts of the mass are filled
up and solidified by the deposit of the precipitate within
their cavities.
A growing accumulation of travertine presents a special
interest to the geologist from the fact that it offers excep-
tional facilities for the preservation of remains of the plants
and animals of the neighbourhood. Leaves from the sur-
rounding trees and shrubs are blown into pools or fall upon
moist surfaces where the precipitation of lime is actively
going on (Fig. 21). Dead insects, snail-shells, birds, small
mammals, and other denizens of the district may fall or be
carried into similar positions. These remains may be
rapidly enclosed within the stony substance before they have
time to decay, and even if they should afterwards moulder
into dust, the sinter enclosing them retains the mould of
their forms, and thus preserves for an indefinite period the
record of their former existence.
A second but less abundant deposit from springs is found
v.] DEPOSITS FROM CHALYBEATE SPRINGS. 79
in regions where the rocks below ground contain decompos-
ing sulphide of iron (p. 184). Water percolating through
FIG. 21. Travertine with impressions of leaves.
such rocks and oxidising the sulphur of that mineral, forms
sulphate of iron (ferrous sulphate) which it removes in solu-
tion. The presence of any notable quantity of this sulphate
is at once revealed by the marked inky taste of the water
and by the yellowish-brown precipitate on the sides and
bottom of the channel. Such water is termed Chalybeate.
When it mixes with other water containing dissolved car-
bonates (which are so generally present in running water), the
sulphate is decomposed, the sulphuric acid passing over to
the lime or alkali of the carbonate, while the iron takes up
oxygen and falls to the bottom as a yellowish-brown pre-
cipitate (p. 172). This interchange of combinations, with
the consequent precipitation of iron-oxide, may continue
for a considerable distance from the outflow of the chaly-
beate water. Nearest the source the deposit of hydrated
8o GEOLOGICAL RECORDS LEFT BY SPRINGS. [CHAP.
ferric oxide or ochre is thickest. It encloses leaves, stems,
and other organic remains, and preserves moulds or casts
of their forms. It also cements the loose sand and shingle
of a river-bottom into solid rock.
One other deposit from spring-water may be enumerated
here. In volcanic regions, hot springs (geysers) rise to the
surface which, besides other mineral ingredients, contain a
considerable proportion of silica (p. 157). This substance is de-
posited as Siliceous Sinter round the vents whence the water
is discharged, where it forms a white stone rising into mounds
and terraces with fringes and bunches of coral-like growth.
Where many springs have risen in the same district, their
respective sheets of sinter may unite, and thus extensive
tracts are buried under the deposit. In Iceland, for ex-
ample, one of the sheets is said to be two leagues long, a
quarter of a league wide, and a hundred feet thick. In
the Yellowstone Park of North America, many valleys are
floored over with heaps of sinter, and in New Zealand other
extensive accumulations of the same material are to be found.
It is obvious that, like travertine, siliceous sinter may readily
entomb and preserve a record of the plants and animals
that lived at the time of its deposition.
Summary. The underground circulation of water pro-
duces changes that leave durable records in geological history.
These changes are of two kinds, (i) Landslips are caused,
by which the forms of cliffs, hills, and mountains are per-
manently altered. Vast labyrinths of subterranean tunnels,
galleries, and caverns are dissolved out of calcareous rocks,
and openings are made from these passages up to the surface
whereby rivers are engulfed. Many of the caves thus
hollowed out have served as dens of wild beasts and dwell-
ing-places for man, and the relics of these inhabitants have
v.] SUMMARY. 8 1
been preserved under the stalagmite of the floors. (2) An
enormous quantity of mineral matter is brought up to the
surface by springs. Most of these solutions are conveyed
ultimately to the sea where they partly supply the substances
required by the teeming population of marine plants and
animals. But under favourable circumstances, considerable
deposits of mineral matter are made by springs, more especi-
ally in the form of travertine, siliceous sinter, and ochre. In
these deposits the remains of terrestrial vegetation, also of
insects, birds, mammals, and other animals are not infre-
quently preserved, and remain as permanent memorials of
the life of the time when they flourished.
CHAPTER VI.
ICE-RECORDS.
ICE in various ways alters the surface of the land. By
eroding even the most durable rocks, and by removing and
piling up elsewhere a vast amount of loose materials, it
greatly modifies the details of a landscape. As it assumes
various forms, so it accomplishes its work with considerable
diversity. The action of frost upon soil and bare surfaces
of rock has already (p. 18) been described. We have now
to consider the action of frozen rivers and lakes, snow and
glaciers, which have each their own characteristic style of
operation, and leave behind them their distinctive contribu-
tion to the geological history of the earth.
Frozen Rivers and Lakes. In countries with a severe
winter climate, the rivers and lakes are frozen over and the
cake of ice that covers them may be more than two feet
thick. When this cake is broken up in early summer, large
masses of it are driven ashore, tearing up the boulders, gravel,
sand, or mud, and pushing them to a height of many feet
above the ordinary level of the water. When the ice melts,
huge heaps of detritus are found to have been piled up by
it, which remain as enduring monuments of its power. Not
only so, but large fragments of the ice that has been formed
CHAP, vi.] FROZEN RIVERS SNOW. 83
along shore and has enclosed blocks of stone, gravel, and
sand, are driven away and may travel many miles before
they melt and drop their freight of stones. On the St.
Lawrence and on the coast of Labrador, there is a constant
transportation of boulders by this means. Further, besides
freezing over the surface, the water not infrequently forms
a loose spongy kind of ice on the bottom (Anchor-ice^ Ground-
ice) which encloses stones and gravel, and carries them up
to the surface where it joins the cake of ice there. This
bottom of ice is formed abundantly on some parts of the
Canadian rivers. Swept down by the current, it accumulates
against the bars or banks, or is pushed over the upper ice,
and from time to time gathers into temporary barriers the
bursting of which may cause destructive floods. In the
river St. Lawrence, banks and islets have been to a large
extent worn down by the grating of successive ice-rafts upon
them.
Snow. On level or gently inclined ground, snow dis-
appears where it falls. But while it remains, it exercises a
protective influence upon the soil and vegetation, shielding
them from the action of frost. On slopes of sufficient decliv-
ity, however, the sheet of snow acquires a tendency to descend
by gravitation. In many cases, it creeps or slides down the
side of a hill or valley, and in so doing moves forward bare
soil, loose stones, or other objects lying on the surface. By
this means, the debris of weathered rock in exposed situa-
tions is gradually pushed down-hill and the rock is bared for
further disintegration. But it is where the declivities are
steep enough to allow the snow to break off in large sheets
and to rush rapidly down that the most striking changes are
observable. Such descending masses are known as Ava-
lanches. Varying from 10 to 50 feet or more in thickness
84 ICE-RECORDS. [CHAP.
and several hundred yards broad and long, they sweep down
the mountain sides with terrific force, carrying away trees,
soil, houses, and even large blocks of rock. The winter of
1884-85 was especially remarkable for the number of ava-
lanches in the valleys of the Alps, and for the enormous
loss of life and property which they caused. Not only
are the declivities bared of their trees, soil, and boulders,
but huge mounds of debris are piled up on the valley below.
Frequently, also, such a quantity of snow, ice, and rubbish
is thrown across the course of a stream as to dam back the
water, which accumulates until it overflows or sweeps away
the barrier. In another but indirect way, snow may power-
fully affect the surface of a district where, by rapid melting,
it so swells the rivers as to give rise to destructive floods.
While, therefore, the influence of snow is on the whole to
protect the surface of the land, it shows itself in mountainous
regions singularly destructive, and leaves as chief memorials
of this destructiveness the mounds and rough heaps of
earth and stones that mark where the down -rushing ava-
lanches have come to rest.
Glaciers and Ice-Sheets leave their record in char-
acters so distinct that they cannot usually be confounded
with those of any other kind of geological agent. The
changes which they produce on the surface of the land may
be divided into two parts: (i) the transport of materials
from the high grounds to lower levels, and (2) the erosion
of their beds. As a glacier descends its valley, it receives
upon its surface the earth, sand, mud, gravel, boulders, and
blocks of rock that roll or are washed down from the slopes
on either side. Most of this rubbish accumulates on the
edges of the glacier, where it is slowly borne to lower levels
as the ice creeps downwards. But some of it falls into the
vi.] GLACIERS AND ICE-SHEETS. 85
crevasses by which the ice is split, and may either be im-
prisoned within the glacier, or may reach the rocky floor
over which the ice is sliding. The rubbish borne onward
upon the surface of the glacier is known as moraine-stuff.
The mounds of it running along each side of the glacier
form lateral moraines, those on the right hand side as we
look down the length of the valley being the right lateral
FIG. 22. Glacier with medial and lateral moraines.
moraine, those on the other side the left lateral moraine.
Where two glaciers unite, the left lateral moraine of the
one joins the right lateral moraine of the other, forming
what is called a medial moraine that runs down the middle
of the united glacier. Where a glacier has many tributaries
bearing much moraine-stuff, its surface may be like a bare
plain covered with earth and stones, so that except where a
yawning crevasse reveals the clear blue gleam of the ice
86
ICE-RECORDS.
[CHAP.
below, nothing but earth and stones meets the eye. When
the glacier melts, the detritus is thrown in heaps upon the
valley, forming there the terminal moraine.
Glaciers, like rivers, are subject to variations of level.
E ven from year to year they slowly sink below their previous
limit or rise above it. The glacier of La Brenva, for example,
on the Italian side of Mont Blanc, subsided no less than 300
feet in the first half of the present century. One notable
consequence of such diminution is that the blocks of rock
FIG. 23. Perched blocks scattered over ice-worn surface of rock.
lying on the edges of a glacier are stranded on the side of
the valley, as the ice shrinks away from them. Such Perched
Blocks or Erratics (Fig. 23), as they are called, afford an
excellent means of noting how much higher and longer a
glacier has once been than it is now. Their great size (some
of them are as large as good-sized cottages) and their
peculiar positions make it quite certain that they could not
have been transported by any current of water. They are
often poised on the tops of crags, on the very edges of
precipices, or on steep slopes where they could never have
vr.] ERRATIC BLOCKS. 87
been left by any flood, even had the flood been capable of
moving them. The agent that deposited them in such
positions must have been one that acted very quietly and
slowly, letting the blocks gently sink into the sites they
now occupy. The only agent known to us that could have
done this is glacier-ice. We can actually see similar blocks
on the glaciers now, and others which have only recently
been stranded on the side of a valley from which the ice
has sunk. In the Swiss valleys, the scattered ice-borne
boulders may be observed by hundreds far above the exist-
ing level of the glaciers and many miles beyond where these
now end. If the origin of the dispersed erratics is self-evident
in a valley where a glacier is still busy transporting them,
those that occur in valleys which are now destitute of glaciers
can offer no difficulty ; they become, indeed, striking monu-
ments that glaciers once existed there.
Scattered erratic blocks offer much interesting evidence
of the movements of the ice by which they were transported.
In a glacier-valley, the blocks that fall upon the ice remain
on the side from which they have descended. Hence, if
there is any notable difference between the rocks of the two
sides, this difference will be recognisable in the composition
of the moraines, and will remain distinct even to the end of
the glacier. If, therefore, in a district from which the glaciers
have disappeared, we can trace up the scattered blocks to
their sources among the mountains, we thereby obtain evi-
dence of the actual track followed by the vanished glaciers.
The limits to which these blocks are traceable do not, of
course, absolutely fix the limits of the ice that transported
them. They prove, however, that the ice extended at least
as far as they occur, but it may obviously have risen higher
and advanced farther than the space within which the blocks
88 ICE-RECORDS. [CHAP.
are now confined. In Europe, some striking examples occur
of the use of this kind of evidence. Thus the peculiar blocks
of the Valais can be traced all the way to the site of the
modern city of Lyons. There can be no doubt that the
glacier of the Rhone once extended over all that interven-
ing country and reached at least as far as Lyons, a distance
of not less than 170 miles from where it now ends. Again,
from the occurrence of blocks of some of the characteristic
rocks of Southern Scandinavia, in Northern Germany,
Belgium, and the east of England, we learn that a great
sheet of ice once rilled up the bed of the Baltic and the
North Sea, carrying with it immense numbers of northern
erratics. In Britain, where there are now neither glaciers
nor snow-fields, the abundant dispersion of boulders from
the chief tracts of high grounds shows that this country
was once in large part buried under ice, like modern Green-
land. The evidence for these statements will be more fully
given in a later part of this volume (chapter xxvi.)
Besides the moraine-stuff carried along on the surface,
loose detritus and blocks of rock are pushed onwards under
the ice. When a glacier retires, this earthy and stony debris,
where not swept away by the escaping river, is left on the
floor of the valley. One remarkable feature of the stones in
it is that a large proportion of them are smoothed, polished,
and covered with fine scratches or ruts, such as would be
made by hard sharp-pointed fragments of stone or grains of
sand. These markings run for the most part along the
length of each oblong stone, but not infrequently cross each
other, and sometimes an older may be noticed partially
effaced by a newer set. The striation of these stones is
a most characteristic mark of the action of glaciers. The
stones under the ice are fixed in the line of least resistance
VI.]
GLACIAL STRIATION.
that is, end on. In this position, under the weight of
hundreds of feet of ice, they are pressed upon the floor over
which the glacier is travelling. Every sharp edge of stone
or grain of sand, pressed along' the surface of a block, or
over which the block itself is slowly drawn, engraves a fine
scratch or a deeper rut. As the block moves onward, it is
more and more scratched, losing its corners and edges,
FIG. 24. Stone smoothed and striated by glacier-ice.
becoming smaller and smoother till, if it should travel far
enough, it might be entirely ground into sand or mud (Fig. 24).
The same process is carried on upon the solid rocks
over which the ice moves. These are smoothed, striated,
and polished by the friction of the grains of sand, pebbles,
and blocks of stone crushed against them by the slowly
creeping mass of ice. Every boss of rock that looks toward
the quarter from which the overlying ice is moving is ground
away, while those that face to the opposite side are more
or less sharp and unworn. The striation is especially note-
worthy. From the fine scratches, such as are made by
grains of sand, up to deep ruts like those of cart-wheels in
unmended roadways, or to still wider and deeper hollows, all
9
ICE-RECORDS.
[CHAP.
the friction -markings run in a general uniform direction,
which is that of the motion of the glacier. Such striated
surfaces could only be produced by some agent with rigidity
FIG. 25. Ice-striation on the floor and side of a valley.
enough to hold the sand-grains and stones in position, and
press them steadily onward upon the rocks. A river polishes
the rocks of its channel by driving shingle and sand across
them j but the currents are perpetually tossing these materials
now to one side, now to another, so that smoothed and
polished surfaces are produced, but with nothing at all re-
sembling striation. A glacier, however, by keeping its
grinding materials fixed in the bottom of the ice, engraves
its characteristic parallel striae and groovings, as it slowly
vi.] EROSION BY GLACIERS. 91
creeps down the valley. All the surfaces of rock within
reach of the ice are smoothed, polished, and striated. Such
surfaces present the most unmistakable evidence of glacier-
action, for they can be produced by no other known natural
agency. Hence, where they occur in glacier valleys, far above
and beyond the present limits of the ice, they prove how
greatly the ice has sunk. In regions also where there are
now no glaciers, these rock-markings remain as almost im-
perishable witnesses that glaciers once existed. By means
of their evidence, for example, we can trace the march of
great ice-sheets which once enveloped the whole of Scandi-
navia and lay deep upon nearly the whole of Britain.
The river that escapes from the end of a glacier is always
muddy. The fine sand and mud that discolour the water
are not supplied by the thawing of the clear ice, nor by the
sparkling brooks that gush out of the mountain-slopes, nor
by the melting of the snows among the peaks that rise on
either side. This material can only come from the rocky
floor 01 the glacier itself. It is the fine sediment ground
away from the rocks and loose stones by their mutual friction
under the pressure of the overlying ice. It is thus a kind
of index or measure of the amount of material worn off the
rocky bed by the grinding action of the glacier. We can
readily see that as this erosion and transport are continually
in progress, the amount of material removed in the course
of time must be very great. It has been estimated, for ex-
ample, that the Justedal glacier in Norway removes annually
from its bed 2,427,000 cubic feet of sediment. At this rate
the amount removed in a century would be enough to fill
up a valley or ravine 10 miles long, 100 feet broad, and 40
feet deep.
In arctic and antarctic latitudes, where the land is
92 ICE-RECORDS. [CHAP.
buried under a vast ice-sheet, which is continually creeping
seaward and breaking off into huge masses that float away
as icebergs, there must be a constant erosion of the terrestrial
surface. Were the ice to retire from these regions, the ground
would be found to wear what is called a glaciated surface ;
that is to say, all the bare rocks would present a character-
istic ice-worn aspect, rising into smooth rounded bosses like
dolphins' backs (roches moutonn'ees), and sinking into hollows
that would become lake -basins. Everywhere these bare
rocks would show the striae and groovings graven upon them
by the ice, radiating generally from the central high grounds,
and thus indicating the direction of flow of the main streams
of the ice-sheet. Piles of earth, ice -polished stones, and
blocks of rock would be found strewn over the country,
especially in the valleys and over the plains. These
materials would still further illustrate the movements of the
ice, for they would be found to be singularly local in char-
acter, each district having supplied its own contribution of
detritus. Thus in a region of red sandstone, the rubbish
would be red and sandy ; in one of black slate, it would be
black and clayey (see chapter xxvi.)
Summary. In this chapter we have seen that ice in
various ways affects the surface of the land and leaves its
mark there. Frost pulverises soil, disintegrates exposed
surfaces of stone, and splits open bare rocks along their
lines of natural joint. On rivers and lakes, the disrupted
ice wears down banks and pushes up mounds of sand, gravel,
and boulders along the shores. In the condition of ava-
lanches, it causes large quantities of earth, soil, and blocks
of rock to be removed from the mountain-slopes and piled
up on the valleys. In the form of glaciers, it transports
the debris of the mountains to lower levels, bearing along
vi.] SUMMARY. 93
and sometimes stranding masses of rock as large as cottages,
which no other known natural agent could transport.
Moving down a valley, it wears away the rocks, giving them
a peculiar smoothed and striated surface which is thoroughly
characteristic. By this grinding action, it erodes its bed and
produces a large amount of fine sediment, which is carried
away by the river that escapes at the end of the glacier.
Land-ice thus leaves thoroughly distinctive and enduring
memorials of its presence in polished and grooved rocks, in
masses of earth, clay, or gravel, with striated stones, and in
the dispersal of erratic blocks from principal masses of high
ground. These memorials may remain for ages after the
ice itself has vanished. By their evidence we know that the
present glaciers of the Alps are only a shrunk remnant of
the great ice -fields which once covered that region; that
the Scandinavian glaciers swept across what is now the bed
of the North Sea as far as the mouth of the Thames ; and
that Scotland, Ireland, Wales, and the greater part of Eng-
land were buried under great sheets of ice which crept
downwards into the North Sea on the one side, and into the
Atlantic on the other.
CHAPTER VII.
THE MEMORIALS OF THE PRESENCE OF THE SEA.
WE have now to inquire how the work of the sea is
registered in geological history. This work is broadly of
two kinds. In the first place, the sea is engaged in wear-
ing away the edges of the land, and in the second place,
being the great receptacle into which all the materials, worn
away from the land, are transported, it arranges these mate-
rials over its floor, ready to be raised again into land at
some future time.
I. Demolition of the Land. In its work of destruc-
tion along the coasts of the land, the sea acts to some extent
(though we do not yet know how far) by chemically dissolv-
ing the rocks and sediments which it covers. Cast-iron
bars, for example, are so corroded by sea-water as to lose
nearly half their strength in fifty years. Doubtless many
minerals and rocks are liable to similar attacks.
But it is by its mechanical effects that the sea accom-
plishes most of its erosion. The mere weight with which
ocean-waves fall upon exposed coasts breaks off fragments
of rock from cliffs. Masses, 13 tons in weight, have been
known to be quarried out of the solid rock by the force
of the breakers in Shetland, at a height of 70 feet above
CHAP. VII.]
BREAKER- ACTION.
95
sea -level. As a wave may fall with a blow equal to a pres-
sure 3 tons on the square foot, it compresses the air in every
cleft and cranny of a cliff, and when it drops it allows the
air instantly to expand again. By this alternate compression
and expansion, portions of the cliff are loosened and removed.
Where there is any weaker part in the rock, a long tunnel
FIG. 26. Buller of Buchan a caldron-shaped cavity 'or blow-hole
worn out of granite by the sea on the coast of Aberdeenshire.
may be excavated, which may even be drilled through to the
daylight above, forming an opening at some distance inland
from the edge of the cliff. During storms, the breakers rush
through such a tunnel, and spout forth from the opening (or
blow-hole) in clouds of spray.
Probably the most effective part of the destructive action
of the sea is to be found in the battery of gravel, shingle,
96 MEMORIALS LEFT BY THE SEA. [CHAP.
and loose blocks of stone which the waves discharge against
cliffs exposed to their fury. These loose materials, caught
up by the advancing breakers and thrown with great force
upon the rocks of a coast-line, are dragged back in the recoil
of the water, but only to be again lifted and swung forward.
In this loud turmoil, the loose stones are reduced in size
and are ground smooth by friction against each other and
upon the solid cliff. The well-rounded and polished aspect
of the gravel on such storm-beaten shores is an eloquent
testimony to the work of the waves. But still more striking,
because more measurable, is the proof that the very cliffs
themselves cannot resist the blows dealt upon them by the
wave -borne stones. Above the ordinary limit reached by
the tides, the rocks rise with a rough ragged face, bearing the
scars inflicted on it by the ceaseless attacks of the air, rain,
frost, and the other agencies that waste the surface of the
land. But all along the base of the cliff, within reach of the
waves, the rocks have been smoothed and polished by the
ceaseless grinding of the shingle upon them, while arches,
tunnels, solitary pillars, half-tide skerries, creeks, and caves
attest the steady advance of the sea and the gradual demoli-
tion of the shore.
Every rocky coast -line exposed to a tempestuous sea
affords illustrations of these features of the work of waves.
Even where the rocks are of the most durable kind, they
cannot resist the ceaseless artillery of the ocean. They are
slowly battered down, and every stage in their demolition
may be witnessed, from the sunken reef, which at some dis-
tance from the shore marks where the coast -line once ran,
up to the tunnelled cliff from which a huge mass was detached
during the storms of last winter. But where the materials
composing the cliffs are more easily removed, the progress
VII.]
BREAKER-ACTION.
97
H
98 MEMORIALS LEFT BY THE SEA. [CHAP.
of the waves may be comparatively rapid. Thus on the east
coast of Yorkshire between Spurn Point and Flamborough
Head, the cliffs consist of boulder-clay, and vary up to more
than 100 feet in height. At high water, the tide rises against
the base of these cliffs, and easily scours away the loose
debris which would otherwise gather there and protect them.
Hence, within historic times, a large tract of land, with its
parishes, farms, villages, and seaports, has been washed away,
the rate of loss being estimated at not less than 2\ yards in
a year. Since the Roman occupation a strip of land between
2 and 3 miles broad is believed to have disappeared.
It is evident that to carry on effectively this mechanical
erosion, the sea-water must be in rapid motion. But in the
deeper recesses of the ocean, where there is probably no
appreciable movement of the water, there can hardly be any
sensible erosion. In truth, it is only in the upper parts of
the sea, which are liable to be agitated by wind, that the
conditions for marine erosion can be said to exist. The
space within which these conditions are to be looked for is
that comprised between the lowest depth to which the grind-
ing influence of waves extends, and the greatest height to
which breakers are thrown upon the land. These limits, no
doubt, vary considerably in different regions. In some parts
of the open sea, as off the coast of Florida, the disturbing
action of the waves has been supposed to reach to a depth
of 600 feet, though the average limit is probably greatly less.
On exposed promontories in stormy seas, such as those of
the north of Scotland, breakers have been known to hurl up
stones to a height of 300 feet above sea-level. But probably
the zone, within which the erosive work of the sea is carried
on, does not as a rule exceed 300 feet in vertical range.
Within some such limits as these, the sea is engaged in
VIL] RATE OF MARINE EROSION. 99
gnawing away the edges of the land. A little reflection will
show us that, if no counteracting operation should come into
play, the prolonged erosive action of the waves would reduce
the land below the sea-level. If we suppose the average rate
of demolition to be 10 feet in a century, then it would take
not less than 52,800 years to cut away a strip one mile
broad from the edge of the land. But while the sea is
slowly eating away the coast-line, the whole surface of the
land is at the same time crumbling down, and the wasted
materials are being carried away by rivers into the sea at
such a rate that, long before the sea could pare away more
than a mere narrow selvage, the whole land might be worn
down to the sea-level by air, rain, and rivers (p. 38).
But there are counteracting influences in nature that
would probably prevent the complete demolition of the land.
What these influences are will be more fully considered in a
later chapter. In the meantime, it will be enough to bear in
mind that while the land is constantly worn down by the
forces that are acting upon its surface, it is liable from time
to time to be uplifted by other forces acting from below.
And the existing relation between the amount and height of
land, and the extent of sea, on the face of the globe, must
be looked upon as the balance between the working of both
these antagonistic classes of agencies.
But without considering for the present whether the
results of the erosion performed by the sea will be inter-
rupted or arrested, we can readily perceive that their tend-
ency is toward the reduction of the level of the land to a
submarine plain (Fig. 28). As the waves cut away slice after
slice from a coast-line, the portion of land which they thus
overflow, and over which they drive the shingle to and fro,
is worn down until it comes below the lower limit of breaker-
ioo MEMORIALS LEFT BY THE SEA. [CHAP.
action, where it may be covered up with sand or mud.
When the abraded land has been reduced to this level, it
FIG. 28. Section of submarine plain." *V. Land cut into caves, tunnels,
sea-stacks, reefs and skerries by the waves, and reduced to a platform
below the level of the sea (s s) on which the gravel, sand, and mud (d)
produced by the waste of the coast may accumulate.
reaches a limit where erosion ceases, and where the sea,
no longer able to wear it down further, protects it from
injury by other agents of demolition.
We see, then, that the goal toward which all the wear
and tear of a coast-line tends, is the formation of a more
or less level platform cut out of the land. Yet an attentive
study of the process will convince us that in the production
of such a platform the sea has really had less to do than
the atmospheric agents of destruction. An ordinary sea-
cliff is not a vertical wall. In the great majority of cases
it slopes seaward at a steep angle ; but if it had been formed,
and were now being cut away, mainly by the sea, it ought
obviously to have receded fastest where the waves attack
it that is, at its base. In other words, if sea-cliffs retired
chiefly because they are demolished by the sea, they ought
to be most eroded at the bottom, and should therefore be
usually overhanging precipices. That this is not the case
shows that some other agency is concerned which causes
the higher parts of a cliff to recede faster than those below.
This agency can be no other than that of the atmospheric
vii.] DEPOSITS FORMED BV THE SEA. J lol
forces air, frost, rain, and springs. These cause the face
of the cliff to crumble down, detaching mass after mass,
which, piled up below, serve as a breakwater, and must be
broken up and removed by the waves before the solid cliif
behind them can be attacked.
II. Accumulations formed by the Sea. It is not
its erosive action that constitutes the most important claim
of the sea to the careful study of the geologist. After all,
the mere marginal belt or fringe within which this action is
confined forms such a small fraction of the whole terrestrial
area of the globe, that its importance dwindles down when
we compare it with the enormously vaster surface over which
the operations of the air, rain, rivers, springs, and glaciers
are displayed. But when we regard the sea as the receptacle
into which all the materials worn off the land ultimately find
their way, we see what a large part it must play in geological
history.
During the last fifteen years great additions have been
made to our knowledge of the sea-bottom all over the world.
Portions of the deposits accumulating there have been
dredged up even from the deepest abysses, so that it is now
possible to construct charts, showing the general distribution
of materials over the floor of the ocean.
Beginning at the shore, let us trace the various types of
marine deposits outward to the floors of the great ocean-
abysses. In many places, the sea is more or less barred
back by the accumulation of sediment worn away from the
land. In estuaries, for example, there is often such an
amount of mud in the water that the bottom on either side
is gradually raised above the level of tide-mark, and forms
eventually a series of meadows which the sea can no longer
overflow. At the mouths of rivers with a considerable current,
102 MEMORIALS LEFT BY THE SEA. [CHAP.
a check is given to the flow of the water when it reaches the
sea, and there is a consequent arrest of its detritus. Hence,
a bar is formed across the outflow of a river, which during
floods is swept seawards, and during on-shore gales is driven
again inland. Even where there is no large river, the smaller
streams flowing off the surface of a country may carry down
sediment enough to be arrested by the sea, and thrown up
as a long bank or bar running parallel with the coast. Be-
hind this bar, the drainage of the interior accumulates in
long lagoons, which find an outflow through some breach
in the bar, or by soaking through the porous materials of
the bar itself. A large part of the eastern coast of the
United States is fringed with such bars and lagoons. A
space several hundred miles long on the east coast of India
is similarly bordered.
But the most remarkable kind of accumulation of ter-
restrial detritus in the sea is undoubtedly that of river-deltas.
Where the tidal scour is not too great, the sediment brought
down by a large river into a marine bay or gulf gradually
sinks to the bottom as the fresh spreads over and mingles
with the salt water. During floods, coarse sediment is swept
along, while during low states of the river nothing but fine
mud may be transported. Alternating sheets of different
kinds of sediment are thus laid down one upon another
on the sea-floor, until by degrees they reach the surface,
and thus gradually increase the breadth of the land. Some
deltas are of enormous size and depth. That of the Ganges
and Brahmaputra covers an area of between 50,000 and
60,000 square miles that is, about as large as England and
Wales. It has been bored through to a depth of 481 feet,
and has been found to consist of numerous alternations of
fine clays, marls, and sands or sandstones, with occasional
vii.] STORM-BEACHES. 103
layers of gravel. In all this great thickness of sediment, no
trace of marine organisms was found, but land-plants and
bones of terrestrial and fluviatile animals occurred.
Turning now to the deposits that are more distinctively
those of the sea itself, we find that ridges of coarse shingle,
gravel, and sand are piled up along the extreme upper limit
reached by the waves. The coarsest materials are for the
most part thrown highest, especially in bays and narrow
.KiG. 29. Storm-beach ponding back a stream and forming a lake j
west coast of Sutherlandshire.
creeks where the breakers are confined within converging
shores. In such situations, during heavy gales, storm-beaches
of coarse rounded shingle are formed sometimes several yards
above ordinary high-tide mark (Fig. 29). Where a barrier
of this kind is thrown across the mouth of a brook, the fresh
water may be ponded back to form a small lake, of which
the outflow usually escapes by percolation through the
shingle. In sheltered bays, behind headlands, or on parts
of a coast -line where tidal currents meet, detritus may
104 MEMORIALS LEFT BY THE SEA. [CHAP.
accumulate in spits or bars. Islands may in this way be
gradually united to each other or to the mainland, while the
mainland itself may gain considerably in breadth. At
Romney Marsh, on the south-east coast of England, for
instance, a tract of more than 80 square miles, which in
Roman times was in great part covered by the sea at high
water, is now dry land, having been gained partly by the
natural increase of shingle thrown up by the waves and
partly by the barriers artificially erected to exclude the sea.
While the coarsest shingle usually accumulates towards
the upper part of the beach, the materials generally arrange
themselves according to size and weight, becoming on the
whole finer as they are traced towards low-water mark. But
patches of coarse gravel may be noticed on any part of a
beach, and large boulders may be seen even below the limits
of the lowest tides. As a rule, the deposits formed along a
beach, and in the sea immediately beyond, include the
coarsest kinds of marine sediment. They are also marked
by frequent alternations of coarse and fine detritus, these
rapid interchanges pointing to the varying action of the
waves and strong shore -currents. Towards the lower limit
of breaker-action, fine gravel and sand are allowed to settle
down, and beyond these, in quiet depths where the bottom
is not disturbed, fine sand and mud washed away from the
land slowly accumulate.
The distance to which the finer detritus of the land is
carried by ocean -currents, before it finds its way to the
bottom, varies up to about 200 miles or more. Within this
belt of sea, the land-derived materials are distributed over
the ocean -floor. Coarse and fine gravel and sand are the
most common materials in the tracts nearest the land.
Beyond these, lie tracts of fine sand and silt with occasional
vii.] DEPOSITS OF THE OCEAN ABYSSES. 105
patches of gravel. Still farther from the land, at depths of
600 feet and upwards, fine blue and green muds are found,
composed of minute particles of such minerals as form
the ordinary rocks of the land. But traced out into the
open ocean, these various deposits of recognisable ter-
restrial origin give place to thoroughly oceanic accumu-
lations, especially to widespread sheets of red and brown
exceedingly fine clay. This clay, the most generally dif-
fused deposit of the deeper or abysmal parts of the sea,
appears to be derived from the decomposition of volcanic
fragments either washed away from volcanic islands or
supplied by submarine eruptions. That it is accumulated
with extreme slowness is shown by two curious and interest-
ing kinds of evidence. Where it occurs farthest removed
from land, great numbers of sharks' teeth, with ear-bones
and other bones of whales, have been dredged up from it,
some of these relics being quite fresh, others partially coated
with a crust of brown peroxide of manganese, some wholly
and thickly enveloped in this substance. The same haul
of the dredge has brought up bones in all these conditions,
so that they must be lying side by side on the red clay floor
of the ocean abysses. The deposition of manganese is no
doubt an exceedingly slow process, but it is evidently faster
than the deposition of the red clay. The bones dredged up
probably represent a long succession of generations of animals.
Yet so tardily does the red clay gather over them, that the
older ones are not yet covered up by it, though they have
had time to be deeply encased in oxide of manganese.
The second kind of evidence of the extreme slowness of
deposit in the ocean abysses is supplied by minute spherules
of metallic iron, which occurring in numbers dispersed through
the red clay, have been identified as portions of meteorites or
io6 MEMORIALS LEFT BY THE SEA. [CHAP.
falling stars. These particles no doubt fall all over the
ocean, but it is only where the rate of deposition of sediment
is exceedingly slow that they may be expected to be detected.
Besides the sediments now enumerated, the bottom of
the sea receives abundant accumulations of the remains of
shells, corals, foraminifera and other marine creatures ; but
these will be described in the next chapter, where an account
is given of the various ways in which plants and animals, both
upon the land and in the sea, inscribe their records in geologi-
cal history. It must also be borne in mind that throughout
all the sediments of the sea-floor, from the upper part of the
beach down to the bottom of the deepest and remotest
abyss, the remains of the plants, sponges, corals, shells,
fishes and other organisms of the ocean may be entombed
and preserved. It will suffice here to remember that various
depths and regions of the sea have their own characteristic
forms of life, the remains of which are preserved in the
sediments accumulating there, and that although gravel,
sand, and mud laid down beneath the sea may not differ in
any recognisable detail from similar materials deposited in
a lake or river, yet the presence of marine organisms in them
would be enough to prove that they had been formed in the
sea. It is evident, also, that if the sea-floor over a wide area
were raised into land, the extent of the deposits would show
that they could not have been accumulated in any mere
river or lake, but must bear witness to the former presence
of the sea itself.
Summary. The sea records its work upon the surface
of the earth in a twofold way. In the first place, in co-
operation with the atmospheric agents of disintegration, it
eats away the margin of the land and planes it down. The
final result of this process if uninterrupted would be to
vii.] SUMMARY. 107
reduce the level of the land to that of a submarine platform,
the position of the surface of which would be determined by
the lower limit of effective breaker-action. In the second
place, the sea gathers over its floor all the detritus worn by .
every agency from the surface of the land. This material
is not distributed at random; it is assorted and arranged
by the waves and currents, the coarsest portions being laid
down nearest the land, and the finest in stiller and deeper
water. The belt of sea-floor within which this deposition
takes place probably does not much exceed a breadth of
200 miles. Beyond that belt, the bottom of the ocean is
covered to a large extent with deposits of red clay derived
from the decomposition of volcanic material and laid down
with extreme slowness. These truly oceanic accumulations
are recognisably distinct from those derived from terrestrial
sources within the narrow zone of deposition near the land.
CHAPTER VIII.
HOW PLANTS AND ANIMALS INSCRIBE THEIR RECORDS IN
GEOLOGICAL HISTORY.
BROADLY considered, there are two distinct ways in which
plants and animals leave their mark upon the surface of the
earth. In the first place, they act directly by promoting or
arresting the decay of the land, and by forming out of their
own remains deposits which are sometimes thick and exten-
sive. In the second place, their remains are transported and
entombed in sedimentary accumulations of many different
kinds, and furnish important evidence as to the conditions
under which these accumulations were formed. Each of
these two kinds of memorial deserves our careful attention,
for, taken together, they comprise the most generally inter-
esting departments of geology, and those in which the history
of the earth is principally discussed. 1
I. We have first to consider the direct action of plants
and animals upon the surface of the globe. This action is
often of a destructive kind, both plants and animals taking
their part in promoting the general disintegration of rocks
and soils. Thus, by their decay they furnish to the soil those
1 In the Appendix a Table of the Vegetable and Animal King-
doms is given, from which the organic grade of the plants and animals
referred to in this and subsequent chapters may be understood.
CHAP, viii.] ACTION OF PLANTS AND ANIMALS. 109
organic acids referred to on p. 32, as so important in in-
creasing the solvent power of water, and thereby promoting
the waste of rocks. By thrusting their roots into crevices
of cliffs, plants loosen and gradually wedge off pieces of rock,
and by sending their roots and rootlets through the soil, they
open up the subsoil to be attacked by air and descending
moisture (p. 21). The action of the common earthworm in
bringing up fine soil to be exposed to the influences of wind
and rain was referred to at p. 24. Many burrowing animals
also, such as the mole and rabbit, throw up large quantities of
soil and subsoil which are liable to be blown or washed away.
On the other hand, the action may be conservative, as,
for instance, where, by forming a covering of turf, vegetation
protects the soil underneath from being rapidly removed, or
where sand-loving plants bind together the surface of dunes,
and thereby arrest the progress of the sand, or where forests
shield a mountain-side from the effects of heavy rains and
descending avalanches.
But it is chiefly by the aggregation of their own remains
into more or less extensive deposits that plants and animals
leave their most prominent and enduring memorials. As
examples of the way in which this is done by plants, refer-
ence may be made to peat-bogs, mangrove-swamps, infusorial
earth, and calcareous sea-weeds.
In temperate and arctic countries, marshy vegetation
accumulates in peat -bogs over areas of many square miles
and to a depth of sometimes 50 feet. These deposits are
largely due to the growth of bog -mosses and other aquatic
plants which, dying in their lower parts, continue to grow
upward on the same spot. On flat or gently-inclined moors,
in hollows between hills, on valley -bottoms, and in shallow
lakes, this marshy vegetation accumulates as a wet spongy
no RECORDS OF PLANTS AND ANIMALS. [CHAP.
fibrous mass, the lower portions of which by degrees become
a more or less compact dark brown or black pulpy substance,
wherein the fibrous texture, so well seen in the upper or
younger parts, in large measure disappears. In a thick bed
of peat, it is not infrequently possible to detect a succession
of plant remains, showing that one
kind of vegetation has given place
to another during the accumulation
of the mass. In Europe, as already
mentioned on p. 6, peat-bogs often
rest directly upon fresh -water marl
containing remains of lacustrine
shells (i in Fig. 30). In every such
case, it is evident that the peat has
FIG. 3 o.-Section of a accumulated on the site of a shallow
peat-bog.
lake which has been filled up, and
converted into a morass by the growth of marsh-plants
along its edges and over its floor. The lowest parts of the
peat may contain remains of the reeds, sedges, and other
aquatic plants which choked up the lake (2, 3). Higher
up, the peat consists almost entirely of the matted fibres of
different mosses, especially of the kind known as Bog-moss
or Sphagnum (4). The uppermost layers (5, 6) may be full
of roots of different heaths which spread over the surface of
the bog.
The rate of growth of peat has been observed in different
situations in Central Europe to vary from less than a foot to
about 2 feet in ten years ; but in more northern latitudes
the growth is probably slower. Many thousand square miles
of Europe and North America are covered with peat -bogs,
those of Ireland being computed to occupy a seventh part
of the surface of the island, or upwards of 4000 square miles.
viii.] PEAT, MANGROVE-SWAMPS, DIATOM-EARTH, in
As the aquatic plants grow from the sides toward the
centre of a shallow lake, they gradually cover over the surface
of the water with a spongy layer of matted vegetation.
Animals, and 'man himself, venturing on this treacherous
surface sink through it, and may be drowned in the black
peaty mire underneath ; and long afterwards, when the
morass has become firm ground, and openings are made in
it for digging out the peat to be used as fuel, their bodies
may be found in an excellent state of preservation. The
peaty water so protects them from decay that the very skin
and hair sometimes remain. In Ireland, numerous skeletons
of the great Irish elk have been obtained from the bogs,
though the animal itself has been extinct since before the
beginning of the authentic history of the country.
Along the flat shores of tropical lands, the mangrove tree
grows out into the salt water, forming a belt of jungle which
runs up or completely fills the creeks and bays. So dense
is the vegetation that the sand and mud, washed into the
sea from the land, are arrested among the roots and radicles
of the trees, and thus the sea is gradually replaced by firm
ground. The coast of Florida is fringed with such man-
grove-swamps for a breadth of from 5 to 20 miles. In
such regions, not only does the growth of these swamps
add to the breadth of the land, but the sea is barred back,
and prevented from attacking the newly -formed ground
inside.
A third kind of vegetable deposit to be referred to here
is that known by the names of infusorial earth, diatom-earth,
and tripoli-powder. It consists almost entirely of the minute
frustules of microscopic plants called diatoms, which are
found abundantly in lakes and likewise in some regions of
the ocean (Fig. 31). These lowly organisms are remarkable
H2 RECORDS OF PLANTS AND ANIMALS. [CHAP.
for secreting silica in their structure. As they die, their singu-
larly durable siliceous remains fall like a fine dust on the
bottom of the water, and accumulate there as a pale grey or
straw-coloured deposit, which, when dry, is like flour, and
in its pure varieties is made almost entirely of silica (90 to
97 per cent). Underneath the peat-bogs of Britain, a layer
of this material is sometimes met with. One of the most
famous examples is that of Richmond, Virginia, where a bed
of it occurs 30 feet thick. 'At Bilin in Bohemia also an
important bed has long been known. The bottom of some
FIG. 31. Diatom-earth from floor of Antarctic Ocean, magnified
300 diameters {Challenger Expedition).
parts of the Southern Ocean is covered with a diatom-ooze
made up mainly of siliceous diatoms, but containing also
other siliceous organisms (radiolarians) and calcareous fora-
minifera (Fig. 31).
Yet one further illustration of plant-action in the build-
ing up of solid rock may be given. Some sea-weeds abstract
from sea-water carbonate of lime, which they secrete to such
an extent as to form a hard stony structure, as in the case
of the common nullipore. When the plants die, their re-
mains are thrown ashore and pounded up by the waves, and
being singularly durable they form a white calcareous sand.
viii.] NULLIPORE-SAND, SHELL-BANKS. 113
By the action of the wind, this sand is blown inland and may
accumulate into dunes. But unlike ordinary sand, it is
liable to be slightly dissolved by rain-water, and as the por-
tion so dissolved is soon redeposited by the evaporation of
the moisture, the little sand-grains are cemented together,
and a hard crust is formed which protects the sand under-
neath from being blown away. Meanwhile rain-water per-
colating through the mounds gradually solidifies them by
cementing the particles of sand to each other, and thick
masses of solid white stone are thus produced. Changes of
this kind have taken place on a great scale at Bermuda,
where all the dry land consists of limestone formed of com-
pacted calcareous sand, mainly the detritus of sea-weeds.
Animals are, on the whole, far more successful than
plants in leaving enduring memorials of their life and work.
They secrete hard outer shells and internal skeletons en-
dowed with great durability, and capable of being piled up
into thick and extensive deposits which may be solidified
into compact and enduring stone. On land, we have an
example of this kind of accumulation in the lacustrine marl
already (pp. 6, 61) described as formed of the congregated
remains of various shells. But it is in the sea that animals,
secreting carbonate of lime, build up thick masses of rock,
such as shell-banks, ooze, and coral-reefs.
Some molluscs, such as the oyster, live in populous
communities upon submarine banks. In the course of
generations, thick accumulations of their shells are formed
on these banks. By the action of currents, also, large
quantities of broken shells are drifted to various parts of
the sea-bottom not far from land. Such deposits of shells,
in situ or transported, may be more or less mixed with or
buried under sand and silt, according as the currents vary
i
H4 RECORDS OF PLANTS AND ANIMALS. [CHAP.
in direction and force. On the other hand, they may be
gradually cemented into a solid calcareous mass, as has been
observed off the coast of Florida, where they form on the
FIG. 32. Recent limestone (Common Cockle, etc., cemented in a
matrix of broken shells).
sea-bottom a sheet of limestone, made up of remains of the
very same kinds of creatures that are living there.
From observations made during the great expedition of
the Challenger^ it has been estimated that in a square mile
of the tropical ocean down to a depth of 100 fathoms there
are more than 16 tons of carbonate of lime in the form of
living animals. A continual rain of dead calcareous organ-
isms is falling to the bottom, where their remains accumulate
as a soft chalky ooze. Wide tracts of the ocean -floor are
covered with a pale grey ooze of this nature, composed
mainly of the remains of the shells of the foraminifer Globi-
gerina (Fig. 33). In the north Atlantic this deposit prob-
ably extends not less than 1300 miles from east to west,
and several hundred miles from north to south.
Here and there, especially among volcanic islands, por-
tions of the sea-bed have been raised up into land, and
masses of modern limestone have thereby been exposed to
CORAL ISLANDS. H5
view. Though they are full of the same kind of shells as
are still living in the neighbouring sea, they have been
cemented into compact and even somewhat crystalline
FIG. 33. Globigerina ooze dredged up by Challenger Expedition from
a depth of 1900 fathoms in the North Atlantic (M).
rock, which has been eaten into caverns by percolating
water, like limestones of much older date. This cementa-
tion, as above remarked, is due to water permeating the
stone, dissolving from." its outer parts the calcareous matter of
shells, corallines, and other organic remains, and redeposit-
ing it again lower down, so as to cement the organic detritus
into a compact stone.
Coral islands offer an impressive example of how exten-
sive masses of solid rock may be built up entirely of the
aggregated remains of animals. In some of the warmer
seas of the globe, and notably in the track of the great
ocean -currents, where marine life is so abundant, various
kinds of coral take root upon the edges and summits of sub-
merged ridges and peaks, as well as on the shelving sea-
bottom facing continents or encircling islands (i in Fig. 34).
These creatures do not appear to flourish at a greater depth
than 15 or 20 fathoms, and they are killed by exposure
n6 RECORDS OF PLANTS AND ANIMALS. [CHAP.
to sun and air. The vertical space within which they live
may therefore be stated broadly as about 100 feet. They
grow in colonies, each composed of many individuals, but
all united into one mass, which at first may be merely a
little solitary clump on the sea-floor, but which, as it grows,
joins other similar clumps to form what is known as a reef.
Each individual secretes from the sea-water a hard calcareous
skeleton inside its transparent jelly-like body, and when it
'dies, this skeleton forms part of the platform upon which the
FIG. 34. Section of a coral-reef. I. Top of the submarine ridge or
bank on which the corals begin to build. 2.. Coral-reef. 3. Talus
of large blocks of coral-rock on which the reef is built outward. 4.
Fine coral sand and mud produced by the grinding action of the
breakers on the edge of the reef. 5. Coral sand thrown up by the
waves and gradually accumulating above their reach to form dry
ground.
next generation starts. Thus the reef is gradually built
upward as a mass of calcareous rock (2), though only its
upper surface is covered with living corals. These creatures
continue to work upward until they reach low-water mark,
and then their further upward progress is checked. But
they are still able to grow outward. On the outer edges of
the reef they flourish most vigorously, for there, amid the
play of the breakers, they find the food that is brought to
them by the ocean-currents. From time to time, fragments
are torn off by breakers from the reef and roll down its steep
front (3). There, partly by the chemical action of the sea-
VIIL] CORAL ISLANDS. 117
water, and partly by the fine calcareous mud and sand (4),
produced by the grinding action of the waves and washed
into their crevices, these loose blocks are cemented into a
firm steep slope, on the top of which the reef continues to
grow outwards. Blocks of coral and quantities of coral-sand
are also thrown up on the surface of the reef, where, by
degrees, they form a belt of low land above the reach of the
waves (5). On the inside of the reef, where the corals can-
not find the abundant food -supply afforded by the open
water outside, they dwindle and die. Thus the tendency
of all reefs must be to grow seawards and to increase in
breadth. Perhaps their breadth may afford some indication
of their relative age.
Where a reef has started on a shelving sea-bottom near
the coast of a continent, or round a volcanic island, the
space of water inside is termed the Lagoon Channel. Where
the reef has been built up on some submarine ridge or peak,
and there is consequently no land inside, the enclosed space
of water is called a Lagoon^ and the circular reef of coral
is known as an Atoll. If no subsidence of the sea-bottom
takes place, the maximum thickness of a reef must be limited
by the space within which the corals can thrive that is, a
vertical depth of about 100 feet from the surface of the sea.
But the effect of the destruction of the ocean-front of the
reef, and the piling up of a slope of its fragments on the
sea-bottom outside, will be to furnish a platform of the
same materials on which the reef itself may grow outward,
so that the united mass of calcareous rock may attain a very
much greater thickness than 100 feet.
It is remarkable how rapidly and completely the struc-
ture of the coral-skeleton is effaced from the coral-rock, and
a more or less crystalline and compact texture is put in its
n8 RECORDS OF PLANTS AND ANIMALS. [CHAP.
place. The change is brought about partly by the action
of both sea-water and rain-water in dissolving and redeposit-
ing carbonate of lime among the minute interstices of the
rock, and partly also by the abundant mud and sand pro-
duced by the pounding action of the breakers on the reef,
and washed into the crevices. On the portion of a reef laid
dry at low-water, the coral-rock looks in many places as solid
and old as some of the ancient white limestones and marbles
of the land. There, in pools, where a current or ripple of
water keeps the grains of coral-sand in motion, each grain
may be seen to have taken a spherical form unlike that of
the ordinary irregularly rounded or angular particles. This
arises because carbonate of lime in solution in the water is
deposited round each grain as it moves along. A mass of
such grains aggregated together is called oolite, from its
resemblance to fish-roe. In many limestones, forming wide
tracts of richly cultivated country, this oolitic structure is
strikingly exhibited. There can be no doubt that in these
cases it was produced in a similar way to that now in pro-
gress on coral-reefs (see p. 188).
In the coral tracts of the Pacific Ocean, there are nearly
300 coral islands, besides extensive reefs round volcanic
islands. Others occur in the Indian Ocean. Coral-reefs
abound in the West Indian Seas, where, on many of the
islands, they have been upraised into dry land, in Cuba to a
height of 1 100 feet above sea-level. The Great Barrier
Reef that fronts the north-eastern coast of Australia is 1250
miles long, and from 10 to 90 miles broad.
There are other ways in which the aggregation of animal
remains forms more or less extensive and durable rocks.
To some of these, references will be made in later chapters.
Enough has been said here to show that by the accumula-
viii.] ENTOMBMENT OF ORGANIC REMAINS. 119
tion of their hard parts animals leave permanent records of
their presence both on land and in the sea.
II. But it is not only in rocks formed out of their remains
that animals leave their enduring records. These remains
may be preserved in almost every kind of deposit, under the
most wonderful variety of conditions. And as it is in large
measure from their occurrence in such deposits that the
geologist derives the evidence that successive tribes of plants
and animals have peopled the globe, and that the climate
and geography of the earth have greatly varied at different
periods, we shall find it useful to observe the different ways
in which the remains both of plants and animals are at this
moment being entombed and preserved upon the land and
in the sea. With the knowledge thus gained, it will be easier
to understand the lessons taught by the organic remains
that lie among the various solid rocks around us.
It is evident that in the vast majority of cases, the plants
and animals of the land leave no perceptible trace of their
presence. Of the forests that once covered so much of Central
and Northern Europe, which is now bare ground, most have
disappeared, and unless authentic history told that they had
once flourished, we might never know anything about them.
There were also herds of wild oxen, bears, wolves, and other
denizens contemporaneous with the vanished forests. But
they too have passed away, and we might ransack the soil
in vain for any trace of them.
If the remains of terrestrial vegetation and animals are
anywhere preserved it must obviously be only locally, but
the favourable circumstances for their preservation, although
not everywhere to be found, do present themselves in many
places if we seek for them. The fundamental condition is
that the relics should, as soon as possible after death, be so
120 RECORDS OF PLANTS AND ANIMALS. [CHAP.
covered up as to be protected from the air and from too
rapid decomposition. Where this condition is fulfilled, the
more durable of them may be preserved for an indefinite
series of ages.
(a) On land, there are various places where the remains
both of plants and animals are buried and shielded from
decay. To some of these reference has already been made.
Thus on the floors of lakes, amid the fine silt, mud, and
marl gathering there, leaves, fruits, and branches, or tree-
trunks, washed from the neighbouring shores, may be im-
bedded, together with insects, birds, fishes, lizards, frogs,
field-mice, rabbits, and other inhabitants. These remains
may of course often decay on the lake-bottom, but where
they sink into or are quickly covered up by the sediment,
they may be effectually preserved from obliteration. They
undergo a change, indeed, being gradually turned into stone,
as will be described in chapter xv. But this conversion
may be effected so gently as to retain the finest microscopic
textures of the original organisms.
In peat-bogs also, as already stated (p. 1 1 1), wild animals
are often engulfed, and their soft parts are occasionally
preserved as well as their skeletons. The deltas of river-
mouths must receive abundantly the remains of animals
swept off by floods. As the carcases float seawards, they
begin to fall to pieces and the separate bones sink to the
bottom, where they are soon buried in the silt. Among the
first bones to separate from the rest of the skeleton are the
lower jaws (pp. 400, 405). We should therefore expect that
were excavations made in a delta these bones would occur
most frequently, the rest of the skeleton being apt to be car-
ried farther out to sea before its bones could find their way
to the bottom. The stalagmite floor of caverns has already
viii.] BONE-CAVES. 121
been referred to (p. 76) as an admirable material for enclosing
and preserving organic remains. The animals that fell into
those recesses, or used them as dens in which they lived or
into which they dragged their prey, have left their bones on
the floors, where, encased in or covered by solid stalagmite,
these relics have remained for ages. Most of our knowledge
of the animals which inhabited Europe at the time when man
appeared, is derived from the materials disinterred from
these bone-caves. Allusion has also been made to the traver-
tine formed by mineral-springs and to the facility with which
leaves, shells, insects, and small birds, reptiles, or mammals
may be enclosed and preserved in it. Thus, while the plants
and animals of the land for the most part die and decay into
mere mould, there are here and there localities where their
remains are covered up from decay and preserved as
memorials of the life of the time.
(&) On the bottom of the sea, the conditions for the pre-
servation of organic remains are more general and favourable
than on land. Among the sands and gravels of the shore,
some of the stronger shells that live in the shallower waters
near land, but often only in rolled fragments, may be covered
up and preserved. It is below tide-mark, however, and
more especially beneath the limit to which the disturbing
action of breakers descends, that the remains of the denizens
of the sea are most likely to be buried in sediment and to be
preserved there as memorials of the life of the sea. It is
evident that hard and therefore durable relics have the best
chance of escaping destruction. Shells, corals, corallines,
spicules of sponges, teeth, vertebrae, and ear -bones of fishes
may be securely entombed in successive layers of silt or
mud. But the vast crowds of marine creatures that have
no hard parts must almost always perish without leaving any
122 RECORDS OF PLANTS AND ANIMALS. [CHAP.
trace whatever of their existence. And even in the case ot
those which possess hard shells or skeletons, it will be easily
understood that the great majority of them must be decom-
posed upon the sea-bottom, their component elements pass-
ing back again into the sea -water from which they were
originally derived. It is only where sediment is deposited
fast enough to cover them up and protect them before they
have time to decay, that they may be expected to be pre-
served.
In the most favourable circumstances, therefore, only a
very small proportion of the creatures living in the sea at
any time leave a tangible record of their presence in the
deposits of the sea-bottom. It is in the upper waters of the
ocean, and especially in the neighbourhood of land, that life
is most abundant. The same region, also, is that in which
the sediment derived from the waste of the land is chiefly
distributed. Hence it is in these marginal parts of the
ocean that the conditions for preserving memorials of the
animals that inhabit the sea are best developed.
As we recede from the land, the rate of deposit of sediment
on the sea -floor gradually diminishes, until in the central
abysses it reaches that feeble stage so strikingly brought
before us by the evidence of the manganese nodules (p. 105).
The larger and thinner calcareous organisms are attacked
by the sea-water and dissolved, apparently before they can
sink to the bottom ; at least their remains are comparatively
rarely found there. It is such indestructible objects as
sharks' teeth and vertebrae and ear-bones of whales that
form the most conspicuous organic relics in those abysmal
deposits.
Summary. Plants and animals leave their records in
geological history, partly by forming distinct accumulations
viir.] SUMMARY. 123
of their remains, partly by contributing their remains to be
imbedded in different kinds of deposits both on land and in
the sea. As examples of the first mode of chronicling their
existence, we may take the growth of marsh-plants in peat-
bogs, the spread of mangrove-swamps along tropical shores,
and the deposition of infusorial earth on the bottom of lakes
and of the sea; the accumulation of nullipore sand into
solid stone, the formation of extensive shell-banks in many
seas, the wide diffusion of organic ooze over the floor of the
sea, and the growth of coral reefs. As illustrations of the
second method, we may cite the manner in which the remains
of terrestrial plants and animals are preserved in peat-bogs,
in the deltas of rivers, in the stalagmite of caverns, and in
the travertine of springs ; and the way in which the hard
parts of marine creatures are entombed in the sediments of
the sea-floor, more especially along that belt fringing the
continents and islands, where the chief deposit of sediment
from the disintegration of the land takes place. Neverthe-
less, alike on land and sea, the proportion of organic remains
thus sealed up and preserved is probably always but an in-
significant part of the total population of plants and animals
living at any given moment.
How the remains of plants and animals when once en-
tombed in sediment are then hardened and petrified, so as
to retain their minute structures, and to be capable of endur-
ing for untold ages, will be treated of in chapter xv.
CHAPTER IX.
THE RECORDS LEFT BY VOLCANOES AND EARTHQUAKES.
THE geological changes described in the foregoing chapters
affect only the surface of the earth. A little reflection will
convince us that they may all be referred to one common
source of energy the sun. It is chiefly to the daily influ-
ence of that great centre of heat and light that we must
ascribe the ceaseless movements of the atmosphere, the
phenomena of evaporation and condensation, the circulation
of water over the land, the waves and currents of the sea,
in short, the whole complex system which constitutes what
has been called the Life of the Earth. Could this influence
be conceivably withdrawn, the planet would become cold,
dark, silent, lifeless.
But besides the continual transformations of its surface
due to solar energy, our globe possesses distinct energy of
its own. Its movements of rotation and revolution, for
example, provide a vast store of force, whereby many of the
most important geological processes are initiated or modi-
fied, as in the phenomena of day and night, and the seasons,
with the innumerable meteorological and other effects that
flow therefrom. These movements, though slowly growing
feebler, bear witness to the wonderful vigour of the earlier
CHAP, ix.] INTERNAL HEAT OF THE GLOBE. 125
phases of the earth's existence. Inside the globe, too,
lies a vast magazine of planetary energy in the form of an in-
terior of intensely hot material. The cool outer shell is but an
insignificant part of the total bulk of the globe. To this cool
part the name of " crust " was given at a time when the earth
was believed to consist of an inner molten nucleus enclosed
within an outer solid shell or crust. The term is now used
merely to denote the cool solid external part of the globe,
without implying any theory as to the nature of the interior.
It is obvious that we are not likely ever to learn by direct
observation what may be the condition of the interior of our
planet. The cool solid outer shell is far too thick to be pierced
through by human efforts ; but by various kinds of observa-
tions, more or less probable conclusions may be drawn with
regard to this problem. In the first place, it has been ascer-
tained that all over the world, wherever borings are made
for water or in mining operations, the temperature increases
in proportion to the depth pierced, and that the average rate
of increase amounts to about- one degree Fahrenheit for
every 64 feet of descent. If the rise of temperature continues
inward at this rate or at any rate at all approaching it, then
at a distance from the surface, which in proportion to the
bulk of the whole globe is comparatively trifling, the heat
must be as great as that at which the ordinary materials of
the crust would melt at the surface. In the second place,
thermal springs in all quarters of the globe, rising sometimes
with the temperature of boiling water, and occasionally even
still hotter, prove that the interior of the planet must be
very much hotter than its exterior. In the third place,
volcanoes widely distributed over the earth's surface throw
out steam and heated vapours, red-hot stones, and streams
of molten rock.
126 VOLCANOES AND EARTHQUAKES. [CHAP.
It is quite certain, therefore, that the interior of the
globe must be intensely hot; but whether it is actually
molten or solid has been the subject of prolonged discus-
sion. Three opinions have found stout defenders, (i) The
older geologists maintained that the phenomena of volcanoes
and earthquakes could not be explained, except on the sup-
position of a crust only a few miles thick, enclosing a vast
central ocean of molten material. (2) This view has been
opposed by physicists who have shown that the globe, if
this were actually its structure, could not resist the attraction
of sun and moon, but would be drawn out of shape, as the
ocean is in the phenomenon of the tides, and that the
absence of any appreciable tidal deformation in the crust
shows that the earth must be practically solid and as rigid
as a ball of glass, or of steel. (3) A third opinion has been
advanced by geologists who, while admitting that the earth
behaves on the whole as a solid rigid body, yet believe that
many geological phenomena can only be explained by the
existence of some liquid mass beneath the crust. Accord-
ingly they suppose that while the nucleus is retained in the
solid state by the enormous superincumbent pressure under
which it lies, and the crust has become solid by cooling,
there is an intermediate liquid or viscous layer which has
not yet cooled sufficiently to pass into the solid crust above,
and does not lie under sufficient pressure to form part of
the solid nucleus below. At present, the balance of evidence
and argument seems to be in favour of the practical rigidity
and solidity of the globe as a whole. But the materials of
its interior must possess temperatures far higher than those
at which they would melt at the surface. They are no doubt
kept solid by the vast overlying pressure, and any change
which could relieve them of this pressure would allow them
ix.] VOLCANOES. 127
to pass into the liquid form. This subject will be again
alluded to in chapter xvi. Meanwhile, let us consider how
the intensely hot nucleus of the planet reacts upon its surface.
Rocks are bad conductors of heat. So slowly is the
heat of the interior conducted upwards by them that the
temperature of the surface of the crust is not appreciably
affected by that of the intensely hot nucleus. But the fact
that the surface is not warmed from this source shows that
the heat of the interior must pass off into space as fast as
it arrives at the surface, and proves that our planet is gradu-
ally cooling. For many millions of years the earth has been
radiating heat into space, and has consequently been losing
energy. Its present store of planetary vitality, therefore,
must be regarded as greatly less than it once was.
VOLCANOES.
Of all the manifestations of this vitality, by far the most
impressive are those furnished by volcanoes. The general
characters of these vents of communication between the
hot interior and cool surface of the planet are doubtless
already familiar to the reader of these chapters the volcano
itself, a conical hill or mountain, formed mainly or entirely
of materials ejected from below, having on its truncated
summit the basin-shaped crater, at the bottom of which lies
the vent or funnel from which, as well as from rents on the
flanks of the cone, hot vapours, cinders, ashes, and streams
of molten lava are discharged, till they gradually pile up
the volcanic cone round the vent whence they escape.
A volcanic cone, so long as it remains, bears eloquent
testimony to the nature of the causes that produced it.
Even many centuries after it has ceased to be active, when
no vapours rise from any part of its cold, silent, and motion-
128 VOLCANOES AND EARTHQUAKES. [CHAP.
less surface, its conical form, its cup-shaped crater, its slopes
of loose ashes, and its black bristling lava-currents remain
as unimpeachable witnesses that the volcanic fires, now
quenched, once blazed forth fiercely. The wonderful groups
of volcanoes in Auvergne and the Eifel are as fresh as if
they had not yet ceased to be active, and might break forth
again at any moment ; yet they have been quiescent ever
since the beginning of authentic human history.
But in the progress of the degradation which everywhere
slowly changes the face of the land, it is impossible that vol-
canic hills should escape the waste which befalls every other
kind of eminence. We can picture a time when the volcanic
cones of Auvergne will have been worn away, and when the
lava-streams that descend from them will be cut into ravines
and isolated masses by the streams that have even already
deeply trenched them. Where all the ordinary and familiar
signs of a volcano have been removed, how can we tell that
any volcano ever existed ? What enduring record do vol-
canoes inscribe in geological history ?
Now, it must be obvious that among the operations
of an active volcano, many of the most striking phenomena
have hardly any importance as aids in recognising the traces
of long extinct volcanic action. The earthquakes and
tremors that accompany volcanic outbursts, the constant and
prodigious out-rushing of steam, the abundant discharge of
gases and acid vapours, though singularly impressive at the
time, leave little or no lasting mark of their occurrence. It is
not in phenomena, so to speak, transient in their effects, that
we must seek for a guide in exploring the records of ancient
volcanoes, but in those which fracture or otherwise alter the
rocks below ground, and pile up heaps of material above.
Keeping this aim before us, we may obtain from an
ix.] VOLCANOES AND VOLCANIC PRODUCTS. 129
examination of what takes place at an active volcano such
durable proofs of volcanic energy as will enable us to recog-
nise the former existence of volcanoes over many tracts of
the globe where human eye has never witnessed an erup-
tion, and where, indeed, all trace of what could be called
a volcano has utterly vanished. A method of observation
and reasoning has been established, from the use of which
we learn that in some countries, Britain for example, though
there is now no sign of volcanic activity, there has been a
succession of volcanoes during many protracted and widely
separated periods, and that probably the interval that has
passed away since the last eruptions is not so vast as that
which separated these from those that preceded them. A
similar story has been made out in many parts of the con-
tinent of Europe, in the United States, India, and New
Zealand, and, indeed, in most countries where the subject
has been fully investigated.
A little reflection on this question will convince us that
the permanent records of volcanic action must be of two
kinds : first and most obvious are the piles of volcanic
materials which have been spread out upon the surface of
the earth, not only round the immediate vents of eruption,
but often to great distances from them ; secondly, the rents
and other openings in the solid crust of the earth caused by
the volcanic explosions, and some of which have served as
channels by which the volcanic materials have been expelled
to the surface.
Volcanic Products. We shall first consider those
materials which are erupted from volcanic vents and are
heaped up on the surface as volcanic cones or spread out as
sheets. They may be conveniently divided into two groups ;
ist, Lava, and 2d, Fragmentary materials.
K
130 VOLCANOES AND EARTHQUAKES. [CHAP.
(i) Lava. Under this name are comprised all the molten
rocks of volcanoes. These rocks present many varieties in
composition and texture, some of the more important of
which will be described in chapter xi. Most of them are
crystalline that is, are made up wholly or in greater part
of crystals of two or more minerals interlocked and felted
together into a coherent mass. Some are chiefly composed
of a dark brown or black glass, while others consist of a
compact stony substance with abundant crystals imbedded in
FIG. 35. Cellular Lava with a few of the cells filled up with infiltrated
mineral matter (Amygdules).
it. In many cases, they are strikingly cellular that is to say,
they contain a large number of spherical or almond-shaped
cavities somewhat like those of a sponge or of bread, formed
by the expansion of the steam absorbed in the molten rock
(Fig. 35 and p. 193). They vary much in weight and in
colour. The heavier kinds are more than three times the
weight of water; or, in other words, they have a specific
gravity ranging up to 3*3 ; and are commonly dark grey to
black. The lighter varieties, on the other hand, are little
more than twice the weight of water, or have a specific
IX.]
LAVA-CURRENTS.
gravity which may be as low as 2*3, while their colours are
usually paler, sometimes almost white.
When lava is poured out at the surface it issues at a
white heat that is, at a temperature sometimes above that
of melting copper, or more than 2204 Fahr. ; but its
surface rapidly darkens, cools, and hardens into a solid crust
which varies in aspect according to the liquidity of the mass.
Some lavas are remarkably fluid, flowing along swiftly like
melted iron ; others move sluggishly in a stiff viscous stream.
In many pasty lavas, the surface breaks up into rough
cindery blocks or scoriae like the slags of a foundry, which
grind upon each other as the still molten stream underneath
creeps forward (p. 193). In general, the upper part of a
lava-stream is more cellular than the central portions, no
doubt because the imprisoned steam can there more easily
expand. The bottom, too, is often rough and slaggy, as the
lava is cooled by contact with the ground, and portions of
the chilled bottom-crust are pushed along or broken up and
involved in the still fluid portion above.
There are thus three more or less well-defined zones in
^
~<fe
FIG. 36. Section of a Lava-current.
a solidified lava-current a cellular or slaggy upper part (c in
I 3 2
VOLCANOES AND EARTHQUAKES. [CHAP.
Fig- 36), a more solid and jointed centre (b\ embracing
usually by much the largest proportion of the whole, and a
cellular or slaggy bottom (a). A rock presenting these
characters tells its story of volcanic action in quite unmis-
takable language. It remains as evidence of the existence
of some neighbouring volcanic vent, now perhaps entirely
covered up, whence it flowed. We may even be able
to detect the direction in which the lava moved. The
cells opened by the segregation and expansion of the steam
FIG. 37. Elongation of cells in direction of flow of a lava-stream.
entangled in the interstices of a mass of lava which is at
rest are, on the whole, spherical. But if the rock is still
moving, the cells will be drawn out and flattened into
almond-shaped vesicles, with their flat sides parallel to the
surface of the lava, and their longer axes ranged in one
general direction, which is that of the motion of the lava
(Fig. 37).
At a volcanic vent, the mass of erupted lava is generally
thickest, and it thins away as its successive streams terminate
on the lower grounds surrounding the cone. But sometimes
a lava -current may flow for 40 miles or more from its
ix.] LAVA-CURRENTS. 133
source, and may here and there attain locally a great thick-
ness by rolling into a valley and filling it up, as has been
witnessed among the Icelandic eruptions. As a rule, where
ancient lava-streams are found to thicken in a certain direc-
tion, we may reasonably infer that in that direction lay the
vent from which they flowed.
Again, sheets of lava that solidify on the slopes of a vol-
canic cone are inclined; they may congeal on declivities
of as much as 30 or 40. If a series of ancient lavas were
observed to slope upward to a common centre, we might
search there for some trace of the funnel from which they
were discharged. But, of course, in proportion to their anti-
quity lava-streams, like every other kind of rock, have suffered
from geological revolutions, among which those that involve
upheaval and dislocation are especially important, so that
the inclination of an ancient lava-bed must not be too hastily
assumed as an indication of the slope of the cone of a vol-
cano. It must be taken in connection with the rest of the
evidence supplied by the whole district.
Where lavas reach the lower grounds beyond the foot of
a volcanic cone, they may spread out in wide nearly horizon-
tal sheets. As current succeeds current, the original features
of the plain may be entirely buried under a mass of lava
many feet thick. If a section could be cut through such an
accumulation, it would be possible to determine the thick-
ness of each successive lava-stream by means of the slaggy
upper and lower surfaces. Here and there, too, where two
eruptions were separated by an interval long enough to allow
the surface of the older mass partially to crumble into soil
and support some vegetation, the layer of burnt soil between
would remain as a witness of this interval.
In other instances, we can understand that in the larger
134 VOLCANOES AND EARTHQUAKES. [CHAP.
hollows of a decaying lava, ponds or lakes might gather, on
the floor of which there might be deposited layers of fine
silt full of terrestrial leaves, insect remains, land and fresh
water shells, and other organic relics of a land -surface. If,
now, a lacustrine accumulation of this kind were to be buried
under a new outburst of lava, it would be sealed up and
might preserve its record intact for vast ages. In any section
cut through such a series of lava-beds by a river, or the sea,
or by man, the layers of silt with their organic remains inter-
calated between the lava-streams would prove the eruptions
to have taken place on land, and to have been separated by
a long interval, during which a lake was formed on the cold
and decomposing surface of the earlier lava.
The conditions under which the volcanic outbursts oc-
curred may thus be inferred, not so much from the nature of
the volcanic materials themselves, as from that of the layers
of sediment that may happen to have been preserved among
them. Seams of red, baked soil, with charred remains of
terrestrial vegetation interposed between the upper and under
sides of successive lavas would point to subaerial eruptions.
Bands of hardened clay or marl, with leaves and fresh-water
shells, would show that the lavas had invaded a lake. Beds
of limestone or other rock, containing corals, sponges, marine
shells, and other traces of the life of the sea, would demon-
strate that the eruptions were submarine. Examples of each
of these varieties of evidence occur abundantly among the
old volcanic tracts of Britain.
(2) Fragmentary Products. These supply some of the
most striking proofs of volcanic energy. They vary in size
from huge blocks of stone weighing many tons down to the
finest dust. The coarsest materials naturally accumulate
round the vent, while the finest may be borne away by wind
ix.] TUFFS. 135
to distances of many hundreds of miles. On the volcano
itself, the stones, ashes, and dust form beds of coarse and
fine texture which, on the outside of the cone, have the
usual slope of the declivity. By degrees, they become
more or less consolidated, and are then known by the
general name of Tuffs.
The materials composing a tuff are generally derived
from lavas. The fine dust, discharged from a large volcano
in such prodigious quantities as to make the sky dark as
midnight for days together, is simply lava that has been
blown into this finely divided condition by the explosion of
the vapours and gases which exist absorbed in it while still
deep down within the earth's crust. The cinder-like frag-
ments, and scoriae or slag, that are ejected in such numbers
and fall back into the crater and upon the outer slopes of
the cone, are pieces of lava frothed up by the expansion of
the imprisoned steam, torn off from the column of lava in
the vent and shot whirling up into the air. Large blocks of
lav, or of the rocks through which the volcanic funnel has
been opened, are often broken off by the force of the ex-
plosions and discharged with the other volcanic detritus
from the vent. These materials descending to the ground,
form successive beds that vary in dimensions according
to the vigour of eruption and their distance from the vent.
Around the focus of activity, there may be thick accumula-
tions of blocks, bombs, and pieces of scoriae mixed with fine
ashes and sand. A notable feature is the generally cellular
character of these stones a, peculiarity which marks them
as made of truly volcanic materials. An examination of the
finest dust likewise discloses the presence of the crystals and
glass that constitute the lava from the explosion of which the
dust was derived (p. 191).
136 VOLCANOES AND EARTHQUAKES. [CHAP.
From the wide area over which the fragmentary materials
ejected by volcanoes are dispersed in the atmosphere before
they fall to the earth, they are more likely than lavas to be
preserved among contemporaneous sedimentary accumula-
tions. They often descend upon lakes, and must there be
interstratified with the mud, marl, or other deposit in progress
at the time. They are also widely diffused over the sea-
floor. Recent dredgings of the ocean -basins have shown
that traces of fine volcanic detritus may be detected even at
remote distances from land. As active volcanoes almost
always rise near the sea, as the oceans are dotted over with
volcanic islands, and as, doubtless, many eruptions take place
on the sea-bottom, there is no difficulty in perceiving why
volcanic particles should be so universally diffused. Geo-
logists can also understand why, in the records of volcanic
history in bygone ages, so large a proportion of the evidence
should be of submarine eruptions.
Beds of tuff often contain traces of the plants or animals
that lived on the surfaces on which the volcanic materials
fell sometimes remains of terrestrial or lacustrine vegetation
and animals ; but in the great majority of instances, shells
and other relics of the inhabitants of the sea.
In a series of layers of tuff round a volcanic orifice, the
memorials of the earliest discharges are of course preserved
in the layers at the bottom. Accordingly, in such situations,
abundant fragments of the rocks of the surrounding country
may be noticed. We could hardly ask for more convincing
evidence of the blowing out of the vent and the ejection of
the rock-fragments from it, before the volcano began to dis-
charge only volcanic materials.
Large blocks of lava ejected obliquely from the crater
may fall beyond the limits of the cone. If a block thus
ix.] RECORDS OF VOLCANIC EXPLOSIONS. 137
discharged should fall into a lake-basin, it would be covered
up in the silt accumulating there, and might be the only re-
maining record of the eruption to which it belonged. In after
times, were the lake-floor laid dry, the stone might be found
in its place, the layers of sediment into which it fell pressed
down by the force with which it landed on them and by its
weight, the later layers mounting over and covering it. Ex-
amples of this kind of evidence may be gathered in many
old volcanic districts. One taken from the coast of Fife is
given in Fig. 38. The
lowest bed there shown
(i) is a brown shaly fire-
clay, about 5 inches thick,
which was once a vege- a /^?^7
table soil, for abundant
., . A i FIG. 38. Volcanic block ejected during
rootlets can be seen the deposition of strata J in water .
branching through it. It
is overlain by a seam of coal (2), 5 or 6 inches thick,
representing the dense growth of vegetation that flourished
upon the soil; the next layer (3) is a green crum-
bling fire-clay about a foot thick, covered by a dark
shale with remains of plants (4). The feature of special
interest in this section is the angular block of lava (dia-
base), weighing about six or eight pounds, which is stuck ver-
tically in bed No. 3. There can be no doubt that this was
ejected by an explosion, of which there is here no other record.
It probably descended with some force, for, as shown in
the drawing, the lower layers of the fire-clay are pressed down
by it, and the coal itself is compressed. We see that the
stone fell before the upper half of the fire-clay was formed,
for the layers of that part of the bed are heaped around the
stone and finally spread over it. There are layers of lava and
138 VOLCANOES AND EARTHQUAKES. [CHAP.
tuff above and below the strata depicted in this section, so
that there is abundant other evidence of preceding and
subsequent volcanic action here (see Fig. 109).
As the fine volcanic dust may be transported by wind
for many hundreds of miles before reaching the surface of
the earth, its presence does not necessarily show that a
volcano existed in the neighbourhood in which it fell. The
fine ashes from the Icelandic volcanoes, for example, have
been found abundantly even as far as Sweden and the
Orkney Islands. But where the fragmentary materials are
of coarse grain, and more especially where they contain
large slags, scoriae, and bombs, and where they are inter-
stratified with sheets of lava, they unquestionably indicate
the proximity of some volcanic vent from which the whole
proceeded.
Volcanic Vents and Fissures. The various materials
ejected from a volcano to the surface may conceivably be
in course of time entirely swept away. Nevertheless, though
every sheet of lava and every bed of tuff is removed, there
will still remain the filled-up funnel or fissure, up which
these materials rose, and out of which they were ejected.
This opening in the solid crust of the earth must evidently
be one of the most durable, as it is certainly one of the fun-
damental features in a volcano. Let us first consider the
vent of an ordinary volcano.
In many instances, there is reason to believe that volcanic
vents are opened along lines of fracture in the earth's crust.
This seems especially to be the case where a group of vol-
canoes runs along one definite line, as is represented in Fig.
39. Along such a fissure, either of older date or due to the
energy of the volcanic explosions themselves, there must be
weaker places where the overlying mass is less able to bear
ix.] VOLCANOES AND FISSURES. 139
the strain of the pent-up vapours underneath. At these
places, after successive shocks, openings will at length be
made to the surface, whence the lavas and ashes will be
emitted, and each such opening will be marked by a cone
of the erupted material.
But in innumerable examples, it is found that a fissure
is not necessary for the formation of a volcano. The effect
of the volcanic explosions is such as to drill a pipe or funnel
even through the solid unfractured crust of the earth. The
volcanic energy, so far from requiring a line of fracture for
its assistance, seems often to have avoided making use of
FIG. 39. Volcanoes on lines of fissure.
such a line, even when it existed. In the volcanic plateaux
of Utah, which are dotted over with little volcanic cones,
and are also traversed by great dislocations, it is noticeable
that the vents are not clustered along the lines of fracture.
In the same region, and also along the courses of the Rhine
and Moselle, volcanic vents have been opened near the brink
of deep ravines rather than at the bottom. These are feat-
ures of volcanic action, for which no satisfactory explanation
has yet been found.
The crater of an active volcano is a hideous yawning
chasm, with rough red and black walls, at the bottom of
which lie steaming pools of molten lava. Every now and
14 VOLCANOES AND EARTHQUAKES. [CHAP.
then, a sharp explosion tears the lava open, sending up a
shower of glowing fragments and hot ashes. These pools
of liquid lava lie evidently on the top of a column of melted
rock which descends in the volcanic chimney to an un-
known depth into the earth's interior. If the volcano were
to become extinct, this lava column would cool and solidify,
and even after the entire destruction and removal of the
cone and crater, would remain as a stump to tell where the
site of the volcano had been. Layer after layer might be
stripped off the surface of the land ; hundreds or thousands
of feet of rock might in this manner be removed, yet, so far
as we know, the stump of the volcano would still be there.
No conceivable amount of waste of the surface of the earth's
crust could remove a vertical column of rock which descends
to an unknown depth into the interior. The site of a vol-
canic vent can never be effaced except by being buried under
masses of younger rock.
Volcanic vents being so durable a testimony to volcanic
action deserve careful attention. At an active volcano, or
even at one which, though extinct, still retains its cone of
erupted materials, we cannot, of course, learn much regard-
ing the shape and size of the funnel, for only the crater, and
at most merely the upper part of the vent, are accessible.
But among volcanic tracts of older date, where the cones
have been destroyed, and where the filled -up funnels are
laid bare, the subterranean architecture of volcanoes is re-
vealed to us. At such places, we are allowed, as it were, to
descend the chimney of a volcano, and to make observations
altogether impossible at a modern volcanic cone.
From observations made at such favourable localities,
it has been ascertained that the funnels of volcanoes are
generally circular though liable to many modifications of
ix.] VOLCANIC VENTS. 141
'outline. They vary indefinitely in diameter, according to
the vigour of the volcanic outbursts that produced them.
The smaller vents are not more than a few yards in width ;
but those of the larger volcanoes which, as in Sumatra and
Java, have sometimes craters comprising an area of 40 square
miles, must have enormously larger funnels.
The materials that fill up a vent are sometimes only frag-
ments of the surrounding rocks. In such cases, we may
suppose that when the volcanic explosions had spent their
force and had blown out an opening to the surface of the
ground, they were not succeeded by the uprise of any solid
volcanic materials ; that, in short, only the first stage in the
establishment of a volcano was reached, and then, owing to
some failure of the subterranean energy at the place, the
operations came to an end. But though the upper part of
the vent might remain open, surrounded with a crater formed
of the fragments into which the rocks were blown by the
explosions, the lower parts would undoubtedly be filled up
by the fall of fragments back again into the vent. And
if all the material ejected to the surface were removed, the
top of this column of fragmentary materials would remain
as an unmistakable evidence of the explosions that had
originated it.
But in the vast majority of cases, the operations at a
volcanic vent do not end with the first explosions. Clouds
of ashes and stones are ejected, and streams of molten lava
are poured forth. In some instances, the chimney may be
finally choked with volcanic blocks, scoriae, cinders, and
ashes, in others with consolidated lava. Examples of both
kinds of infilling are found, and also others where the two
forms of volcanic material occur together in the same vent.
A volcanic chimney filled up in this way with volcanic
142
VOLCANOES AND EARTHQUAKES. [CHAP.
materials, and exposed by the removal of the lava or ashes
thrown out to the surface is known as a Neck. As these
materials are usually harder and more durable than the sur-
rounding rocks, they project above the general surface of the
ground. The stump of the volcano is left as a hill, the form
and prominence of which will chiefly depend upon the nature
of the material ; hard tough lava will rise abruptly, as a crag
or hill, above the surrounding country, while consolidated
ashes, scoriae, and other fragmentary stuff will give a smoother
and less marked outline.
These features will be best understood from a series of
diagrams. We may take, by way of illustration, a neck com-
FIG. 40. Outline of a Volcanic Neck.
posed mainly of fragmentary ejections, but with a plug of
lava reaching its summit. The usual outlines of such a
neck are represented in
Fig. 40. There is nothing
in the general form of this
hill to suggest a volcanic
origin ; yet, if we examine
its structure and that of the
ground around it, we may
FIG. 41. Ground-plan of the struc- find them tQ be ag ^
ture of the Neck shown in Fig. 40. . .
sented in Fig. 41, where
the surrounding rocks are supposed to consist of various
sandstones, clays, limestones, and other sedimentary deposits
IX.]
VOLCANIC NECKS.
143
(a\ through which the volcanic vent (, c) has been drilled.
The neck is represented as elliptical in cross-section, com-
posed mainly of consolidated volcanic ashes and blocks (b\
but with a mass of lava (c) in the centre. The structure of
the hill is explained in the vertical section, Fig. 42. We
d
FIG. 42. Section through the same Neck as in Figs. 40 and 41.
there see that the vent has been blown through the surround-
ing strata, and has been filled up mainly with fragmentary
materials (d, b) ; but that through its centre there has risen a
column or plug of lava (c\ which not improbably marks the
last effort of the volcano to force solid ejections to the
surface. The line ss indicates the present surface of the
ground, after the prolonged waste during which all the vol-
canic cone has been removed. But we can in imagination
restore the original surface, which may have been somewhat
as shown by the dotted lines, the position of the crater being
indicated at *, and its crest on either side at d, d. No trace
is here left of the original volcanic cone, and though the pro-
gress of superficial degradation would remove still more of
the neck, the downward continuation of the volcanic column
must always remain. A volcanic neck is thus one of the
most enduring and unmistakable evidences of the site of a
volcano (see p. 273).
144 VOLCANOES AND EARTHQUAKES. [CHAP.
Besides funnels or vents, other openings are made by
volcanic explosions in the crust, which serve as recep-
tacles of lava and ashes, and remain as durable memorials
of volcanic action. Of these the most important are fissures,
which are formed in large numbers in and around a volcanic
cone, but may also arise at a distance from any actual
volcano. During- the convulsions of a volcano, the cone
and the surrounding country are split by lines of fissure,
which not infrequently radiate from the centre of disturbance,
somewhat as cracks do in a pane of glass through which a
stone is thrown. Sometimes the two sides of a fissure close
together again, leaving no superficial trace of the dislocation.
More frequently steam and various volcanic vapours escape
from the chasm, and may deposit along the walls sublimates
of different minerals, such as common salt, chloride of
iron, specular iron, sulphur, or sal-ammoniac. These de-
posited substances may even continue to grow there until
they entirely fill up the space between. In such cases, the
line of fissure is marked by a vertical or steeply inclined band
of minerals interposed between the ends of the rocks that
have been ruptured and separated. But in most instances,
the opening is filled up by the rise of lava from below. At
night, the vents opened on the outside of an active volcano
may be traced from afar by the glow of the white-hot lava
that rises in them to within a short distance from the surface.
When the lava cools and solidifies in these fissures, it forms
wall -like masses, known as dykes. Inside many volcanic
craters, the walls are traversed with dykes which, though on
the whole tending to keep a vertical direction, may curve
about irregularly according to the form of the vents into
which the lava rose. Like the necks above described, dykes
form enduring records of volcanic action. The superficial
IX.]
VOLCANIC DYKES.
145
cones and craters may disappear, but the subterranean lava-
filled fissures will still remain as records of volcanic action.
In some volcanic regions, where enormous floods of lava
have been poured forth, no great central cones have existed.
Such regions extend as vast black plains of naked rock,
mottled with shifting sand-hills, or as undulating tablelands
FIG. 43. Volcanic dykes rising through the bedded tuff of a crater.
carved by running water into valleys and ravines, between
which the successive sheets of lava are exposed in terraced
hills. Beyond the limits over which the lava- sheets are
spread, dykes of the same kinds of lava rise in abundance
to the surface. There can be no doubt that the dykes do
not terminate at the edge of the lava-fields, but pass under-
neath them. Indeed, as they increase in number in that
direction, they are probably more abundant underneath the
146 VOLCANOES AND EARTHQUAKES. [CHAP.
lava than outside of the lava -fields. Sometimes sections
are exposed showing how, after rising in a fissure, the lava
has spread out on either side as a sheet. In these vast
lava-plateaux or deserts, the molten rock, instead of issuing
from one main central Etna or Vesuvius, appears to have
risen in thousands of fissures opened in the shattered crust,
and to have welled forth from numerous vents on these
fissures, spreading out sheet after sheet till, like a rising
lake, it has not only overflowed the lower grounds, but
even buried all the minor hills. Such appears to have
been the history of vast tracts in Western North America.
The area which has there been flooded with lava has been
estimated to be larger than that of France and Great
Britain together, and the depth of the total mass of lava
erupted reaches in some places as much as 3700 feet.
Some rivers have cut gorges in this plain of lava, laying bare
its component rocks to a depth of 700 feet or more. Along
the walls of these ravines we see that the lava is arranged in
parallel beds or sheets often not more than 10 or 20 feet
thick, each of which, of course, represents a separate outpour-
ing of molten rock.
Except where such deep sections have been cut through
them by rivers, recent lava-floods can only be examined along
their surface, and we are consequently left chiefly to inference
regarding their probable connection with fissures and dykes
underneath. But in various parts of the world, lava-plains
of much older date have been so deeply eroded as to expose
not only the successive sheets of lava but the floor over
which they were poured, and the abundant dykes which
no doubt served as the channels wherein the lava rose
towards the surface, till it could escape at the lowest levels,
or at weaker or wider parts of the fissures. In Western
ix.] EARTHQUAKES. 147
Europe, important examples of this structure occur, from the
north of Ireland through the Inner Hebrides and the Faroe
Islands to Iceland. This volcanic belt presents a succession
of lava-fields which even yet, in spite of the enormous waste,
are in some places more than 3000 feet thick. The sheets
of lava are nearly flat, and rise in terraces one over another
into green grassy hills, or into the dark fronts of lofty sea-
washed precipices. Where this thick cake of lava has been
stripped off during the degradation of the land, thousands of
dykes are exposed, and many of these traverse at least the
lower parts of the sheets of lava. They form, as it were, the
subterranean roots of which these sheets were the subaerial
branches ; and even where the whole of the material that
reached the surface, more than 3000 feet thick, has been
worn away, the dykes still remain as evidence of the reality
and vigour of the volcanic forces.
EARTHQUAKES.
The rise of hot springs and the explosions of volcanoes
furnish impressive testimony to the internal heat of our
planet ; but they are by no means the only proofs that the
pent-up energy of the interior of the globe reacts upon the
outer surface. By means of delicate instruments, it can be
shown that the ground beneath our feet is subject to con-
tinual tremors which are too feeble to be perceived by the
unaided senses. From these minuter vibrations, movements
of increasing intensity can be detected up to the calamitous
earthquake, whereby a country is shaken to its foundations,
and thousands of human lives with much valuable property
are destroyed. We do not yet know by what different
causes these various disturbances are produced. Some of
the fainter tremors may arise from such influences as changes
148 VOLCANOES AND EARTHQUAKES. [CHAP.
of temperature and atmospheric pressure, and the rise and
fall of the tides. But the more violent must be assigned
to causes working within the earth itself. The collapse of
the roofs of underground caverns, the sudden condensation
of steam or explosion of volcanic vapours, the snap of rocks
that can no longer resist the strain to which they have been
subjected within the earth's crust these and other influ-
ences may at different times come into play to determine
convulsive earthquake shocks. Without, however, entering
into the difficult question of the causes of the movements,
we may inquire into their effects in so far as these register
their passing in the annals of geological history.
Their awful suddenness and devastation have invested
earthquakes with a high importance in the popular estimate
of the forces by which the surface of the globe is modified.
Yet if we judge of them by their permanent effects, we must
give them a comparatively subordinate place among these
forces. After some of the most destructive earthquakes
recorded in human history, hardly any trace of the calamity
is to be seen, save in shattered and prostrate houses. But
when these buildings have been repaired or rebuilt, no one
visiting the ground might be able to detect any trace of the
earthquake that shattered or overthrew them.
Yet severe earthquakes do not pass without their self-
written chronicle which, though often evanescent, is at the
time conspicuous enough. Landslips are caused, large
masses of earth and blocks of rock being shaken down
from higher to lower levels ; the ground is rent, and the
fissures are sometimes subsequently widened and deepened
by rain and runnels into ravines. But more important are
the marked changes of level that occasionally accompany
earthquake -shocks. In some cases, the ground is raised
ix.] SLOW UPHEAVAL AND SUBSIDENCE. 149
for several feet, so that along maritime tracts there is a gain
of land from the sea ; in others, the ground sinks, and the
sea flows in upon the land. Yet it is evident that unless
these changes are actually witnessed as the accompaniments
of the earthquakes, they may take place without retaining
any evidence that they were produced by such a cause.
The convulsion of an earthquake, notwithstanding the havoc
it may bring to the human population of a country, does
not always record itself in distinctive and enduring char-
acters in geological history. Some of its most noticeable
effects, also, are not due directly to its own action, but to
the operations of the waters of the land and of the sea
which, when disturbed by the shock, not infrequently acquire
increased vigour in their own peculiar forms of activity.
The great waves set in motion by an earthquake roll over
the low lands bordering the sea, and may cause vastly more
destruction than is done by the mere shock of the earth-
quake itself.
It is perhaps not so much by earthquakes, as by quiet
hardly perceptible movements, that the relative positions
of sea and land are undergoing change at the present time.
In some parts of the world, the land is gradually rising, in
others, it is slowly sinking. Proofs of elevation are supplied
by lines of barnacles or rock -boring shells, now standing
above the reach of the highest tides ; by caves that have
obviously been scooped out by the sea, but now stand at
a higher level than the waves can reach ; and by deposits
of sand, gravel, and shells which were evidently accumulated
on a beach, but which now rise above the level where
similar materials are now being accumulated (raised beaches).
Evidences of subsidence are furnished by traces of old land-
surfaces trees with roots in situ, and beds of peat, lying
150 VOLCANOES AND EARTHQUAKES. [CHAP.
below the limits of the tides (submerged forests). But it
must be more difficult to prove subsidence than elevation,
for as the land sinks, its surface is carried below the waves,
which soon efface the evidence of its terrestrial character.
The time within which man has been observing and
recording the changes of the earth's surface forms but an
insignificant fraction of the ages during which geological
history has been in progress. We cannot suppose that
during this brief period he has had experience of every kind
of geological process by which the outlines of land and sea
are modified. There may be great terrestrial revolutions
which happen so rarely that none has occurred since man
began to take note of such things. Among these revolu-
tions, of which he has had as yet no experience, the most
gigantic is the formation of a mountain -chain. That the
various mountain -chains of the globe are of very different
ages, and that some of the most gigantic of them are, com-
pared with others, of recent date are facts in the history of
the globe which will be more fully referred to in later pages ;
but so far as human history or tradition go, man has never
witnessed the uprise of a range of mountains. The crust
of the earth has been folded and crumpled on the most
colossal scale, some parts having been pushed for miles
away from their original position ; it has been rent by pro-
found fissures, on each side of which the rocks have been dis-
placed for many thousand feet ; and it has been so broken,
crushed, and sheared that its component rocks have in some
places assumed a structure entirely different from what they
originally possessed. But of all these colossal mutations
there is no human experience. We are driven to reason
regarding them from the record of them preserved among
the rocks, and from the analogies that can be suggested by
ix.] SUMMARY. 151
experiments devised to imitate as far as possible the
processes of nature. To this subject we shall return in
chapter xiii.
Summary. The enduring records left by volcanoes,
whence their former existence in almost all regions of the
world may be demonstrated, are to be sought partly in the
materials which they have brought up to the surface, and
partly in the funnels and fissures by which they have dis-
charged these materials. Of the former kind of evidence
lava furnishes a conspicuous example ; its internal crystal-
line or glassy structure, and its steam-cavities, the cellular
slaggy upper and under parts of the sheets in which it lies,
are all proofs of its former molten condition. A succession
of lava beds, piled one above another, marks a series of
volcanic eruptions, and the nature of the layers of non-vol-
canic material intercalated between them may indicate the
conditions under which the eruptions took place, whether
on land, in lakes, or in the sea. The fragmentary products
consolidated into beds of tuff are likewise characteristic of
volcanoes ; they consist mainly of lava -dust with cindery
scoriae, slags, and blocks ; they accumulate most deeply
and in coarsest material at and immediately around the
volcanic vents, but their finer particles may be carried to
enormous distances ; they are especially liable to be inter-
calated with contemporaneous sedimentary deposits in lakes
and on the sea-floor.
The vents through which lava and ashes are ejected to
the surface form the most permanent record of volcanoes,
for, being filled up with volcanic rocks, they cannot be
destroyed by the mere denudation of the surface, and can
only disappear by being buried under later accumulations.
Such " necks " consist sometimes of lava, sometimes of con-
152 VOLCANOES AND EARTHQUAKES. [CHAP. ix.
solidated volcanic debris, or of both kinds of material
together, and remain as the stumps of volcanoes, where
every other trace of volcanic action may have passed away.
Not less enduring are the dykes or wall -like masses of lava
which have risen and solidified in open fissures. Enormous
sheets of lava appear to have flowed out from such fissures
in regions where the volcanic energy never produced any
great central cone.
Earthquakes do not impress their mark upon geological
history so indelibly as might be supposed. In spite of the
destruction which they cause to human life and property,
it is by such direct changes as landslips, rents of the ground,
and the upheaval or depression of land, and by such indirect
changes as may be produced by derangements of rivers,
lakes, and the sea that earthquakes leave their chief record
behind them. Some of the most important changes of level
now going on are effected quietly and almost imperceptibly,
some regions being slowly elevated, and others gradually
depressed. But the time within which man has been an
observer and recorder of nature is too brief to have supplied
him with experience of all the ways in which the internal
energy of the globe affects its surface. In particular, he
has never witnessed the production of a mountain -chain,
nor any of the plications, fractures, and displacements
which the crust of the earth has undergone. Regarding
these revolutions we can only reason from the records of
them in the rocks, and from such laboratory experiments
as may seem most closely to imitate the processes of nature
that were concerned in their production.
PART II.
ROCKS, AND HOW THEY TELL THE
HISTORY OF THE EARTH.
CHAPTER X.
THE MORE IMPORTANT ELEMENTS AND MINERALS OF THE
EARTH'S CRUST.
IN the foregoing Part of this volume, we have been engaged
in considering the working of various processes by which
the surface of the earth is modified at the present time,
and some of the more striking ways in which the record of
these changes is preserved. We have seen that, on the whole,
it is by deposits of some kind, laid down in situations where
they can escape destruction, that the story of geological
revolution is chronicled. In one place, it is the stalagmite
of a cavern, in another, the silt of a lake -bottom, in a
third, the sand and mud of the sea-floor, in a fourth, the
lava and ashes of a volcano. In these and countless other
examples, materials are removed from one place and set
down in another, and in their new position, while acquiring
novel characters, they retain more or less distinctly the
record of their source and of the conditions under which
their transference was effected.
154 ELEMENTS OF EARTH'S CRUST. [CHAP.
In these chapters, reference has intentionally been
avoided as far as possible to details that required some
knowledge of minerals and rocks, in order that the broad
principles of geology, for which such knowledge is not
absolutely essential, might be clearly enforced. It is
obvious, however, that as minerals and rocks form the
records in which the history of the earth has been preserved,
this history cannot be profitably studied until some acquaint-
ance with these materials has been made. What now lies
before the reader, therefore, in order that he may be able
to apply the knowledge he has gained of geological pro-
cesses to the elucidation of former geological periods, is
to make himself familiar with, at least, the more common
and important minerals and rocks. This he can only do
satisfactorily by handling the objects themselves, until he
acquires such an acquaintance with them as to be able to
recognise them where he meets with them in nature.
At first, the number and variety of these objects may
appear to be almost endless, and the learner may be apt
to despair of ever mastering more than an insignificant
portion of the wide circle of inquiry and observation which
they present. But though the detailed study of this subject
is more than enough to tax the whole powers of the most
indefatigable student, it is not by any means an arduous
labour, and assuredly a most interesting one, to acquire so
much knowledge of the subject as to be able to follow
intelligently the progress of geological investigation, and
even to take personal part in it. This accordingly is the
task to which he is invited in this and the following chapters.
Before considering the characters presented by the
various rocks that form the visible part of the earth's crust,
we may find it of advantage to inquire into the general
x.] METALLOIDS AND METALS. 155
chemical composition of rocks, for by so doing we learn
that though the chemist has detected more than sixty sub-
stances which he has been unable to decompose, and which,
therefore, he calls elements, only a small proportion of these
enter largely into the composition of the outer part of the
globe. In fact, there are only about sixteen elements that
play an important part as constituents of rocks ; these to-
gether constitute about ninety-nine parts of the terrestrial
crust. Half of them are metals; and the other half are
metalloids or non-metals, as in the two subjoined lists, the
most abundant being in each case placed first.
METALLOIDS OR NON-METALS. METALS.
Symbol
Atomic
' Weight.
Symbol
' Weigh<
Oxygen .
15-96
Aluminium .
Al
27-30
Silicon .
Si
28-00
Calcium
Ca
39-90
Carbon .
C
11-97
Magnesium .
Mg
23-94
Sulphur.
s
3I-98
Potassium
K
39 '04
Hydrogen
H
I '00
Sodium
Na
22-99
Chlorine
Cl
35*37
Iron
Fe
55-90
Phosphorus .
P
30-96
Manganese .
Mn
54-80
Fluorine
F
19-10
Barium .
Ba
136-80
Some of those elements occur in the free state, that is,
not combined with any other element. Carbon, for instance,
is found pure in the form of the diamond, and also as
graphite. But in the great majority of cases, they assume
various combinations. Most abundant are oxides, or com-
pounds of oxygen with another element. Compounds of
sulphur and a metal are known as sulphides ; and similar com-
pounds with chlorine are chlorides. Some of the compounds
form further combinations with one or more elements. Thus
the acid -forming oxides unite with water to form what are
called acids, which, combining with metallic oxides or bases,
156 ELEMENTS OF EARTH'S CRUST. [CHAP.
form with them compounds termed salts. Sulphur and
oxygen, for example, uniting in certain proportions with
water, constitute sulphuric acid (H 2 SO 4 ) which, parting with
its displaceable hydrogen and combining with the metal
calcium, forms the salt known as calcium-sulphate, or sul-
phate of lime (CaSO 4 ).
METALLOIDS. Of the non- metallic elements, by far
the most abundant and important is Oxygen. In its free
state, it exists as a gas which has been detected by itself at
active volcanic vents. But with such rare exceptions, it is
always found mixed or combined with one or more elements.
Thus, mixed with nitrogen, it constitutes the atmosphere, of
which it forms not less than 23 per cent by weight. It takes a
still larger share in the composition of water, which consists
of 88' 88 per cent of oxygen and 1 1 1 2 of hydrogen. There
is a continual removal of oxygen from air and water in the
processes of weathering described in chapter ii. Substances
which can take more of this element abstract it especially
from damp air or from water. A knife or any other piece
of iron, for example, will remain unchanged for an indefinite
length of time if kept in dry air ; but as soon as it is exposed
to moisture, in which there is always some dissolved air, it
begins to rust. The familiar brown crust which then forms
on its surface, and slowly eats into the very centre of the iron,
is due to a chemical union of oxygen with the iron, forming
an oxide of iron with water. Among the rocks of the earth's
crust, a large proportion are liable to undergo a similar change,
and so enormous has been the extent of this change in the
past history of the earth, that somewhere about one-half of
the outer and accessible part of the crust consists of oxygen,
which was probably at first in the atmosphere.
Next in importance to oxygen among the metalloids, is
x.]
SILICON AND SILICA.
157
Silicon, which is never met with in the free state. It has
been artificially obtained, however, in the form of a dull
brown powder. In nature, it always occurs united with
oxygen, forming the familiar substance known as silica
(SiO 2 ), which constitutes more than a half of all the known
part of the earth's crust. Silica or silicic acid is indeed the
fundamental compound of the crust, forming by itself entire
masses of rock, and entering as a principal constituent into
FlG. 44. Group of Quartz-crystals (Rock-crystal).
the majority of rocks. It occurs abundantly as the mineral
quartz, the colourless transparent forms of which are known
as rock-crystal (Fig. 44), and also in combination with various
metallic bases as the important family of silicates. It is
present in solution in most natural waters, both those of the
land and of the sea, and is secreted by plants (diatoms,
grasses) and animals (radiolarians, sponges). It is thus ready
to be carried by percolating water into the heart of rocks,
and to be deposited in their interstices and cavities. Its
158 ELEMENTS OF EARTH'S CRUST. [CHAP.
hardness and durability eminently fit it for the important
part it plays in binding the materials of rocks together, and
enabling them better to resist the decomposing effects of
air and water.
Carbon, though found in a nearly pure state in the clear
gem called diamond, and also in the black opaque mineral,
graphite, more usually occurs mixed with various impurities,
as in the different kinds of coal. This element has a high
importance in nature, because it is the fundamental substance
made use of by both plants and animals to build up their
structures, and because it serves as a bond of connection
between the organic and the inorganic worlds. In union
with oxygen, carbon forms the widely -diffused gaseous com-
pound known as carbon-dioxide (CO 2 ), which occurs in the
proportion of about four parts in every ten thousand parts
of ordinary atmospheric air. From the air it is abstracted
and decomposed by living plants in presence of sunshine,
the oxygen being in great measure sent back into the atmos-
phere, while the carbon with some oxygen, nitrogen, and
hydrogen is built up into the various vegetable cells and
tissues. When we look at a verdant landscape or a bound-
less forest, it is a striking thought that all this vegetation
has been chiefly constructed out of the small proportion of
invisible carbon-dioxide present in the atmosphere. The
vast numbers of beds of coal imbedded in the earth's crust
have, in like manner, been derived from the atmosphere
through the agency of former tribes of plants. Not only
in beds of coal, but still more prevalently in masses of
limestone, carbon enters into the composition of rocks.
Carbon -dioxide, as was pointed out in chapter ii., is ab-
stracted by rain in passing from the clouds to the earth, and
is also supplied by decomposing plants and animals in the
x.] CARBON, SULPHUR. 159
soil. It is readily dissolved in water, and forms with it car-
bonic acid, CO(OH) 2 , which has been referred to as so power-
ful a solvent of the substance of many rocks. This acid
unites with a number of alkaline and earthy bases to form
the important family of Carbonates. Of these the most
abundant is calcium-carbonate, or carbonate of lime (CaCO 3 ),
which consists of 44 per cent carbon-dioxide, and 56 per
cent lime. This carbonate not only occurs abundantly
diffused through many rocks, but in the form of limestone
builds up by itself thick mountainous masses of rock many
hundreds of square miles in extent. It is abstracted by plants
to form calcareous tufa (chapter v.), but far more abundantly
by animals, especially by the invertebrata, as exemplified by
the familiar urchins, corallines, and shells of the sea-shore.
The limestones of the earth's crust appear to have been mainly
formed of the calcareous remains of animals. Hence we
perceive that the two forms in which carbon has been most
abundantly stored up in the earth's crust have been prin-
cipally due to the action of organised life ; coal being chiefly
carbon that has been taken out of the atmosphere by plants,
and limestone consisting of carbon-dioxide, to the extent of
nearly one-half, which has been secreted from water by the
agency of animals.
Sulphur is found in the free state, more particularly at
volcanic vents, in pale yellow crystals or in shapeless masses
and grains j but it chiefly occurs in combination. Some of
its compounds are widely diffused among plants and animals.
The blackening of a silver spoon by a boiled egg is an illus-
tration of this diffusion, for it arises from the union of the
sulphur in the egg with the metal. Combinations with a
metal (sulphides) and combinations with a metal and oxygen
(sulphates) are the conditions in which sulphur chiefly exists.
160 ELEMENTS OF EARTH'S CRUST. [CHAP.
Hydrogen is a gas which has been detected in the free
state at active volcanic vents ; but otherwise it occurs
chiefly in combination with oxygen as the oxide water (H 2 O),
of which it constitutes about one-ninth, or 11-12 per cent,
by weight. It also enters into the composition of plant and
animal substances, and forms with carbon the important
group of bodies known as Hydrocarbons^ of which mineral
oil and coal-gas are examples. In smaller quantity, it is
found united with sulphur (sulphuretted hydrogen, H 2 S),
with chlorine (hydrochloric acid, HC1), and a few other
elements.
>* Chlorine is a transparent gas of a greenish-yellow colour,
but except possibly at active volcanic vents it does not occur
in the free state ; united with the alkali metals, potassium,
sodium, and magnesium, it forms the chief salts of sea-water.
The most important of these salts, sodium-chloride, or com-
mon salt (NaCl) contains 60*64 per cent of chlorine, and
forms 2-64 per cent by weight of sea-water. This salt is
found diffused in microscopic particles in the air, especially
near the sea, and beds of it hundreds of feet thick occur
in many parts of the world among the sedimentary rocks
that constitute most of the dry land.
Phosphorus does not occur free; it has so strong an
affinity for oxygen that it rapidly oxidises on exposure to
the air, and even melts and takes fire. Its most frequent
combination is with oxygen and calcium, as calcium -phos-
phate or phosphate of lime (Ca 3 (PO 4 ) 2 , p. 182). Though for
the most part present in minute proportions, it is widely dif-
fused in nature. It occurs in fresh and sea water, in soil and
in plants, especially in their fruits and seeds ; it is supplied
by plants to animals for the formation of bones, which when
burnt are found to consist almost entirely of phosphate of lime.
x.] METALS. 161
Fluorine also is never met with uncombined ; it never
unites with oxygen, forming in this respect the sole exception
among the elements ; its most frequent combination as a
rock constituent is with calcium, when it forms the mineral
Fluor-spar (CaF 2 ). Like phosphorus, it is widely diffused in
minute proportions in the waters of some springs, rivers,
and the sea, and in the bones of animals.
To these metalloids we may add the colourless, tasteless
gas Nitrogen, which, though not largely present in the
earth's crust, constitutes four-fifths by volume or 77 per cent
by weight of the atmosphere. It does not enter into com-
bination so readily as the other elements above enumerated,
but it is always found in the composition of plants and is
a constituent of many animal tissues. It is the principal
ingredient of the substance called ammonia, which is pro-
duced when moist organic matter is decomposed in the air.
In many rocks composed wholly or in great part of organic
remains, such, for instance, as peat and coal, nitrogen is a
constant constituent.
METALS. Though so large a proportion of the known
terrestrial elements are metals, these are much less abundant
in the earth's crust than the metalloids. The most frequent
are Aluminium, Calcium, and Magnesium. The substances
most familiar to us as metals occupy an altogether sub-
ordinate part among rocks, the most abundant of them
being Iron.
Aluminium never occurs in the free state, but can be
artificially separated from its compounds when it is seen to
be a white, light, malleable metal. It is almost always united
with oxygen as the oxide of Alumina (A1 2 O 3 ) which occurs
crystallised as the ruby and sapphire, but is for the most part
united with silica, and in this form constitutes the basis of
M
162 ELEMENTS OF EARTH'S CRUST. [CHAP.
the great family of minerals known as the Silicates of
Alumina, or Aluminous Silicates. These silicates generally
contain some other ingredient which is more liable to de-
composition, and when they decay and their more soluble
parts are removed, they pass into clay, which consists chiefly
of hydrated silicate of alumina.
Calcium is not met with uncombined, but has been
artificially isolated and found to be a light, yellowish metal,
between gold and lead in hardness. It occurs in nature
chiefly combined with carbonic acid as a carbonate, and with
sulphuric acid as a sulphate, to both of which substances
reference has already been made; it is also present in
many silicates. So abundant is calcium-carbonate or car-
bonate of lime in nature that it may be detected in most
natural waters, which dissolve it and carry it in solution into
the sea. Its presence in rocks may be detected by a drop
of any mineral acid, when the liberated carbon -dioxide
escapes as a gas with brisk effervescence. Calcium-sulphate
is likewise a common constituent of terrestrial waters, especi-
ally of those which in household management are called
hard ; it constitutes not less than 3^6 per cent of the salts
in ordinary sea-water, and when sea-water is evaporated this
sulphate (gypsum), being least soluble, is the first to be pre-
cipitated in minute crystals resembling in shape those shown
in Fig. 62.
Magnesium is likewise only isolated artificially, when it
appears as a soft, silver-white, malleable and ductile metal.
It occurs in sea-water combined with chlorine as magnesium-
chloride, which constitutes io'S per cent of the total propor-
tion of salts. It unites with carbonic acid as a carbonate,
which with carbonate of lime forms the widely diffused rock
called magnesian limestone or dolomite ; it also enters into
x.] ALKALI METALS, IRON. 163
the composition of the Magnesian Silicates which are only
second in importance to those of alumina.
Potassium and Sodium (alkali metals) are only obtain-
able in the free state by chemical processes, when they are
found to be white, brittle metals that float on water, and
rapidly oxidise if exposed to the air. Combined with chlorine,
sodium forms the familiar chloride known as common salt,
which constitutes 777 per cent of the salts of sea-water, is
abundantly present in salt lakes, and occurs in extensive beds
among the rocks of the dry land. Potassium-chloride like-
wise occurs in the sea and may be obtained from the ashes
of burnt sea-weed. Enormous deposits of it, combined with
chlorides of sodium and magnesium, have been met with in
Germany (Stassmrt). Potassium also exists in the sea in
combination with sulphuric acid as potassium-sulphate or sul-
phate of potash, which amounts to about 2 "4 of the total salts
of sea-water. Sulphates of potassium, sodium, magnesium,
and calcium form thick masses of rock in the Stassfurt
deposits. Potassium and sodium in combination with silica
form silicates which enter largely into the composition of
many rocks. They are readily attacked by water contain-
ing carbonic acid, giving rise to what are called carbonates
of the alkalies, or alkaline carbonates, which are removed in
solution. By this means, carbonate of potash is introduced
into soil, where it is taken up by plants into their leaves and
succulent parts. When wood is burnt, this carbonate in
considerable quantity may be dissolved with water out of
the ash.
Iron is found in the free or native state in minute grains
and large blocks in some volcanic rocks, also in granules of
" cosmic dust," probably of meteoric origin, and in fragments
of various size which have undoubtedly fallen upon the
164 ELEMENTS OF EARTH S CRUST. [CHAP.
earth's surface from the regions of space. There is reason
to believe that much of the solid interior of the globe
may consist of native iron and other metals. But it is
in combination that iron is chiefly of importance in the
earth's crust. It has united with oxygen to form several
abundant oxides. The protoxide or ferrous oxide (FeO)
contains the lowest proportion of oxygen, and being, there-
fore, prone to take up more, gives rise to many of the
processes of decay included under the general name of
weathering. It is readily dissolved by organic and other
acids, and is then removed in solution, but on exposure
rapidly oxidises and passes into the highest oxide, known as
the peroxide or sesquioxide of iron or ferric oxide (Fe 2 O 3 ),
which, being the permanent insoluble form, is found abund-
antly among the rocks of the earth's crust. Iron is the great
colouring matter of nature ; its protoxide compounds give
greenish hues to many rocks, while its peroxide colours them
various shades of red which, when the peroxide is combined
"with water, pass into many tints of brown, orange, and yellow.
Manganese is commonly associated with iron in minute
proportion in many lavas and other crystalline rocks; its
oxides resemble those of iron in their modes of occurrence.
Barium and Calcium are called metals of the alkaline
earths. The former can only be obtained in a free state
by artificial means, when it appears as a pale yellow very
heavy metal which rapidly tarnishes. In nature it chiefly
occurs as the sulphate, barytes, or heavy spar (BaSO 4 ), a
mineral of frequent occurrence in veins associated with
metallic ores (p. 182).
Passing now from the simple elements we have next to
note the mineral-forms in which they appear as constituents
of the earth's crust. A mineral may be defined as an in-
x.] MINERALS DEFINED. 165
organic substance, having theoretically a definite chemical
composition, and in most cases, also, a certain geometrical
form. It may consist of only one element, for example,
the diamond, sulphur, and the native metals, gold, silver,
copper, etc. But in the vast majority of cases, minerals
consist of at least two, usually more, elements in definite
chemical proportions. In the following short list of the more
important minerals of the earth's crust they are arranged
chemically, according to the predominant element in them,
or the manner in which the combinations of the elements
have taken place, so that their leading features of composi-
tion may be at once perceived. The two elements, Carbon
and Sulphur, in their native or uncombined state, sometimes
form considerable masses of rock. Some of the native
metals, also, may be enumerated as rock-constituents when
they occur in sufficient quantities to be commercially import-
ant. Gold, for example, is found in grains and strings, in
veins of quartz, and in irregular pieces or nuggets dispersed
through the gravel deposits of regions where gold-bearing
quartz-veins traverse the solid rocks. Omitting, however, the
minerals formed of a single element, we may pass on to
combinations of two or more elements, and consider first
those in which oxygen is combined with some other element,
forming what are commonly grouped together as oxides.
Then will come the Silicates, or combinations of silica with
one or more bases, followed by the Carbonates, or combina-
tions of carbon-dioxide with some base ; the Sulphates, or
compounds of sulphuric acid and a base ; the Fluorides, or
compounds of fluorine and a metal ; the Chlorides, or com-
pounds of chlorine and a metal ; and the Sulphides, or com-
pounds of sulphur and a metal.
One of the most obvious features in a crystal of any
i66
ELEMENTS OF EARTH'S CRUST. [CHAP.
mineral is the regular and sharply-defined edges and corners
which it presents. Take a piece of rock-crystal or quartz,
for example (Fig. 44), and you will find it to consist of six
sides or faces, forming what is called a prism, and bevelled
off at the end into a six-sided cone, called a pyramid. If
you examine a large collection of similar crystals you may
find no two of them exactly alike, yet they agree in pre-
senting a six-sided figure. Again, procure a piece of the
common mineral calcite, either a whole crystal (Figs. 59, 60),
or a portion of a crystalline
mass (Fig. 45) ; break it and
you will find each fragment
to possess the same form, that
of a rhombohedron ; crush
one of these fragments and
you will observe that each
little grain of the powder pre-
serves the same shape. The
rhombohedron, therefore, is
FIG. 45. Calcite (Iceland spar), called the fundamental crys-
ne form Qf the mineml
The property so strikingly
shown in calcite, of breaking along definite crystalline planes,
is termed cleavage. So perfect is the cleavage of calcite that
the crystallised mineral can hardly be broken, except along
the planes that define the rhombohedron. Many minerals
cleave more or less easily in one or more directions, and
break irregularly in others. The cleavage affords a guide to
the proper crystalline form of a mineral.
Though there are many hundreds of varieties of crystal-
line form, they may all be reduced to six primary types or
systems. These are distinguished from each other by the
showmg its characteristic rhom-
bohedral cleavage.
x.] CRYSTALLINE FORMS OF MINERALS. 167
number and position of their axes, which are mathematical
straight lines, intersecting each other in the interior of a
crystal, and connecting the centres of opposite flat faces of
the crystal, or opposite angles or corners. The six systems,
with their axes, are enumerated in the subjoined list.
I. Isometric (monometric, cubical, tesseral, regular). In this system
there are three axes which are of the same length, and inter-
sect each other at a right angle. The cube, octahedron, and
dodecahedron are examples (Fig. 46). Crystals of this system
b
FIG. 46. Cube (a), octahedron v ), dodecahedron (c\
are distinguished by their symmetry, their length, breadth,
and thickness being equal. Common salt, fluor-spar (Fig. 64),
and magnetite (Fig. 54) are illustrations.
II. Tetragonal (dimetric). The axes are three in number, and
intersect each other at a right angle, but one of them, called
the vertical axis, is longer or shorter than the other two,
which are lateral axes. Hence a crystal belonging to this
system may either be oblong or squat (Fig. 47).
FIG. 47. Tetragonal prism (b) FIG. 48. Orthorhom-
and pyramid (a). bic prism.
III. Orthorhombic (trimetric) has the three axes intersecting each
other at a right angle, but all of unequal lengths. The
1 68
ELEMENTS OF EARTH S CRUST.
[CHAP.
rectangular and rhombic prisms, and the rhombic octahedron
belong to this system (Fig. 48).
IV. Hexagonal. This is the only system with four axes (Fig. 49).
The lateral axes are all equal, intersect at right angles the
FIG. 49. Hexagonal prism (a), rhombohedron (l>), and
scalenohedron (c).
vertical axis (which is longer or shorter than they are), and form
with each other angles of 60. Water, for instance, crystal-
lises in this system, and the six-rayed star of a snow-flake is an
illustration of the way in which the lateral axes are placed.
Quartz is an example (Fig. 44), also calcite (Figs. 59, 60).
V. Monoclinic, with all the axes of unequal length. One of the
lateral axes cuts the vertical axis at a right angle, the other
intersects the vertical axis obliquely. Augite (Fig. 50), Horn-
blende (Fig. 57), and Gypsum (Fig. 62) are examples.
VI. Triclinic, the most unsymmetrical of all the systems, all the
axes being unequal and placed obliquely to each other (Fig. 51).
FIG. 50. Monoclinic prism.
Crystal of Augite.
FIG. 51. Triclinic prism.
Crystal of Albite felspar.
Every mineral that takes a crystalline form belongs to
one or other of these six systems, and through all its
x.] ORIGIN OF CRYSTALLISED MINERALS. 169
varieties of external form the fundamental relations of the
axes remain unchanged.
Some minerals have crystallised out of solutions in
water. How this may take place can be profitably studied
by dissolving salt, sugar, or alum in water, and watching
how the crystals of these substances gradually shape them-
selves out of the concentrated solution, each according to
its own crystalline pattern. Other minerals have crystallised
from hot vapours (sublimation), as may be observed at the
fissures of an active volcano. Others have crystallised out
of molten solutions, as in the case of lava. Thoroughly fused
lava is a glassy or vitreous solution of all the mineral substances
that enter into the composition of the rock, and when it
cools, the various minerals crystallise out of it, those that
are least fusible taking form first, the most fusible appearing
last ; but a residue of non-crystalline glass sometimes remain-
ing even when the rock has solidified.
It is evident that minerals can only form perfect crystals
where they have room and time to crystallise. But where
they are crowded together, and where the solution in which
they are dissolved dries or cools too rapidly, their regular
and symmetrical growth is arrested. They then form only
imperfect crystals, but their internal structure is crystalline,
and if examined carefully will be found to show that in the
attempt to form definite crystals each mineral has followed
its own crystalline type. These characters are of much im-
portance in the study of rocks, for rocks are only large
aggregates of minerals, wherein definite crystals are excep-
tional, though the structure of the whole mass may still be
quite crystalline.
But minerals also occur in various indefinite or non-
crystalline shapes. Sometimes they are fibrous or disposed
170 ELEMENTS OF EARTH'S CRUST. [CHAP.
in minute fibre-like threads (Fig. 56) ; or concretionary when
they have been aggregated into various irregular concretions
of globular, kidney -shaped, grape -like, or other imitative
shapes (Figs. 61, 64, 65, 75); or stalactitic (Fig. 20) when
they have been deposited in pendent forms like stalactites ;
or amorphous when they have no definite shape of any kind,
as, for instance, in massive ironstone.
Oxides occur abundantly as minerals. The most import-
ant are those of Silicon (Quartz) and Iron (Haematite,
Limonite, Magnetite, Titanic Iron).
QUARTZ (Silica, Silicic Acid, SiO 2 ), already alluded to,
is the most abundant mineral in the earth's crust. It occurs
crystallised, also in various crystalline and non-crystalline
varieties. In the crystallised form as common quartz it is,
when pure, clear and glassy, but is often coloured yellow, red,
green, brown, or black from various impurities. It crystal-
lises in the six-sided prisms and pyramids above referred to,
the clear colourless varieties being rock-crystal (Fig. 44).
When purple it is called amethyst ; yellow and smoke-
coloured varieties, found among the Grampian Mountains,
are popularly known as Cairngorm stones. In many places,
silica has been deposited as chalcedony, in translucent masses
with a waxy lustre, and pale grey, blue, brown, red, or black
colours. Deposits of this kind are not infrequent among
the cavities of rocks. The common pebbles and agates
with concentric bands of different colours are examples of
chalcedony, and show how the successive layers have been
deposited from the walls of the cavity inwards to the centre
which is often filled with crystalline quartz (Fig. 52). The
dark opaque varieties are called jasper.
Quartz can be usually recognised by its vitreous lustre and
hardness ; it cannot be scratched with a knife, but easily
x.] QUARTZ. 17 1
scratches glass, and it is not soluble in the ordinary acids.
It is an essential constituent of many rocks, such as granite
FIG. 52. Section of a pebble of chalcedony. The outer banded layers
are chalcedony, the interior being nearly filled up with crystalline
quartz.
and sandstone. Silica being dissolved by natural waters,
especially where organic acids or alkaline carbonates are
present, is introduced by permeating water into the heart of
even the most solid rocks. Hence it is found abundantly
in strings and veins traversing rocks, also in cavities and
replacing the forms of plants and animals imbedded in
sedimentary deposits. Soluble silica is abstracted by some
plants and animals and built up into their organic structures
(diatoms, radiolarians, sponges).
Four minerals composed of Oxides of Iron occur abund-
antly among rocks. The peroxide is found in two frequent
forms, one without water (Haematite), the other with water
(Limonite). The peroxide and protoxide combine to form
Magnetite, and a mixture of the peroxide with the per-
oxide of the metal titanium gives Titanic Iron.
172 ELEMENTS OF EARTHS CRUST. [CHAP.
HEMATITE or Specular Iron (Fe 2 O 3 = Fe7oO3o) occurs
in rhombohedral crystals that can with difficulty be scratched
with a knife; but is more usually found in a massive
condition with a compact, fibrous, or granular texture, and
dark steel-grey or iron-black colour, which becomes bright
red when the mineral is scratched or powdered. The
earthy kinds are red in colour, and it is in this earthy form
FIG. 53. Piece of haematite, showing the nodular external form and
the internal crystalline structure.
that haematite plays so important a part as a colouring
material in nature. Red sandstone, for example, owes its
red colour to a deposit of earthy peroxide of iron round
the grains of sand. Haematite occurs crystallised in fissures
of lavas as a product of the hot vapours that escape at these
places; but is more abundant in beds and concretionary
masses (Fig. 53) among various rocks.
LIMONITE or Brown Iron-ore differs from Haematite in
being rather softer, in containing more than 14 per cent of
water which is combined with the iron to form the hydrated
peroxide, in being usually massive or earthy, in presenting a
dark brown to yellow colour (ochre), and in giving a yellowish-
brown to dull yellow powder when scratched or bruised. It
may be seen in the course of being deposited at the present
x.] IRON OXIDES. 173
time through the action of vegetation in bogs and lakes,
hence its name of Bog-iron ore (chapter viii.), likewise
in springs and streams where the water carries much sul-
phate of iron. The common yellow and brown colours of
sandstones and many other rocks are generally due to the
presence of this mineral.
MAGNETITE (Fe 3 O 4 ) occurs crystallised in octahedrons
and dodecahedrons of an iron-black colour, giving a black
powder when scratched. It is found abundantly in many
rocks (schists, lavas, etc.), sometimes in large crystals (Fig.
FIG. 54. Octahedral crystals of magnetite in chlorite schist.
54), sometimes in such minute form as can only be detected
with the microscope. It also forms extensive beds of a
massive structure. Its presence in rocks may be detected by
its influence on a magnetised needle. By pounding basalt
and some other rocks down to powder, minute crystals and
grains of magnetite may be extracted with a magnet.
TITANIC IRON (FeTi) 2 O 3 ) occurs in iron-black crystals
like those of haematite, from which they may be distinguished
by the dark colour and metallic lustre of its surface when
scratched. Though it occurs in beds and veins in certain
kinds of rock (schists, serpentine, syenite), its most generally
ELEMENTS OF EARTH S CRUST.
[CHAP.
diffused condition is in minute crystals and grains scattered
through many crystalline rocks (basalt, diabase, etc.)
MANGANESE OXIDES are commonly associated with
those of iron in rocks. They are liable to be deposited in
the form of bog-manganese, under conditions similar to those
FIG. 55. Dendritic markings due to arborescent deposit of earthy
manganese oxide.
in which bog-iron is thrown down. Earthy manganese oxide
(wad) not infrequently appears between the joints of fine-
grained rocks in arborescent forms that look so like plants
as to have been often mistaken for vegetable remains.
These plant-like deposits are called Dendrites or dendritic
markings (Fig. 55).
Silicates. Compounds of Silica with various bases
x.] FELSPARS. 175
form by far the most numerous and abundant series of
minerals in the earth's crust. They may be grouped accord-
ing to the chief metallic base in their composition, the most
important are the Silicates of Alumina, and the Silicates of
Magnesia. Of the aluminous silicates we need consider
here only the Felspars, Zeolites, and Mica. Among the
magnesian silicates it will be enough to note the leading
characters of Hornblende, Augite, Olivine, Talc, Chlorite,
and Serpentine. When the learner has made himself so
familiar with these as to be able readily to recognise them,
he may proceed to the examination of others.
FELSPARS. This family of minerals plays an important
part in the construction of the earth's crust, for it constitutes
the largest part of the crystalline rocks which, like lava, have
been erupted from below ; is found abundantly in the great
series of schists; and by decomposition has given rise to
the clays, out of which so many sedimentary rocks have been
formed. The felspars are divided into two series, according
to crystalline form Orthoclase and Plagioclase.
Orthoclase or potash -felspar contains about i6'Sg per
cent of potash, crystallises in monoclinic or oblique rhombic
prisms, but also occurs massive ; is white, grey, or pink in
colour ; has a glassy lustre ; can with difficulty be scratched
with a knife, but easily with quartz. Associated with quartz,
it is an abundant ingredient of many ancient crystalline
rocks (granite, felsite, gneiss, etc.) In the form of sanidine
it is an essential constituent of many modern volcanic rocks.
Plagioclase. Under this name are grouped several
species of felspar which, differing much from each other in
chemical composition, agree in crystallising in the same type
or system, which is that of a triclinic or oblique rhomboidal
prism. As abundant ingredients of rocks they commonly
1 76 ELEMENTS OF EARTH'S CRUST. [CHAP.
appear as clear, colourless, or white glassy strips, on the flat
faces of which a fine minute parallel ruling may be detected
with the naked eye, or with a lens. This striation or lamella-
tion is a distinctive character, which proves the crystals in
which it occurs not to be orthoclase. The plagioclase fel-
spars occur as essential constituents of many volcanic rocks,
and also among ancient eruptive masses and schists. Among
them are Microdine or Potash-felspar, with 15 per cent of
potash; Albite or Soda-felspar, containing nearly 12 per cent
of soda (Fig. 51); Anorthite or Lime-felspar, with 20*10 per
cent of lime ; Soda-lime felspar. Lime-soda felspar a group
of felspars containing variable proportions of soda, lime, and
sometimes potash ; the chief varieties are Oligodase (Silica,
62-65 per cent), Andesine (Silica, 58-61 per cent), Labrador-
ite (Silica, 50-56 per cent).
ZEOLITES, a characteristic family of minerals, composed
FIG. 56. Cavity in a lava, filled with zeolite which has crystallised in
long slender needles.
essentially of silicate of alumina and some alkali with water ;
often marked by a peculiar pearly lustre, especially on certain
X.] HORNBLENDE, AUGITE. 177
planes of cleavage ; usually found filling up cavities in rocks
where they have been deposited from solution in water.
Some of the species commonly crystallise in fine needles or
silky tufts. The zeolites have obviously been formed from
the decomposition of other minerals, particularly felspars.
They are especially abundant in the steam-cells of old lavas
in which plagioclase felspars prevail, either lining the walls
of the cavities, and shooting out in crystals or fibres towards
the centre (Fig. 56), or filling the cavities up entirely.
MICA, a group of minerals (monoclinic) specially dis-
tinguished by their ready cleavage into thin, parallel, usually
elastic silvery laminae, They are aluminous silicates with
potash (soda), or with magnesia and ferrous oxide, and
always with water. They occur as essential constituents of
granite, gneiss, and many other eruptive and schistose rocks,
also in worn spangles in many sedimentary strata (micaceous
sandstone). Among their varieties the two most important
are Muscovite (white mica, potash -mica), and Black mica
(magnesia-mica, Biotite).
HORNBLENDE or AMPHIBOLE, a silicate of magnesia, with
lime, iron-oxides, and sometimes alumina, occurs in mono-
clinic (oblique rhombic) prisms, also colum-
nar, fibrous, and massive. It is divisible into
(1) a group of pale-coloured varieties, con-
taining little or no alumina, white or pale
green in colour, often fibrous (Tremolite,
Actinolite, Asbestus\ found more particularly
among gneisses and associated rocks, and
(2) a dark group containing 5 to 18 per cent FlG ' 57- Horn-
blende crystal,
of alumina, which replaces the other bases ;
dark green to black in colour, in stout, dumpy prisms (Fig.
57), and in columnar or bladed aggregates (Common horn-
N
178 ELEMENTS OF EARTH'S CRUST. [CHAP.
blende). Abundant in many eruptive rocks, and also forming
almost entire beds of rock among the crystalline schists.
AUGITE (PYROXENE), in composition resembles horn-
blende ; indeed, they are only different forms of the
same substance, differing slightly in crystalline form, horn-
blende being the result of slow and augite of rapid crystal-
lisation. Like hornblende, also, augite occurs in two groups :
(l) pale non- aluminous, found more especially among
gneisses, marbles, and associated rocks ; and (2) dark green
or black (Fig. 50), occurring abundantly in many eruptive
rocks, such as black heavy lavas (basalts, etc.)
OLIVINE (PERIDOT) (SiO 2 41*01, MgO 49*16, FeO 9*83),
occurs in small orthorhombic prisms and glassy grains in
basalts and other lavas ; of a pale yellowish-
green or olive-green colour, whence its name.
These grains can often be readily detected
on the black ground of the rock, through
which they are abundantly dispersed.
Olivine is liable to alteration, and especi-
ally to conversion into serpentine by the
FIG. 58 -Olivine influence of percolating water (Fig. 58).
crystal ; the light
portions repre- CHLORITE (SlO 2 25-28, Al^ 19-23,
sent the unde- FeO 15-29, MgO 13-25, H 2 O 9-12) is a dark
composed mm- o ii ve _g reen hydrated magnesian silicate,
eral, the shaded .
parts show the ^ 1S so so ^ as to " e easi ly scratched with
conversion of the the nail, and occurs in small six-sided
olivine into ser- tables, also in various scaly and tufted ag-
gregations diffused through certain rocks.
It appears generally to be the result of the alteration of some
previous anhydrous magnesian silicate, such as hornblende.
SERPENTINE (Mg 3 Si 2 O 7 +2H 2 O) is another hydrated mag-
nesian silicate, containing a little protoxide of iron and
x.] CALCITE. 179
alumina, usually massive, dark green but often mottled with
red. It occurs in thick beds among schists, is often asso-
ciated with limestones, and may be looked for in all rocks
that contain olivine, of the alteration of which it is often
the result. In many serpentines, traces of the original
olivine crystals can be detected.
Carbonates. Though these are abundant in nature, only
three of them require notice here as important constituents of
the earth's crust, those of lime, lime and magnesia, and iron.
CALCITE (calcium-carbonate, carbonate of lime, CaCO 3 )
crystallises in the fourth system, and has for its fundamental
crystalline form the rhombohedron, as already mentioned
(p. 1 66). When quite pure it is transparent (Iceland spar,
Fig. 45), with the lustre of glass ; but more usually is opaque
and white. Its crystals, where the chief axis is shorter
than the others, sometimes take the form of flat rhombo-
hedrons (nail-head spar, Fig. 59) ; where, on the other hand,
that axis is elongated, they present pointed pyramids (scaleno-
FIG. 59. Calcite in the form of "nail-head spar."
hedrons, dog-tooth spar, Fig. 60). The mineral occurs also
in fibrous, granular, and compact forms. The decomposition
of silicates containing lime by permeating water gives rise
i8o ELEMENTS OF EARTH* S CRUST. [CHAP.
to calcium-carbonate, which is removed in solution. Being
readily soluble in water containing carbonic acid, it is found
in almost all natural waters, by which it is introduced into
the cavities of rocks. Some plants and many animals secrete
large quantities of carbonate of lime, and their remains are
aggregated into beds of limestone, which is a massive and
more or less impure form of calcite. Calcite is easily
FIG. 60. Calcite in the form of dog-tooth spar.
scratched with a knife, and is characterised by its abundant
effervescence when acid is dropped upon it.
A less frequent and stable form of calcium-carbonate is
Aragonite which crystallises in orthorhombic forms, but is
more usually found in globular, dendritic, coral -like, or
other irregular shapes, and is rather harder and heavier
than calcite.
DOLOMITE assumes a rhombohedral crystallisation, and
is a compound of 54*4 of magnesium - carbonate, with 45-6
of calcium -carbonate. It is rather harder than calcite, and
does not effervesce so freely with acid. It occurs in strings
and veins like calcite, but also in massive beds having a
prevalent pale yellow or brown colour (owing to hydrated
x.]
SIDERITE, GYPSUM.
181
peroxide of iron), a granular and often cavernous texture,
and a tendency to crumble down on exposure.
SIDERITE (chalybite, spathic iron, ferrous carbonate,
FeCO 3 ), another rhombohedral carbonate, contains 62 -per
cent of ferrous oxide or pro-
toxide of iron. In its crystal-
line form it is grey or brown,
becoming much darker on ex-
posure as the protoxide passes
into peroxide. It also occurs
mixed with clay in concre-
tions and beds, frequently
associated with remains of
plants and animals (Sphcero-
siderite, Clay-ironstone, Figs.
61, 65).
Sulphates. Two sul-
phates deserve notice for their
importance among rock- FIG. 61. Sphserosiderite or Clay-
masses those Of lime and ir nstone concretion enclosing
portion of a fern,
baryta.
GYPSUM (hydrous calcium - sulphate, CaSO 4 +2HO 2 )
occurs in monoclinic crystals, commonly with the form of
right rhomboidal prisms (Fig. 62, a), which not infrequently
appear as macks or twin -crystals (Fig. 62, b\ When pure
it is clear and colourless, with a peculiar pearly lustre
(Selemte); it is found fibrous with a silky sheen (Satin-
spar}, also white and granular (Alabaster). It is so soft as
to be easily cut with' a knife or even scratched with the
finger-nails. It is readily distinguished from calcite by its
crystalline form, softness, and non-effervescence with acid.
When burnt it becomes an opaque white powder (plaster of
182
ELEMENTS OF EARTH S CRUST.
[CHAP.
Paris). Gypsum occurs in beds associated with sheets of
rock-salt and dolomite (pp. 63, 206) ; it is soluble in water,
and is found in many springs and rivers, as well as in the
sea. One thousand parts of water at 32 Fahr. dissolve 2*05
parts of sulphate of lime ; but the solubility of the substance
is increased in the presence of common salt, a thousand
FIG. 62. Gypsum crystals.
parts of a saturated solution of common salt taking up as
much as 8-2 parts of the sulphate.
Anhydrous calcium-sulphate or Anhydrite is harder and
heavier than gypsum, and is found extensively in beds asso-
ciated with rock-salt deposits. By absorbing water, it in-
creases in bulk and passes into gypsum.
BARYTES (Heavy spar, barium-sulphate, BaSO 4 ), the usual
form in which the metal barium is distributed over the globe,
crystallises in orthorhombic prisms which are generally tabu-
lar ; but most frequently it occurs in various massive forms.
The purer varieties are transparent or translucent, but in
general the mineral is dull yellowish or pinkish white, with a
x.] FLUORITE, HALITE, PYRITE. 183
vitreous lustre, and is readily recognisable from other similar
substances by its great weight ; it does not effervesce with
acids. Barytes is usually met with in veins traversing rocks,
especially in association with metallic ores.
Phosphates. Only one of these requires to be enume-
rated in the present list of minerals the phosphate of lime
or Apatite.
APATITE (tricalcic phosphate, phosphate of lime, p. 160)
crystallises in hexagonal prisms which, as minute colourless
FIG. 63. Group of fluor-spar crystals.
needles, are abundant in many crystalline rocks; it also
occurs in large crystals and in amorphous beds associated
with gneiss. It is soluble in water containing carbonic acid,
ammoniacal salts, common salt, and other salts. Hence its
introduction into the soil, and its absorption by plants, as
already mentioned (p. 160).
Fluorides. The only member of this family which
occurs conspicuously in the mineral kingdom is calcium
fluoride or FLUOR-SPAR (Fluorite, CaF 2 ) which, in the form of
colourless, but more commonly light green, purple, or yellow
1 84 ELEMENTS OF EARTH* S CRUST. [CHAP. x.
cubes, is found in mineral veins not infrequently accompany-
ing lead-ores (Fig. 63).
~ Chlorides. Reference has already been made to the
only chloride which occurs plentifully as a rock -mass, the
chloride of sodium, known as HALITE or Rock-salt (NaCl,
chlorine 60-64, sodium 39-36). It crystallises in cubical
forms, and is also found massive in beds that mark the
evaporation of former salt-lakes or inland seas (p. 207).
r* Sulphides. Many combinations of sulphur with the
metals occur, some of them of great commercial value \ but
the only one that need be mentioned here for its wide diffu-
sion as a rock-constituent is the iron-disulphide (FeS 2 ), in
which the elements are combined in the proportion of 46-7
iron and 53-3 sulphur. This substance assumes two crystal-
line forms : (i) PYRITE which occurs in cubes and other
forms of the first or monometric system, of a bronze-yellow
colour and metallic lustre, so hard as to strike fire with steel,
and giving a brownish-black powder when scratched. This
mineral is abundantly diffused in minute grains, strings, veins,
concretions (Fig. 64, c\ and crystals in many different kinds
of rocks ; it is usually recognisable by its colour, lustre, and
hardness ; (2) MARCASITE (white pyrite) crystallises in the
tetragonal system, has a paler colour than ordinary pyrite,
and is much more liable to decomposition. This form,
rather than pyrite, is usually associated with the remains of
plants and animals imbedded among rocks. The sulphide
has no doubt often been precipitated round decaying organ-
isms by their effect in reducing sulphate of iron. By its ready
decomposition, marcasite gives rise to the production of sul-
phuric acid and the consequent formation of sulphates.
One of the most frequent indications of this decomposition
is the rise of chalybeate springs (p. 79).
CHAPTER XL
THE MORE IMPORTANT ROCKS AND ROCK-STRUCTURES IN
THE EARTH'S CRUST.
FROM the distribution of the more important elements in
the earth's crust and the mineral forms which they assume,
we have now to advance a stage farther and inquire how
the minerals are combined and distributed so as to build up
the crust. As a rule, simple minerals do not occur alone in
large masses ; more usually they are combined in various
proportions to form what are known as Rocks. A rock may
be defined as a mass of inorganic matter, composed of one
or more minerals, having for the most part a variable
chemical composition, with no necessarily symmetrical ex-
ternal form, and ranging in cohesion from loose or feebly
aggregated debris up to the most solid stone. Blown sand,
peat, coal, sandstone, limestone, lava, granite, though so
unlike each other, are all included under the general name
of Rocks.
In entering upon the study of rocks, it is desirable to be
provided with such helps as are needed for determining
leading external characters ; in particular, a hammer to de-
tach fresh splinters of rock, a pocket-knife for trying the
hardness of minerals, a small phial of dilute hydrochloric
1 86 ROCK-STRUCTURES. [CHAP.
acid for detecting carbonate of lime, and a pocket lens.
The learner, however, must bear in mind that the thorough
investigation of rocks is a laborious pursuit, requiring
qualifications in chemistry and mineralogy. He must not
expect to be able to recognise rocks from description until
he has made good progress in the study. He will find much
advantage in procuring a set of named specimens, and
making himself familiar with such of their characters as he
can himself readily observe.
Great light has in recent years been thrown upon the
structure and history of rocks by examining them with the
microscope. For this purpose, a thin chip or slice of the
rock to be studied is ground smooth with emery and water,
and after being polished with flour-emery upon plate-glass,
the polished side is cemented with Canada balsam to a piece
of glass, and the other side is then ground down until the
specimen is so thin as to be transparent. Thin^ sections of
rock thus prepared reveal under the microscope the minutest
kinds of rock -structure. Not only can the component
minerals be detected, but it is often possible to tell the order
in which they have appeared, and what has been the probable
origin and history of the rock. Some illustrations of this
method of investigation will be given in a later part of the
present chapter. It will be of advantage to begin by taking
note of some of the more important characters of rocks, and
of the names which geologists apply to them.
Sedimentary composed of sediment which may be
either a mechanically suspended detritus, such as mud, sand,
or gravel ; or a chemical precipitate, as rock-salt and cal-
careous tufa. The various deposits which are accumulated
on the floors of lakes, in river-courses, and on the bed of
the sea, are examples of sedimentary rocks.
XI.]
CONCRETIONS.
187
Fragmental) Clastic composed of fragments derived from
some previous rock. All ordinary detritus is of this nature.
Concretionary composed of mineral matter which has
been aggregated round some centre so as to form rounded
FIG. 64. Concretions.
a, b, " Fairy stones ;" c, Pyrite, showing internal radiated structure.
or irregularly -shaped lumps. Some minerals, particularly
pyrite (Fig. 64, c), marcasite, siderite, and calcite, are fre-
quently found in concretionary forms, especially round some
organic relic, such as a shell or plant (Fig. 61). In alluvial
clay, calcareous concretions which often take curious imitative
shapes, are known as " fairy stones " (Fig. 64, a, b, see p. 235).
1 88 ROCK-STRUCTURES. [CHAP.
When nodules of limestone, ironstone, or cement-stone
are marked internally by cracks which radiate towards, but
FIG. 65. Section of a septarian nodule, with coprolite of a
fish as a nucleus.
do not reach, the outside, and are filled up with calcite or
other mineral, they are known as Septaria or septarian
nodules (Fig. 65).
Oolitic made up of spherical grains, each of which has
been formed by the deposition of successive coatings of
mineral matter round some grain of sand, fragment of shell,
or other foreign particle ( Fig. 66). A rock with this structure
looks like fish-roe, hence the name oolite or roe-stone ; but
when the granules are like peas, the rock becomes pisolitic
(pea -stone, Fig. 67). This peculiar structure is produced
in water (springs, lakes, or enclosed parts of the sea), wherein
dissolved mineral matter (usually carbonate of lime) is so
abundant as to be deposited in thin pellicles round the grains
of sediment that are kept in motion by the current (p. 118).
Stratified^ Bedded arranged in layers, strata, or beds
XI.]
AQUEOUS, UNSTRATIFIED.
189
lying generally parallel to each other, as in ordinary sediment-
ary deposits (Fig. 79, p. 227).
FIG. 66. Piece of oolite.
Aqueous laid down in water, comprising nearly the
whole of the sedimentary and stratified rocks.
FIG. 67. Piece of pisolite.
Unstratified, Massive having no arrangement in definite
layers or strata. Lavas and the other eruptive rocks are
examples (chapter xiv.)
IQO ROCK-STRUCTURES. [CHAP.
Eruptive^ Igneous forced upwards in a molten or
plastic condition into or through the earth's crust. All
lavas are Eruptive or Igneous rocks. In the same division
must be classed granite and allied masses, which have been
thrust through rocks at some depth within the earth's crust.
Crystalline consisting wholly or chiefly of crystals or
crystalline grains which have taken their forms by crystal-
lising where they are now found. Rocks of this nature
may have arisen from (a) igneous fusion, as in the case of
lavas, where the minerals have separated out of a molten
glass, or what is called a Magma ; (b) aqueous solution, as
where crystalline calcite forms stalactite and stalagmite in a
cavern ; (c) sublimation, where the materials have crystallised
out of hot vapours, as in the vents and clefts of volcanoes.
By the aid of the microscope it can often be ascertained
that the crystals or crystalline grains in a rock, as they were
crystallising out of their solution,
have enclosed various foreign
bodies. Among the objects thus
taken up are minute globules of
gas, which are prodigiously abund-
ant in certain minerals in some
lavas; liquids, usually water, en-
closed in cavities of the crystals,
but not quite filling them, and
FIG. 68. -Cavities in quartz leayi ft minute freely _ moving
containing liquids (magnified).
bubble (Fig. 68); glass, filling
globular spaces and probably portions of the original glassy
magma, out of which the crystals formed; crystals and
crystallites (rudimentary crystalline forms, Fig. 69) of other
minerals. Thus a crystal, which to the eye may appear quite
free from impurities, may be found to be full of various kinds
XL] GLASSY, PORPHYRITIC. I9 1
of enclosures. Obviously the study of these enclosures can-
FIG. 69. Crystallites (highly magnified).
not but throw light on the conditions under which the rocks
enclosing them were produced.
Glassy, Vitreous having a structure and aspect like that
of artificial glass. Some lavas, obsidian for example, are
natural glasses, and look not unlike masses of dark bottle-
glass. In almost all cases, however, they contain dispersed
crystals, crystallites, or other enclosures, and these substances
have sometimes multiplied to such an extent as to take the
place of the original glass. When a glass is thus converted
into a dull, opaque, stony, or lithoid substance, it is said to be
devitrified. The microscope enables us to detect traces of
an original glassy condition in many crystalline eruptive
rocks which, consequently, are thus ascertained to have
been once molten glass that has been devitrified by the
development of crystals and crystallites.
Porphyritic composed of a compact or crystalline base
or matrix, through which are scattered conspicuous crystals
much larger than those of the base, and generally of some
felspar. Many eruptive rocks have this structure and are
sometimes spoken of as porphyries or as being porphyritic.
The large crystals have probably been formed in the rock
while still in a mobile state within the earth's crust, while
the minuter crystals of the base have been developed during
ROCK-STRUCTURES.
[CHAP.
the consolidation of the rock at the surface ; in the succes-
sive zones of growth which porphyritic crystals often present,
FIG. 70. Porphyritic structure.
we may note by the enclosed minerals the stages of consoli-
dation of the rock.
Spherulitic composed of or containing small pea-like
FIG. 71. Spherulites and fluxion-structure. A, Spherulites, as seen
under the microscope (with polarised light). B, Fluxion structure of
obsidian, as seen under the microscope.
globular bodies (spherulites) which show a minutely fibrous
internal structure radiating from the centre (Fig. 7 1, A). This
XL] VESICULAR, AMYGDALOIDAL. 193
structure is particularly observable in vitreous rocks, where
it appears to be one of the stages of devitrification.
Vesicular containing spherical cavities. In many erup-
tive rocks (as in modern lavas) the expansion of interstitial
steam, while the mass was still in a molten condition, has
produced this cellular structure (Fig. 35), the vesicles have
usually remarkably smooth walls; they may form a com-
paratively small part of the whole mass, or they may so
increase as to make pieces of the rock capable of floating on
water. Where the vesicular structure is conjoined with
more solid parts, as in the irregular slags of an iron furnace,
it may be called slaggy. Where, as in the scoriae of a
volcano, the cellular and solid parts are in about equal pro-
portions, and the vesicles vary greatly in numbers and size
within short distances, the structure may be termed scoria-
ceous. The lighter and more froth-like varieties that can float
on water are said to be pumiceous, or to have the characters
of pumice (p. 214). Exposed to the influence of percolating
water, vesicular rocks have had their vesicles filled up by the
deposition of various minerals from solution, especially quartz,
calcite, and zeolites. These substances first begin to encrust
the walls of the cells, and as layer succeeds layer they gradu-
ally fill the cells up (Fig. 52); as the cells have not infre-
quently been elongated in one direction by the motion of
the rock before consolidation was completed (Fig. 37), the
mineral deposits in them, taking their exact moulds, appear
as oval or almond-shaped bodies. Hence rocks which have
been treated in this way are called Amygdaloids, and the
kernels filling up the cells are termed Amygdules (Fig. 35).
An amygdaloidal rock, therefore, was originally a molten
lava, rendered cellular by the expansion of its absorbed
steam and gases, its vesicles having been subsequently filled
o
194
ROCK-STRUCTURES.
[CHAP.
up by the deposit in them of mineral matter, often derived
out of the surrounding rock by the decomposing and re-
arranging action of percolating water.
Fluxion -structure an arrangement of the crystallites
and crystals of an eruptive rock in streaky lines, the minuter
forms being grouped round the larger, indicative of the
internal movement of the mass previous to its consolidation.
The lines are those in which the particles moved past each
other, the larger crystals giving rise to obstructions and
eddies in the flow of the smaller objects past them. This
structure is characteristic of many once molten rocks ; it
is well seen in obsidian (Fig. 71, B).
Schistose, Foliated consisting of minerals that have
crystallised in approximately parallel, wavy, and irregular
FIG. 72. Schistose structure.
laminae, layers, or folia (Fig. 72). Such rocks are called
generally schists. They have, in large measure, been formed
by the alteration or metamorphism of other rocks of various
kinds (chapter xiii.)
Various schemes of classification of rocks are in use
XL] SEDIMENTARY ROCKS. 195
among geologists, some based on mode of origin, others on
mineral composition or structure. For the purpose of the
learner, perhaps the most instructive and useful arrangement
is one which as far as possible combines the advantages of
both these systems. Accordingly, in the following account of
the more important rocks which enter into the structure of the
earth's crust, a threefold subdivision will be adopted into : (i)
sedimentary rocks ; (ii) eruptive rocks ; (iii) schistose rocks.
(i.) SEDIMENTARY ROCKS.
This division includes the largest number, and to the
geologist the most important of the rocks accessible to our
notice. It comprises the various deposits that owe their
origin to the decay of the surface of the land, and which
are laid down on the land or over the bed of the sea,
together with all those which are directly or indirectly due to
the growth of plants and animals. It thus embraces those
which constitute the main mass of the earth's crust so far as
known to us, and which contain the evidence whence the
geological history of the earth is chiefly worked out. It is,
therefore, worthy of the earliest and closest attention of the
student.
Sedimentary rocks, being due to the deposition of some
kind of sediment or detritus, are obviously not original or
primitive rocks. They have all been derived from some
source, the nature of which, if not its actual site, can usually
be easily determined. In no case, therefore, can a sedi-
mentary rock carry us back to the beginning of things ; it
is itself derivative and presupposes the existence of some
older rock or material from which it could be derived.
One of their most obvious characters is that, as a rule, they
196 SEDIMENTARY ROCKS. [CHAP.
are stratified. They have been deposited, usually in water,
sometimes in air, layer above layer, and bed above bed, each
of these strata marking a particular interval in the progress
of deposition (chapter xii.) As regards their mode of
origin, they may be subdivided into three great sections :
(i) fragmental or clastic, composed of fragments of pre-
existing rocks ; (2) chemically precipitated, as in the deposits
from mineral springs ; and (3) formed of the remains of
organisms, as in peat and coral-rock.
(i.) Fragmental or Clastic Rocks.
These are masses of mechanically -formed sediment,
derived from the destruction of older rocks ; they vary in
coherence from loose sand or mud up to the most compact
sandstone or conglomerate ; they are accumulating abund-
antly at the present time in the beds of rivers and lakes, and
on the floor of the sea, and they have been formed in a
similar way all over the globe from the earliest periods of
known geological history. Some of the more frequent kinds
are the following :
CLIFF -DEBRIS coarse angular rubbish, including large
blocks of stone, disengaged by the weather from cliffs and
other bare faces of rock. This kind of detritus is formed
abundantly in rugged and mountainous regions, especially
where the action of frost is severe ; it slides down the
slopes and accumulates at their foot, unless washed away
by torrents. In glacier-valleys it descends to the ice, where,
gathering into moraines (chapter vi.), it is transported to
lower levels. The perched blocks of such valleys are some
of the larger fragments of this cliff-debris left stranded by
the ice, and from around which the smaller detritus has
been washed away (Fig. 23).
XL] SOIL, BRECCIA, GRAVEL. 197
SOIL, SUBSOIL, described in chapter ii., represent the re-
sult of the subaerial decomposition of the surface of the land.
BRECCIA a rock composed of angular fragments.
Such a rock shows that its materials have not travelled far ;
otherwise, they would have lost their edges, and would
have been more or less rounded. Ordinary cliff- debris
may consolidate into a breccia, more especially where it
falls into water and is allowed to gather on the bottom.
FIG. 73. Brecciated structure volcanic breccia, a rock composed of
angular fragments of lava, in a paste of finer volcanic debris.
The angular fragments shot out of a volcano often accumu-
late into volcanic breccia (Fig. 73). A rock with abundant
angular fragments is said to be brecciated.
GRAVEL loose rounded water-worn detritus, in which
the pebbles range in average size between that of a small
pea and that of a walnut ; where they are larger they form
shingle. They may consist of fragments of any kind of
rock, though having resulted from more or less violent water-
action, as a rule, pieces of only the more durable stones are
found in them. Quartz and other siliceous materials, from
198 SEDIMENTARY ROCKS. [CHAP.
their great hardness, are better able to withstand the grind-
ing to which the detritus on an exposed sea-shore, or in the
bed of a rapid stream, is subjected. Hence quartzose and
siliceous pebbles are the most frequent constituents of gravel
and shingle.
CONGLOMERATE a name given to gravel and shingle
when they have been consolidated into stone, the pebbles
being bound together by some kind of paste or cement-
FIG. 74. Conglomerate.
ing material, which may be fine hardened sand, clay, or
some calcareous, siliceous, or ferruginous cement (Fig. 74).
As above remarked with regard to gravel, the component
materials of conglomerate may have been derived from any
kind of rock, but siliceous pebbles are of most common
occurrence. Different names are given to conglomerates,
according to the nature of the pebbles, as quartz-conglomer-
ate, flint-conglomerate, limestone-conglomerate.
SAND a name given to fine kinds of detritus, the grain
of which may vary from the size of a small pea down to
minute particles that can only be detected with a lens. In
XL] SAND, SANDSTONE. 199
general, for the reason already assigned in the case of
gravel, the component grains of sand are of quartz or
of some other durable material. Examined with a good
magnifying glass, they are seen to be rounded, water-worn,
or sometimes angular, unworn particles of indefinite shapes
which, except in their smaller size, resemble those of gravel-
stones. Sand may be formed by the disintegration of the
surface of rocks exposed to the weather, more especially in
dry climates, where there is a great difference between the
temperature of day and night (p. 17). The loosened
particles are blown away by the wind, and may be heaped
up into great sand-wastes, as in the tracts known as deserts.
On a sea-coast, where a sandy beach is liable to be laid bare
and exposed to be dried between tides by breezes blowing
from the sea, the upper particles of sand are lifted up by
the wind and borne away landward, to be piled up into
dunes (p. 27). In some places, the materials are derived
mainly from the remains of calcareous sea-weeds, shells,
corallines, and other calcareous organisms exposed to the
pounding action of the surf. A sand composed of such
materials speedily hardens into a more or less coherent and
even compact limestone, for rain falling on it dissolves some
carbonate of lime which, being immediately deposited again,
as the moisture evaporates, coats the grains of sand and
cements them together. At Bermuda, as already stated, all
the rock above sea-level has been formed in this way, and
some of it is hard enough to make a good building stone
(p. 112). Ordinary siliceous or quartzose sand remains
loose, unless its grains are made to cohere by some kind of
cement, when it becomes sandstone.
SANDSTONE consolidated sand. The grains are chiefly
quartz, but may include particles of any other mineral or
200 SEDIMENTARY ROCKS. [CHAP.
rock ; they are bound together by some kind of cement
which has either been laid down with them at the time of
their deposition, or has subsequently been introduced by
water permeating the sand. The cementing material may
be argillaceous that is, some kind of clay ; or calcareous,
consisting of carbonate of lime ; or ferruginous, composed
mainly of peroxide of iron ; or siliceous, where silica has been
deposited in the interstices of the mass. The colours of
sandstone vary chiefly with the nature of this cementing
material. The hydrous peroxide of iron colours them
shades of yellow and brown; the anhydrous peroxide of
iron gives them different tints of red ; the mineral glauconite
gives them a greenish hue. Some varieties of sandstone
are named after a conspicuous component or structure ; thus
micaceous sandstone is distinguished by abundant spangles
of mica deposited along the bedding planes, whereby the
rock can be split up into thin layers ; freestone a thick-
bedded sandstone that does not tend to split up in any one
direction, and can therefore be cut into blocks of any size
and form; glauconitic sandstone (green sand), containing
green grains and kernels of glauconite ; quartzose sandstone,
conspicuously composed of quartz-grains ; grit a sandstone
formed of coarse or sharp, somewhat angular grains of
quartz.
GREYWACKE a greyish, compact, granular rock, com-
posed of rounded or subangular grains of quartz and other
minerals or rocks, cemented together in a compact paste ;
it differs from sandstone chiefly in its darker colour, in
the proportion of other grains than those of quartz, and in
the presence of a tough cement.
The rocks above enumerated represent the coarser and
more durable kinds of detritus derived from the weathering
XL] CLAYS. 201
of the surface of the land ; but during the progress of the
decomposition from which these materials are derived some
of the component ingredients of the rocks decay into clay,
or what is called argillaceous sediment. This more parti-
cularly occurs in the case of felspars and other aluminous
silicates, the decomposition of which produces minute
particles capable of being lifted up and carried a great
distance by running water. Hence argillaceous sediment,
being finer in grain, travels farther, on the whole, than quartz-
ose sediment; and beds of clay denote, generally, deeper
and stiller water than beds of sand.
CLAY a fine-grained argillaceous substance, derived from
the decay and hydration of aluminous silicates, white when
pure, but usually mixed with impurities, which impart to it
various shades of grey, green, brown, red, purple, or blue ;
it usually contains interstitial water, and when wet can be
kneaded between the fingers ; when dry it is soft and friable,
and adheres to the tongue. Shaken with water it becomes
MUD, and even a small quantity will make a glass of water
turbid, so fine are the particles of which it is composed.
KAOLIN the name given to the white purer forms of
clay, resulting from the decomposition of the felspars of
granite or similar rocks ; it is sometimes called China-clay,
from its use in the manufacture of porcelain.
FIRE-CLAY a white, grey, yellow, or black clay, nearly
free from alkalies and iron, and capable of standing a great
heat without fusing; it is abundantly found underneath
coal-seams, where it represents the ancient soil on which the
plants grew that have been converted into coal.
BRICK-CLAY a name commonly applied to any clay,
loam, or earth from which bricks can be made ; these de-
posits are always more or less sandy and impure clays.
202 SEDIMENTARY ROCKS. [CHAP.
MUDSTONE a compact solidified clay or clay -rock,
having little or no tendency to split into thin laminae.
SHALE clay that has become hard and splits into thin
laminae which lie parallel with the planes of deposit. A
thoroughly fissile shale can be subdivided into leaves as thin
as fine cardboard. This is the common form which the clays
of the older geological formations have assumed. Grada-
tions can be traced from shale by additions of sand into
fissile sandstones, of calcareous matter into limestone, of
carbonate of iron into ironstone, and of carbonaceous matter
into coal. These passages are interesting as indications of
the conditions under which the rocks were formed. Where,
for example, shale shades off into coral -limestone, we see
that mud gathered over one part of the sea -floor, while not
far off, probably in clearer water, corals flourished and built
up a limestone out of their remains.
LOESS a pale somewhat calcareous and sandy clay,
found in regions where it has probably been accumulated
by the drifting action of the wind. It is sufficiently coherent
to be capable of excavation into tunnels and passages, and
in China is even dug out into houses and subterranean
villages. It occupies parts of the valleys of the Rhine,
Danube, Mississippi, and other large rivers, but also crosses
watersheds (p. 471).
Fragmental rocks of volcanic origin may be enumerated
here. They consist partly of materials ejected in fragment-
ary form from volcanic vents, and partly of the detritus
derived from the disintegration of volcanic rocks already
erupted to the surface. They are comprised under the
general name of TUFF.
BOMBS round elliptical or discoidal pieces of lava which
xi.] VOLCANIC FRAGMENTAL ROCKS. 203
have been ejected in a molten state from an active vent,
and have acquired their form from rapid rotation in the air
during ascent and descent. They are often very cellular or
even quite empty inside. Where the large ejected stones
are of irregular forms, and appear to have been thrown out
in an already solidified condition, as from the consolidated
crust of the lava-plug,, or from the sides of the funnel or
crater, they are called VOLCANIC BLOCKS (p. 135).
LAPILLI ejected pieces of lava, usually vesicular or
porous, from the size of a pea to a walnut.
VOLCANIC ASH the fine dust produced by the explosion
of the superheated steam absorbed in molten lava. Under
the microscope, it is often found to consist of minute grains
of glass, and, in such cases, shows that the lava from which
it was derived rose from below in the condition of a liquid
glassy magma. In other instances, it is made up of the
crystallites and crystals arising from the devitrification of
the glass. It consolidates into a more or less coherent mass,
which is known as TUFF, and which may receive some dis-
tinctive name according to the nature of the lava that has
supplied it, as basalt-tuff and trachyte-tuff. Most tuffs con-
tain angular and vesicular pieces of lava, and sometimes pass
into coarse breccias. In many cases, they enclose the remains
of plants and animals which, if of terrestrial kinds, indicate
that the eruptions took place on land ; if of marine species,
that the volcanoes were probably submarine.
AGGLOMERATE a coarse, usually unstratified accumula-
tion of blocks of lava and other rocks, filling up the chimney
or neck of a volcanic vent.
(2.) Rocks formed by Chemical Precipitation.
In chapter v. it was pointed out that all natural waters
204 CHEMICAL PRECIPITATES. [CHAP.
contain in solution invisible mineral matter which they have
dissolved out of the rocks of the earth's crust, and that the
quantity of this material is sometimes so great that it is
precipitated into visible form as the water evaporates. The
substance most abundantly dissolved and deposited is car-
bonate of lime. Others of frequent occurrence are sulphate
of lime, chloride of sodium, silica, carbonate of magnesia,
and various salts of iron. Among the rocks of the earth's
crust, considerable masses of these substances have been
piled up by chemical precipitation.
LIMESTONE compact or crystalline calcium-carbonate,
which may be nearly pure, or may contain sand, clay, or
other impurity, and may consequently pass into sandstone,
shale, or other sedimentary rock. Probably the great majority
of the limestones in the earth's crust have been formed by
the agency of animals, as more particularly referred to at p.
209. We are here concerned only with those which have
been deposited from chemical solution. The most familiar
example of this kind of limestone is afforded by stalactites
and stalagmite, which have already been described (chapter
v. and Fig. 20). Large masses of it have been deposited
by calcareous springs and streams. At first, it is a fine
white milky precipitate, but gradually crystals of calcite
shape themselves and grow out of it, with their vertical axes
usually at right angles to the surface of deposit. In a verti-
cal stalactite, consequently, the prisms radiate horizontally
from the centre outwards ; on a horizontal surface of stalag-
mite they diverge perpendicular to the floor. A mass of
limestone, not originally crystalline, may thus acquire a
thoroughly crystalline internal structure by the action of in-
filtrating water in dissolving and redepositing the carbonate
of lime in a crystalline condition.
XL] LIMESTONE, DOLOMITE. 205
Limestones vary greatly in texture and purity. Some
are snow-white and distinctly crystalline; others are grey,
blue, yellow, or brown, dull and compact, and full of various
impurities. They may usually be detected by the ease with
which they can be scratched, and their copious effervescence
when a drop of weak acid is put on the scratched surface.
Pure limestone dissolves entirely in hydrochloric acid, so
that the amount of residue is an indication of the proportion
of insoluble impurity. Among the varieties of limestone the
following may be named : Oolite a limestone composed of
minute spherical grains like the roe of a fish, each grain being
composed of concentrically deposited layers or shells of
calcite (Fig. 66) ; Pisolite a similar rock, where the grains
are as large as peas (Fig. 67); Travertine or calcareous tufa
a white porous crumbling rock which, by infiltration of
carbonate of lime, may acquire a compact texture, and
become suitable for building-stone (p. 77); Hydraulic lime-
stone containing 10 to 30 per cent of fine sand or clay,
and having the property, after being burnt, of hardening
under water into a firm compact mortar.
DOLOMITE, MAGNESIAN LIMESTONE this substance has
been already referred to as a mineral (p. 180); but it also
occurs in large masses as a white or yellowish crystalline or
compact rock. The white varieties look like marble. The
yellow and brown kinds contain various impurities, and are
coloured by iron oxide. Dolomite differs from limestone
in its greater hardness and feebler solubility in acid, in its
frequently cellular or cavernous texture, its tendency to
assume spherical, grape-shaped, or other irregular concre-
tionary forms (Fig. 75), and its proneness to crumble down
into loose crystals. It occurs in beds not uncommonly
associated with gypsum and rock-salt, and in such conditions
206 CHEMICAL PRECIPITATES. [CHAP.
it may have been deposited first as limestone which, by the
chemical action of the magnesian salts in the saline water,
FIG. 75. Concretionary forms assumed by Dolomite, Magnesian
Limestone, Durham.
had its carbonate of lime partially replaced by carbonate of
magnesia. It is also found in irregular bands traversing
limestone which, probably by the influence of percolating
water containing carbonate of magnesia in solution, has been
changed into dolomite.
GYPSUM is not only a mineral (p. 181) but also a rock,
white, grey, brown, or reddish in colour, granular to com-
pact, sometimes fibrous or coarsely crystalline in texture, and
consists of sulphate of lime. It is easily scratched with the
nail, and is not affected by acids, being thus readily distin-
guishable from limestone. It is found in beds or veins,
xr.] GYPSUM, ROCK-SALT, IRONSTONE. 207
especially associated with layers of red clay and rock-salt,
and in these cases has evidently resulted from the evapora-
tion of water containing it in solution, as in sea-water. The
lime-sulphate being less soluble than the other constituents
is precipitated first.' Hence in a thick series of alternations
of beds of gypsum (or anhydrite) and rock-salt, each layer
of sulphate of lime indicates a new supply of water into
the natural reservoirs where the evaporation took place.
The overlying bed of salt, usually much thicker than the
gypsum, points to the condensation of the water into a
strong brine, from which the salt was ultimately precipitated.
And the next sheet of sulphate of lime tells how, by the
breaking down of the barrier, renewed supplies of salt water
were poured into the basin.
ROCK-SALT occurs in beds or layers, from less than an
inch to hundreds or even thousands of feet in thickness.
One mass of salt in Galicia is more than 4600 feet thick,
and a still thicker mass occurs near Berlin. When quite
pure, rock-salt is clear and colourless, but it is usually more
or less mixed with impurities, particularly with red clay, as
above remarked. It has been formed in inland salt lakes
or basins by the evaporation and concentration of the saline
water. It is being deposited at the present time in the
Dead Sea, the Great Salt Lake, and the salt lakes so frequent
in the desert regions of continents, where the drainage does
not flow outwards to the sea.
IRONSTONE. Various minerals are included under this
name as large rock-masses. One of the most important of
them is Hematite (p. 172), which occurs in large beds and
veins, as well as filling up caverns in limestone. Limonite
or bog-iron-ore is formed in lakes and marshy places (p. 62),
and occurs in beds among other sedimentary accumulations.
208 ORGANICALLY DERIVED ROCKS. [CHAP.
Magnetite (p. 173) is found in beds and huge wedge-shaped
masses among various crystalline rocks, as in Scandinavia,
where it sometimes forms an entire mountain. Carbonate
of iron (Siderite, Sphaerosiderite, Clay-ironstone) occurs in
concretions and beds among argillaceous deposits (Figs. 61,
65). In the Coal-measures, for example, it is largely de-
veloped, much of the iron of Britain being obtained from this
source. As many ironstones are largely due to the influence
of plants and animals, the rock is alluded to again on p. 2 1 1 .
SILICEOUS SINTER a white powdery to compact and
flinty deposit from the hot water of springs in volcanic dis-
tricts, consisting of 84 to 91 per cent of silica, with small
proportions of alumina, peroxide of iron, lime, magnesia, and
alkali, and from 5 to 8 per cent of water. It accumulates
in basin-shaped cavities round the mouths of hot springs and
geysers, and sometimes forms extensive terraces and mounds,
as at the geyser regions of Iceland, Wyoming, and New
Zealand.
VEIN-QUARTZ a massive form of quartz, which occurs
in thin veins and in broad dyke-like reefs, traversing especi-
ally the older rocks.
(3.) Rocks formed of the Remains of Plants
or Animals.
In chapter viii. an account was given of the manner in
which extensive accumulations are now being formed of the
remains of plants and animals. Similar deposits have con-
stantly been accumulated from an early period in the history
of the earth. Regarding them with reference to their mode
of origin, we observe that in some cases they have been piled
up by the unremitting growth and decay of the organisms
upon the same site. In a thick coral-reef, for example, the
xi.] LIMESTONE FORMED OF ORGANIC REMAINS. 209
living corals now building on the surface are the descendants
of those whose skeletons form the coral -rock underneath.
In other cases, the remains of the organisms are broken up
and carried along by moving water, which deposits them
elsewhere as a sediment. Strictly speaking these last de-
posits are fragmental, and might be classed with those
described at p. 196 ; they pass into ordinary sand, sandstone,
clay, or shale. But it will be more convenient to class to-
gether all the rocks which consist mainly of organic remains,
whether they have been directly built up by the organisms,
or have only been formed out of their detrital remains.
LIMESTONE. As carbonate of lime is so largely secreted
by animals in their hard parts which are more or less durable,
it is naturally the most common substance among rocks of
organic origin. By far the larger proportion of the lime-
stones of the earth's crust have been formed out of the
remains of marine animals. The following are some of the
more important or interesting varieties : Shell-marl a soft
white earthy crumbling deposit formed chiefly of fresh-water
shells (p. 62). By subsequent infiltration it may be hardened
into a compact stone when it is known as fresh-water lime-
stone ; Calcareous sand a mass of broken-up shells, calcare-
ous algae, and other calcareous organisms (p. 199), often
cemented by percolating water into solid stone ; Coral-rock
a limestone formed by the continuous growth of corals
and cemented into a solid compact and even crystalline rock
by the washing of calcareous mud into its interstices and the
permeation of sea -water and rain-water through it, whereby
crystalline calcite is deposited within it (p. 115); Chalk a
soft, white rock, soiling the fingers, formed of a fine calcareous
powder of remains of foraminifera, shells, etc. (see Ooze, p.
1 14); Crinoidal limestone composed chiefly of the calcareous
210 ORGANICALLY DERIVED ROCKS. [CHAP.
joints of the marine creatures known as crinoids, with fora-
minifera, shells, corals, and other organisms. A limestone
composed in great part of organic remains may show little
trace of its origin on a fresh fracture of the stone ; but a
FIG. 76. Weathered surface of crinoidal limestone.
weathered surface will often reveal its true nature, the fossils
being better able to withstand the action of the atmosphere
than the surrounding matrix which is accordingly removed,
leaving them standing out in relief (Fig. 76).
PEAT a yellow, brown, or black fibrous mass of com-
pressed and somewhat altered vegetation. It occurs in
boggy places in temperate latitudes where it largely consists
of bog-mosses and other marshy plants (p. 109). Its upper
parts are loose and full of the roots of living plants, while
the bottom portions may be compact and black like clay,
and with little trace of vegetable structure.
LIGNITE or Brown Coal is a more compressed and chemi-
cally changed condition of vegetation. It varies in colour
from yellow to deep brown or black and may be regarded
XL] COAL, IRONSTONE. 211
as an intermediate stage between peat and coal. It occurs
in beds intercalated between layers of shale, clay, and sand-
stone.
COAL a compact, brittle, black, or dark brown stone,
formed of mineralised vegetation, and found in beds or seams
usually resting on clay, and covered with sandstone, shale,
etc. There are many varieties of coal differing from each
other in the relative proportions of their constituents.
Caking-coal, such as is ordinarily used in England, con-
tains from 75 to 80 per cent of carbon, 5 or 6 per cent of
hydrogen, and 10 or 12 per cent of oxygen, with some sul-
phur and other impurities. Anthracite^ the most thoroughly
mineralised condition of vegetation, is a hard, brittle, lustrous
substance, from which the hydrogen and oxygen have been
in great measure driven away, leaving 90 per cent or more
of carbon.
IRONSTONE. Reference was made at pp. 62, 79, to iron-
stone precipitated from chemical solution. This precipita-
tion is often caused through the medium of decomposing
organic matter. Organic acids, produced by the decay of
plants in marshy places and shallow lakes, attack the salts of
iron contained in the rocks or detritus of the bottom, and
remove the iron in solution. On exposure, the iron oxidises
and is thrown down as a yellow or brown precipitate of
limonite or bog -iron -ore (pp. 172, 207). Clay -ironstone ,
composed of a mixture of carbonate of iron, with clay and
carbonaceous matter, occurs abundantly with remains of
plants, shells, fishes, etc., in the Coal-measures, and has, no
doubt, been also formed through the agency of organic acids
which, passing into carbonic acid, have given rise to the solu-
tion and subsequent deposit of the iron as carbonate mingled
with mud and with entombed plants and animals.
212 ERUPTIVE ROCKS. [CHAP.
FLINT. Some siliceous deposits, due to organic agency,
have been already referred to at p. in. Besides these,
mention may be made of Flint, which occurs as dark
lumps and irregular nodular sheets in chalk and other lime-
stones, frequently enclosing urchins, shells, and other
organisms, which are sometimes converted into flint. Its
mode of origin is not yet thoroughly understood, but there
is reason to regard it as due to the abstraction of silica from
sea-water, either directly, by such animals as sponges, or
indirectly, by the decomposition of animal remains.
GUANO a brown, light, powdery deposit, formed of
the droppings of sea-birds in rainless tracts of the west
coasts of South America and Africa. It has a great com-
mercial value as an important manure.
BONE -BEDS deposits composed of fragmentary or
entire bones of fish, reptiles, or higher animals. The floors
of some caverns are covered with stalagmite, so full of
pieces of the bones of cave-bears, hyaenas, and other extinct
and living species, as to be called Bone-breccia. Layers of
stone, full of the coprolites (fossil excrement) of various verte-
brate animals, have, in recent Jyears, been largely worked
as sources of phosphate of lime for the manufacture of arti-
ficial manures.
(ii.) ERUPTIVE ROCKS.
Under this division are grouped all the massive rocks
which have been erupted from underneath into the crust
or to the surface of the earth. They are composed chiefly
of silicates of alumina, magnesia, lime, potash, and soda,
with different proportions of free silica, magnetic or other
oxide of iron, and phosphate of lime. The principal silicate
XL] CLASSIFICATION OF ERUPTIVE ROCKS. 213
is generally some felspar, the number of eruptive rocks
without felspar being comparatively small. The felspar is,
in different rocks, conjoined with mica, hornblende, augite,
magnetite, or other minerals.
No perfectly satisfactory classification of the eruptive
rocks has yet been devised; they have been grouped
according to their presumed mode of origin, some being
classed as hypogene, from their supposed origin, deep within
the earth's crust, others as volcanic, from having been ejected
by volcanoes. They have, likewise, been arranged accord-
ing to their chemical composition, and also with reference
to their internal structure. In the following enumeration
of some of the more abundant and important varieties, it
may be enough to adopt an arrangement in three sections,
according to the nature of the predominant silicate : viz.
(i) orthoclase rocks; (2) plagioclase rocks; and (3) olivine
and serpentine rocks. It has already been pointed out
that the original condition of many lavas and other erup-
tive rocks has been that of molten glass, their present stony
structure being due to the more or less complete devitrifica-
tion and disappearance of the glass by the development of
crystals and crystallites out of it during the process of cool-
ing and consolidation. Though there is no evidence that
all crystalline eruptive rocks have once been in the state
of molten glass, it may be useful to begin with the vitreous
varieties, which we know to represent the earliest forms of
many that are now quite crystalline.
(i.) Orthoclase Rocks.
In this section the prevalent silicate is orthoclase, either
in its common, dull, white, or pink form, or in the glassy
condition (sanidine). In many of the rocks, free quartz
214 ORTHOCLASE ROCKS. [CHAP.
occurs either in irregular crystalline blebs or in definite
crystals, which frequently take the form of double pyramids.
Among other minerals, hornblende, white and black mica,
and apatite are of common occurrence. The rocks of this
division are the most acid of the eruptive series that is,
they contain the largest proportion of silica or silicic acid,
sometimes more than 75 per cent. Some of them (granite)
are only found as masses that have consolidated deep be-
neath the surface ; others (trachyte, rhyolite, obsidian) are
abundant as superficial volcanic products.
OBSIDIAN a black, brown, or greenish (sometimes
yellow, blue, or red) glass, breaking with a shell-like or con-
choidal fracture and into sharp splinters, which are trans-
lucent at the edges. Examined in a thin section under the
microscope, the rock is found to owe its usual blackness to
the presence of minute opaque crystallites which are crowded
through it, not infrequently drawn out into streaky lines, and
curving round any larger crystal that may be embedded in
the mass. These arrangements, called fluxion -structure
(p. 194), have evidently been caused by the movement of the
rock while still in a fused state, the crystallites and other
objects being borne onward by the currents of molten glass.
In some obsidians, little spherulites of a dull grey enamel-
like substance have made their appearance as stages in the
devitrification of the rock (Fig. 71); but the mass has
consolidated before the stony condition could be completed.
In other instances, the whole rock has passed into a stony
enamel-like mass (pearlstone). Where a still molten ob-
sidian has been frothed up by the expansion of steam or
gas through it, so as to become a spongy cellular substance
which will float on water, it is called pumice. Obsidian
occurs in many volcanic regions, sometimes as streams of
XL] TRACHYTE, FELSITE, SYENITE. 215
lava which have been poured forth at the surface, some-
times in dykes and veins, and often in fragments ejected
with the other detritus that now forms tuffs.
TRACHYTE a compact porphyritic rock, consisting
mainly of orthoclase (sanidine), with some plagioclase and
usually with some hornblende, or with augite, mica, magne-
tite, or other minerals; having a peculiar matrix which,
under the microscope, is found to consist mainly of minute
felspar-crystallites. Large crystals of orthoclase (sanidine)
are frequent, and also scales of dark mica. This rock is
found abundantly among some of the younger volcanic
regions of the world, where it occurs in lava-streams and
also in intrusive sheets and dykes. QUARTZ -TRACHYTE
(Liparite, Rhyolite) is a rock composed of a compact, often
rough and somewhat porous base, through which are
scattered crystals of felspar and blebs of quartz, often also
with hornblende and mica.
FELSITE an exceedingly close-grained rock, composed
of an intimate mixture of quartz and orthoclase. The felspar
often occurs as large disseminated crystals, giving the porphy-
ritic structure. Where the quartz appears as distinct blebs
or crystals (sometimes double pyramids) the rock becomes
QUARTZ-PORPHYRY. The felsites and quartz-porphyries play
an important part among the eruptive rocks of older geo-
logical time, occurring both in the form of lavas erupted
to the surface and of intrusive masses that have consoli-
dated below ground. Many of them can be proved to have
been originally in the condition of molten glass which has
been devitrified.
SYENITE a thoroughly crystalline rock, consisting essen-
tially of orthoclase and hornblende, and distinguished from
granite chiefly by the absence or small amount of quartz.
216
ORTHOCLASE ROCKS.
[CHAP.
It occurs in bosses and veins which have been erupted into
older rocks.
GRANITE a thoroughly crystalline compound of felspar,
quartz, and mica, the individual minerals being large enough
to be distinctly recognised by the naked eye. Sometimes
large crystals of felspar are porphyritically scattered through
the rock. Granite occurs in large eruptive masses which
have been intruded into many different kinds of rocks, also
in smaller bosses and veins. Round the outside of a mass
FIG. 77. Group of crystals of felspar, quartz, and mica, from a cavity in
the Mourne Mountain granite.
of granite, there frequently diverge from it dykes and veins
(p. 272) which, where of great width, may be merely prolonga-
tions of the granite ; but which, when of small dimensions,
are apt to appear as felsite or quartz-porphyry. There can
be no doubt that such fine-grained veins are actually portions
of the same mass of rock as the granite, so that granite and
felsite or quartz-porphyry are only different conditions of
the same substance, the differences being probably due to
XL] PLAGIOCLASE ROCKS. 217
variations in the circumstances under which the cooling and
consolidation took place. The crystalline-granular structure
is so distinctive of granite that the name granitic 01 granitoid
is often applied to it. The constituent minerals have not
had room to assume perfect crystallised shapes, but occa-
sionally they have been able to shoot out in perfect crystals
where cavities occur. Fig. 77, for example, shows a group
of the ordinary crystals of this rock which have crystallised
in a cavity of the granite of the Mourne Mountains, Ireland.
(2.) Plagioclase Rocks.
In this Section, the felspar is some variety of plagioclase,
and the other most frequent silicate is either augite or horn-
blende. Though free quartz occurs in some of the rocks,
they contain generally so much less silica than the orthoclase
rocks as to be called basic compounds. A similar range of
texture can be observed in them to that characteristic of the
orthoclase series, from a true glass up to a thoroughly crys-
talline granitoid rock. Some of them, more especially the
coarsely crystalline varieties, are probably of deep-seated
origin; others (and these include the great majority) are
truly volcanic ejections which have risen in volcanic pipes
and fissures, and have been poured forth at the surface as
actual lava-streams.
BASALT-ROCKS a group of rocks consisting of plagio-
clase, augite, olivine, and magnetite or titaniferous iron, to
which apatite and other minerals may be added. These
rocks range in texture from a black glass up to a coarsely
crystalline mass wherein the component minerals are dis-
tinctly visible to the naked eye. Different names are em-
ployed to distinguish these varieties. Basalt-glass is a
general epithet to denote the vitreous varieties. These are
218 PLAGIOCLASE ROCKS. [CHAP.
particularly to be observed along the edges of dykes and
other intrusive masses, where they represent the outer surface
of the basalt that was suddenly chilled and consolidated by
coming in contact with the cold walls of the vent into
which it was injected, and where they no doubt show what
was the original state of the whole basalt before devitrifica-
tion converted the rock into its present crystalline structure.
Basalt a black, compact, heavy, homogeneous rock, break-
ing with a conchoidal fracture, showing sometimes large
porphyritic crystals of plagioclase, olivine, or augite, but
too fine-grained for the component minerals of the base to
be determined except with the microscope. The coarser
varieties, where the minerals can be recognised with the
naked eye, are known as dolerite. The basalt -rocks are
pre-eminently volcanic lavas, occurring both as intrusive
masses that consolidated underground, and as sheets that
were poured out in successive streams at the surface.
The black, compact kinds (true basalt) are particularly
prone to assume columnar forms (Fig. 78), whence colum-
nar rocks are sometimes spoken of as basalfic. In some
varieties of basalt the mineral Leucite takes the part of
the plagioclase; and in others this is done by another
mineral, Nepheline.
DIABASE a name given to some ancient basalt-rocks
in which, owing to alteration of their augite or olivine, a
greenish chloritic discoloration has often taken place. The
lavas of early geological time are to a large extent diabase.
ANDESITE is closely allied to basalt; but contains no
olivine. It sometimes includes free quartz and hornblende
may be substituted in it for augite. Hornblende-andesite
and augite-andesite are lavas which have been extensively
erupted in later geological time.
XI.]
PLAGIOCLASE ROCKS.
219
220 OLIVINE AND SERPENTINE ROCKS. [CHAP.
DIORITE a crystalline aggregate of plagioclase and
hornblende, usually with magnetite and apatite, sometimes
with augite and mica. The hornblende is black or dark
green and often more or less decomposed, giving rise to a
greenish chloritic discoloration of the felspar. From its
prevalent green colour, the rock was formerly known as
"greenstone." It occurs in intrusive masses, and seems
generally if not always to have consolidated below ground
instead of being poured out at the surface.
GABBRO, DIALLAGE-ROCK a thoroughly crystalline grani-
toid aggregate of plagioclase and the variety of augite known
as diallage, which appears in distinct brown or greenish
crystals, with a peculiar metalloidal or pearly lustre ; it is
found in bosses associated with granite, gneiss, etc., and also
sometimes with volcanic rocks.
(3.) Olivine and Serpentine Rocks.
In this group may be included a comparatively small
number of rocks which consist principally of olivine, and
which by gradual alteration pass into serpentine (Fig. 58).
OLIVINE-ROCKS (Peridotites) are liable to remarkably rapid
changes of texture and composition. In some places they
are mainly made up of olivine, augite, or hornblende, magne-
tite, and brown mica, but some of these minerals may dis-
appear and some felspar may take their place. They
are intrusive masses which appear to have been generally
injected into the crust in connection with volcanic eruptions,
rather than to have been poured out at the surface in true
lava-streams.
SERPENTINE a compact, dull, or faintly glimmering rock,
with a general dark dirty green colour, variously mottled,
greasy to the touch, easily scratched and giving a white
XL] SCHISTS. 221
powder which does not effervesce with acids. It is a mass-
ive form of the mineral serpentine described on p. 178;
frequently containing disseminated crystals of the minerals
bronzite, enstatite, and chromic iron, and veins of a delicately
fibrous silky variety of serpentine known as chrysotile.
Many serpentines were originally olivine- rocks which, by
hydration and alteration of their magnesian silicates, have
assumed their present characters. Serpentine occurs in
bosses, dykes, and veins, which were evidently of eruptive
origin and were at first probably olivine -rocks; it is also
found in thick beds associated with limestones and crystal-
line schists, where it may be a metamorphosed sedimentary
rock.
(iii.) THE SCHISTS
AND THEIR ACCOMPANIMENTS.
This section includes a remarkable series of rocks of
which the leading character is the possession of a schistose
or foliated character (Fig. 72). They are, in their more
typical varieties, distinctly crystalline; but some of them
shade off into ordinary fragmental rocks, such as shale and
sandstone. Several of them agree- in chemical and mineral
composition with some of the eruptive rocks already enumer-
ated, but differ from these in the peculiar foliated arrange-
ment of their minerals, though gradations can also be traced
between them.
In the schists, therefore, we see an assemblage of rocks
which, though possessing distinct characters of their own,
may yet be observed to shade off into fragmental rocks on
the one side, and into eruptive rocks on the other. In
chapter xiii. some further account of the schists will be given.
222 SCHISTS. [CHAP.
and it will there be shown on what grounds they have been
regarded as metamorphic or altered rocks. For the present,
in taking notice of their composition and structure, it will
be enough to state that in many cases they can be shown
to be more or less altered and crystalline, but originally sedi-
mentary rocks ; in other instances, they are crystalline erup-
tive masses, which have been subjected to such enormous
pressure and shearing, that a foliated structure and recrys-
tallisation of minerals have been superinduced in them.
CLAY-SLATE a hard fissile clay-rock, through which
minute scales of mica and crystals or crystallites of other
minerals have been developed; generally bluish-grey to
purple or green, and splitting into thin parallel leaves. As
this rock often contains remains of marine animals and
plants, and is interstratified with bands of sandstone, grit,
conglomerate, and limestone, it was undoubtedly, at first, in
the condition of soft mud on the sea-bottom. Sometimes the
organic remains in it are so curiously elongated or distorted
in one general direction as to show that the rock has been
drawn out by intense pressure and shearing (Figs. 98, 103,
104). The planes along which clay-slate splits are generally
independent of the original surfaces of deposit, sometimes
cross these at a right angle, and have been superinduced in
the rock by mechanical movements, as explained in chapter
xiii. Different varieties of clay-slate have received special
names. Roofing-slate is the fine compact durable kind, em-
ployed for roofing purposes and also for the manufacture of
cisterns, chimney-pieces, writing - slates ; Alum-slate dark,
carbonaceous, and pyritous, the iron disulphide oxidising into
sulphuric acid, and giving rise to an efflorescence of alum ;
Whet-slate, honestone exceedingly hard, fine-grained, and
suitable for making hones ; sometimes owing its hardness to
XL] AMPHIBOLITES, MICA-SCHIST. 223
the presence of microscopic crystals of garnet ; Chiastolite-
slate containing disseminated crystals of chiastolite, and
found especially around eruptive bosses of granite. By in-
crease of its mica-flakes a clay-slate passes into a Phyllite,
which has a more silvery sheen, and represents a farther
stage of metamorphism. Phyllite, by increase of the mica,
becomes Mica-slate. Clay-slate occurs extensively among
the older geological formations in all parts of the world.
AMPHIBOLITES rocks composed mainly of hornblende,
but with quartz, orthoclase, and other minerals in minor pro-
portions ; sometimes they are massive and granular (Horn-
blende-rocK), and in this condition may represent original
eruptive rocks; more usually they are schistose (Horn-
blende-schist). Gradations can be followed from a hornblende-
schist into a massive rock (diorite, diabase, etc.), so that there
can be no doubt that, in these cases at least, the schistose
structure is not original but has been superinduced. Am-
phibolites occur among the crystalline schists in most parts
of the world as occasional bands or bosses, perhaps originally
of an eruptive nature, but more or less affected by the
movements that have induced the schistose structure.
CHLORITE-SCHIST a scaly, schistose aggregate of green-
ish chlorite with quartz, and often with felspar, mica, and
octahedra of magnetite (Fig. 54) ; it occurs in beds associ-
ated with gneiss and other schists.
MICA- SCHIST (MiCA- SLATE) a schistose aggregate of
quartz and mica, the two minerals being arranged in irregular
but nearly parallel wavy folia. The rock splits along the
laminae of mica, so that its flat surfaces have a bright silvery
sheen, and the quartz is not well seen except on the cross
fracture, where only the thin edges of the mica -plates pre-
sent themselves. Mica-schist is often remarkably crumpled
224 SCHISTS AND THEIR ACCOMPANIMENTS. [CHAP.
or puckered a structure bearing witness to the intense
compression it has undergone. It abounds in most regions
where schists are extensively developed (chapter xvi.)
GNEISS a schistose aggregate of orthoclase, quartz, and
mica, varying in texture from a fine-grained rock up to a
coarse crystalline mass which, in hand specimens, may not
be distinguishable from granite. There is no difference, in-
deed, as regards composition, between gneiss and granite ;
gneiss may be called a foliated granite. There is good
reason to believe that some gneisses at least have been
made out of granite by the process of shearing above
referred to. Gneiss occurs abundantly among the oldest
known rocks of the earth's crust, and may be found in most
large regions of crystalline schists (chapter xvi.)
A few rocks which are commonly associated with the
schists, or with evidence of metamorphism, may be noticed
here marble, quartzite, and schistose conglomerate.
MARBLE a crystalline granular aggregate of calcite,
white when pure, and having the texture of loaf-sugar, but
passing into various colours according to the nature of the
impurities. It occurs in beds among the schists, and is no
doubt a limestone, formed either by chemical precipitation
or by organic agency, which has been metamorphosed by
heat and pressure into its present thoroughly crystalline
character. Some of the fossiliferous limestones through
which the Christiania granite rises have been changed into
crystalline marble, but their original corals and shells have
not been wholly effaced (see chapter xiv.)
QUARTZITE a hard, compact, granular rock, composed
of adherent quartz-grains, and breaking with a characteristic
lustrous fracture. It occurs in beds and thick masses, asso-
ciated with slates, mica-schists, and limestones ; it sometimes
XL] SCHISTOSE GRIT AND CONGLOMERATE. 225
contains organic remains; and has evidently at one time been
a siliceous sand, which owes its present firm texture to subse-
quent metamorphism.
SCHISTOSE GRIT and CONGLOMERATE. Interstratified
with clay-slates and mica-schists there are sometimes found
beds of grit and conglomerate, the grains and pebbles of
which consist of quartz or other durable material, while the
paste in which these rolled fragments have been imbedded
has been metamorphosed into a slate or schist, so that the
original clay or mud has passed into a more or less crystal-
line micaceous substance that wraps round the pebbles as a
kind of glaze. The original fragmental character of such
rocks admits of no doubt ; they were obviously at one time
sheets of fine and coarse gravel mixed with sandy mud; and
their presence among schistose rocks furnishes additional
corroborative evidence of the original sedimentary character
of some of these rocks.
PART III.
THE STRUCTURE OF THE CRUST OF
THE EARTH.
CHAPTER XII.
SEDIMENTARY ROCKS THEIR ORIGINAL STRUCTURES.
HAVING in the two foregoing chapters considered the more
important elementary substances of which the earth's crust
is composed and their combinations in minerals and rocks,
we have to inquire how these minerals and rocks have
been put together so as to build up the crust A very little
examination will suffice to show us that the upper or outer
parts of the solid globe consist chiefly of sedimentary rocks.
All over the plains and low grounds of the earth's surface,
which cover so large a proportion of the whole area of the
land, some kind of sediment underlies the soil clay, sand,
gravel, limestone. It is for the most part only in hilly or
mountainous regions that anything has been pushed up from
below, so as to indicate the nature of the materials under-
neath ; but everywhere we encounter proofs that the sedi-
mentary rocks do not remain as they were deposited. In
the first place, most of them were laid down on the sea-floor,
and they have been upraised into land. In the next place,
HAP. XII.]
STRATIFICATION.
227
not only have they been upheaved, they have not infre-
quently been bent, broken, and crushed, until sometimes
their original condition can no longer be determined. More-
over they have been invaded by masses of lava and other
eruptive rocks, which have been thrust in among them and
have often burst through them to form volcanoes at the
surface. We must now endeavour to form as clear a con-
ception as possible of what, after all these changes, the
present structure of the crust actually is. In this chapter,
therefore, we may examine some of the leading characters
of sedimentary rocks in the architecture of the crust, more
particularly those which have been determined by the con-
ditions under which the rocks were formed. In the next
chapter we shall consider some of the more important
characters which have been superinduced upon the rocks
since their formation.
STRATIFICATION. It has been shown (p. 53) that one of
the most distinctive features in sedimentary rocks is that they
are stratified that is, are ar-
ranged in layers one above
another. As those at the
bottom must have been de-
posited before those at the
top, a succession of layers of
stratified rocks forms a record
of deposition, in which the
early stages are chronicled by
the lower, and the later stages
by the upper layers. An illus-
tration of this kind of record
FIG. 79. Section of stratified
rocks.
has already been given in the introductory chapter. As a
further example, the accompanying section (Fig. 79) may be
228 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
taken. At the bottom lies a bed (a) of dark shale or clay with
fragments of crinoids, corals, shells, and other marine organ-
isms. Such a bed unmistakably points to a former muddy sea-
floor, on which the creatures lived whose remains have been
preserved in the hardened mud or shale. The next bed ()
is one of limestone full of similar organic remains ; it shows
that the supply of mud which had previously made the
water turbid and had been slowly gathering in successive
layers on the bottom now ceased. The water became clear
and much better fitted for the life of the crinoids, corals, and
shells. These creatures accordingly flourished abundantly,
living and dying on the spot generation after generation,
until their accumulated remains had built up a solid sheet
of limestone several feet thick. But once more muddy
currents spread over the place, and from the cloud of sus-
pended mud there slowly settled down the layer of blue clay
(<r) which overlies the limestone. As hardly any remains of
organisms are to be seen in it, we may infer that the inroad
of mud killed them off. Next, owing to some new shifting
of the currents, a quantity of sand was brought in and spread
out over the mud, forming the sandstone beds (d). The
sea in which these various strata were deposited was prob-
ably shallow ; or its floor may have been gradually rising.
At all events, the last layers of sand could have been only
slightly below the surface of the water, for they are immedi-
ately covered by a hardened silt or fire-clay (e) which, from
the abundant roots and rootlets that run through it in all
directions, was clearly once a soil whereon plants grew. It
was probably part of a mud-flat, on which vegetation spread
seaward from the land where the water shallowed, as happens
at the present day among the tropical mangrove-swamps
(p. in). The plants that grew on this soil have formed the
xii.] LAMINAE, STRATA, BEDS. 229
coal-seam (/), no doubt representing the growth of a long
period of time. But the existence of the coal-jungle came
to an end probably by a sinking of the ground beneath the
water. Mud, once more carried hither from the neighbour-
ing land, settled down upon the submerged vegetation and
formed the clay (g). But that land plants still abounded in
the immediate neighbourhood is shown by their numerous
remains in this clay. We notice too that the salts of iron
dissolved in the water were eliminated by the decaying
plants and animals and were precipitated in the form of
carbonate, so as to form concretions round occasional dead
shells, fishes, fern-fronds, and seed -cones. What were the
immediately succeeding events in this ancient history we
cannot tell ; the layer next in order is a coarse conglomerate
(/;), originally gravel, which must have been swept along by a
swift current that tore away the upper part of the clay-beds
(g) and any strata which may once have overlain them.
The whole stratified part of the earth's crust is composed
of materials which in this way may be made to tell their
story. In forcing them to yield up their records of the
ancient changes of which they are memorials, scope is
afforded for the most accurate and laborious investigation
and for the closest reasoning from the facts collected. At
the same time, it is obvious that the pursuit is one which
constantly exercises the imagination, and that, indeed, it
cannot be adequately followed unless, by the proper use of
the imagination, the former conditions of the earth's surface
are vividly realised.
The thinnest layers of a stratified rock form laming
such as the thin paper-like leaves into which shale can be
split. A number of laminae may be united in a stratum
or bed which may vary from less than an inch to many feet
230 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
in thickness. It is only the finer kinds of sedimentary rock
m that, as a rule, are laminated. In most
cases a stratum or bed is the thinnest
subdivision ; it can usually be separated
easily from those above and below it,
and it may generally be regarded as mark-
12 ing one continued phase of deposit, while
M the break between it and the next bed
3 above or below probably denotes an in-
a terruption of the deposit. The study of
the relations of strata to each other is
7 called Stratigraphy.
Layers of deposit usually lie parallel
s with each other, their flat surfaces marking
s the general floor of the water at the time of
\.i
J 1 1 J . I
3 their formation (Fig. 80). But sometimes
a series of layers may be found inclined
at various angles to what was obviously
the original general plane of deposition.
In Fig. 8 1, for example, a series of strata
is presented, which are distinguished by
a diagonal lamination. This is known as
false bedding or current-bedding. As ex-
plained in chapter iii. (p. 50), it has been
FIG. 80. Section
showing alternation
of beds
15. Shale. i 4 . seam of caused b ? the P ushin g of layers of sedi-
wkh S s eptarian I3 nod S u h ies e ment over the advancing front of a stra-
i2. Sandstone, n. Mud- turn, and may be compared to the oblique
stone. 10. Limestone. *
9 . ciay. s. Sandstones, bedding often to be seen in an earthwork.
7. bandy clays. 6. Lime-
stone with parting of sucn as a railway embankment, the upper
shale. 5. Shale. 4. Lime- *
stone. 3 . shale with surface of which may be in a general sense
cement - stone passing *
down into sandstone ( 2 ), parallel with the flat bottom of the valley,
which graduates into L J '
fine conglomerate (i). w hile the successive layers of which the
XII.]
FALSE BEDDING, RIPPLE-MARKS.
231
mound is made are inclined at angles of 30 or more. False
bedding is interesting as affording some indication of the
FIG. 81. False-bedded sandstone.
nature and direction of the currents by which the sediment
was transported.
PROOFS OF FORMER SHORES. Along the margin of the
sea, of lakes, and of rivers, several interesting kinds of mark-
ings may be seen impressed on surfaces of sand or mud from
which the water has retired. Every one who has walked on a
sea-beach is familiar with the ripple-marks left by the retreat-
ing tides upon the bare sands. They are produced by the
oscillation of the water driven into movement by wind play-
ing over its surface. They are usually effaced by the next
advancing tide; hence, out of the same sand new sets
of ripple-marks are made by each tide. But we can under-
stand that now and then under peculiarly favourable con-
ditions the markings may not be destroyed. If, for in-
stance, they were made in a kind of muddy sand which, in
the interval between two tides and under a strong sun, could
232 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
become hard and coherent on the surface, and if the next
tide advanced so quietly as not to disturb them, but to lay
down upon them a fresh layer of sand or mud, they might
be covered up and preserved. They would then remain
as a memorial of the shallow rippling water and bare sandy
shore where they had been formed.
Now evidence of this kind regarding the conditions of
FIG. 82. Ripple-marked surface of sandstone.
deposition occur abundantly among sedimentary rocks (Fig.
82). Ripple-marked surfaces may be traced one over another
for many hundred feet in a thick series of sandstones. They
bring clearly to the mind that the strata on which they lie
were accumulated in shallow water, along beaches that were
often laid dry.
LAND-SURFACES. Other traces of exposure to the air
may be noticed, where ripple -mark is abundant, in what
xii.] SUN-CRACKS. 233
are termed sun-cracks, rain-prints, and footprints. Those
who have observed what takes place in muddy places during
dry weather will remember that, as the mud dries and con-
tracts, it splits up into a network of cracks ; and that, on its
hardened surface, it retains impressions of the feet of birds
or of insects that may have walked over it while still soft.
The geological history recorded at such places cannot be
mistaken; first, the rainy period, with the rush of muddy
FIG. 83. Cast of a sun-cracked surface preserved in the next
succeeding layer of sediment.
water down the slopes and the formation of pools in which the
mud is allowed to settle ; then the season of warm weather
when the pools gradually dry up and birds seek their edges
to drink. If by any means a layer of sediment could be
laid down upon one of these desiccated basins so gently as
not to efface its peculiar markings, the cracked surface of
mud, with its footprints, would contain a perfectly intelli-
gible record of the changes which it had witnessed (Fig. 83).
Now surfaces of this kind abound among the sediment-
ary rocks of the earth's crust. They are found upon strata
234 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
that, from the presence of marine organic remains in them,
were certainly deposited under the sea. But these strata
cannot have accumulated in deep water; they must have
been formed along flat shores, where the sheets of sand
and mud were liable from time to time to be laid bare to
the sun and wind, where animals of various kinds left their
footmarks or trails on the still soft sediment, where the
evaporation and desiccation were so rapid as to cause the
exposed mud to harden on the surface and to crack up into
irregular polygonal cakes, and where the next succeeding
layers of sediment were deposited so gently as to cover up
and preserve the sun-cracked surfaces.
One further piece of evidence to indicate land-surfaces,
or, at least, shore-surfaces, in a series of aqueous sedimentary
FIG. 84. Rain-prints on fine mud.
strata, is that furnished by rain-prints. A brief shower of
rain leaves upon a smooth surface of fine sand or mud a
xii.] RAIN-PRINTS, CONCRETIONS. 235
series of small pits, each of which is the imprint of a de-
scending raindrop (Fig. 84). Where this takes place along
the edge of a muddy pool which is rapidly being dried up,
the prints of the drops may remain quite distinct on the
hardened surface of mud. And here, again, we can sup-
pose that if another layer of mud were gently deposited above
this surface the rain-prints would be sealed up and preserved.
We might even be able to tell from what quarter the wind
blew that brought the rain-cloud. If, for example, the rain-
prints were ridged up on one side in one general direction
this would show that the shower fell aslant and with some
force, and that the side on which the mud round the im-
prints was forced up was that towards which the rain was
driven. Such indications of ancient weather may here and
there be detected among stratified rocks.
CONCRETIONS. Another original characteristic of many
sedimentary rocks is a concretionary structure, particularly
observable in clays, limestones, and ironstones. In many
cases, the concretions have gathered round some frag-
ment of a plant or an animal. Clay -ironstone and im-
pure limestone have been aggregated in this manner into
spherical or elliptical forms, which are of frequent occur-
rence in clay or shale. Flint has also gathered round
some organic nucleus, which it has often entirely replaced.
But many concretions may be found where no organic frag-
ment as a starting-point can be detected. Some of the most
curious are the so-called Fairy-stones (Fig. 64), found in
alluvial clays, with so many imitative shapes, which have
been popularly supposed to be works of human or even
preternatural construction. They have probably been pro-
duced by the irregular cementing of clay, owing to the
spread of carbonate of lime through it, carried down by
236 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
permeating water. Some of the most extraordinary con-
cretionary masses are to be seen in certain magnesian lime-
stones, which appear to be built up of petrified lumps of coral,
bunches of grapes, cannon-balls, and other objects (Fig. 75).
In reality, all these diversified figures are due to the irregu-
larly varied way in which a concretionary structure has been
developed in the limestone.
ASSOCIATION OF STRATA. Certain kinds of sedimentary
rocks are apt to occur together to the exclusion of others.
This association depends on the circumstances of deposition.
Ironstone concretions, for example, are met with much more
frequently among clays or shales than in any other strata,
because it was during the deposit of fine mud with abundant
decomposing organic matter that the most favourable con-
ditions were supplied for the precipitation of carbonate of
iron. Clays and limestones frequently alternate, as also do
sandstones and conglomerates, because the circumstances
of deposition were somewhat alike. But we need not
expect to encounter a bed of coarse conglomerate in a
group of fine clays, for the current that was strong enough
to sweep along the stones of the conglomerate was too
powerful to allow the fine silt to lie undisturbed. For a
similar reason, we should be surprised to meet with a layer
of well stratified shale in a mass of conglomerate. The
agitated water in which these coarse materials were heaped
up would have swept away any fine sediment and prevented
it from being deposited. In all cases, the manner in which
the different kinds of sediment are associated with each other
leads us back directly to the original conditions of deposit,
and is only intelligible in proportion as these conditions
are clearly realised.
RELATIVE AREAS OF STRATIFIED ROCKS. Moreover,
xii.] DISPOSITION OF MECHANICAL SEDIMENT. 237
some kinds of sedimentary material must obviously spread
over wider areas than others. The coarse gravel and shingle
of the present beach do not extend far seawards ; they are
confined to the margin of the land. Sand covers the sea-
floor over a much wider area ; and beyond the limits of the
sand, in the deeper and stiller water, mud is allowed to accu-
mulate. Roughly speaking, therefore, the area of the distri-
bution of sediment is in inverse proportion to the coarseness of
the materials. The same law has regulated the accumulation
of detritus from early geological time. Coarse conglomer-
ates, which represent ancient shingles and gravels, thicken
and thin out rapidly, and do not usually cover a large area ;
they pass laterally and vertically into grits and sandstones
which have a much wider distribution, and these again shade
off into clays and shales that range also over large areas.
CHRONOLOGICAL VALUE OF STRATA. No clue has yet
been found to determine the length of time required for the
accumulation of a stratum or group of strata; but some
indications are afforded of relative lapse of time. Here
and there, for instance, vertical trunks of trees are met with
standing in their positions of growth, but imbedded in solid
sandstone (Fig. 85). These plants, sometimes 20 feet or
more in height, prove that a mass of sand of that depth
must have been accumulated around them before they had
time to decay. We know little about the durability of the
submerged trees ; but they probably could not have lasted
many years unless covered up by sediment. Finely lami-
nated clays were evidently deposited with extreme slowness.
Beds of limestone, composed of the crowded remains of
successive generations of marine creatures, must also have
required prolonged periods of time for their growth.
No reliable inference can be drawn from the mere thick-
238 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
nesses of strata as to the lapse of time which they represent.
A mass of sandstone 20 feet thick may have accumulated
FIG. 85. Vertical trees (Sigillaria) in sandstone, Swansea (Logan).
round a submerged tree in a few years. On the other
hand, a corresponding depth of fine laminated clay may
have required tenfold more time for its deposition. But
the same thickness of rock composed of alternations of
shale and limestone might represent a still longer period.
For it is obvious that the change from one kind of sediment
to another must often have been brought about by an ex-
tremely gradual modification of the geography of the region
from which the supply of sediment was derived. Hence
the interval between two beds or groups of beds, differing
much from each other in mineral composition, may have
been considerably longer than the time required for the
xii.] CONDITIONS OF SEDIMENTATION. 239
actual deposition of the strata of either or both beds or
groups of beds.
Sedimentary rocks have been accumulated to a depth of
many thousand feet and over areas many thousands of
square miles in extent. On any probable estimate, the
deposition of such vast masses of sediment must have
demanded enormous periods of time. Side by side with
the growth of mechanical sediments, there must have been a
corresponding wasting of land. Every bed of conglomerate,
sand, or mud represents, at least, an equivalent amount of
rock worn away from the land and transported as sediment
to the floor of the sea. During such prolonged ages as
these changes required, there was ample time for the outburst
of many successive volcanoes, for the passage of many earth-
quake-shocks, and for the subsidence or upheaval of many
parts of the earth's crust.
PROOFS OF SUBSIDENCE. A mass of sedimentary mate-
FiG. 86. Hills formed out of horizontal sedimentary rocks.
rial of great thickness which, from the remains of sun-cracks
and other evidence, was obviously deposited in shallow
water near land can only have been accumulated on an
area that was gradually sinking. Suppose, for instance, that
240 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
a hill formed out of such strata rises a thousand feet above
the valley at its foot, and that proofs of deposition in shallow
water can be detected from the lowest beds all the way up
to the highest. The lowest beds having once been close to
the surface, as shown by the sun-cracks and other evidence,
could only be covered with hundreds of feet of similar strata
by a gradual sinking of the ground, during which fresh
sediment was poured in, so that, although the original bottom
sank a thousand feet, the water may never have become
sensibly deeper, the rate of deposit of sediment having, on
the whole, kept pace with that of the subsidence.
OVERLAP. During such tranquil movements, as the area
of land lessens and that of the sea increases, the later sedi-
mentary accumulations must needs extend beyond the limits
of the older ones. Suppose, for instance, that such a sloping
land-surface as that represented in the section (x, Fig. 87)
FIG. 87. Section of overlap.
were slowly to subside beneath the sea, the first formed
Strata (a) will be covered and overlapped by the next series
(b\ and these in turn will be similarly concealed by the
following group (c). This structure is termed Overlap, and
may usually be regarded as evidence of a gentle subsidence
of the area of deposit
CONFORMABILITY,UNCONFORMABILITY. When stratified
deposits are laid down regularly and continuously upon each
other, with no interruption of their generally level position,
XII.]
UNCONFORMABILITY.
241
they are said to be conformable. In the section Fig. 80, for
instance, the series of sediments there represented has
evidently been deposited under the same general conditions.
The nature of the sediment has of course varied from time
to time ; limestones, shales, and sandstones have alternated
with each other ; but there has been no marked interrup-
tion or disturbance in their sequence. Suppose, however,
that owing to subterranean movements, a series of rocks is
shifted from its original position, and after being uplifted,
is exposed to the wearing action of the sea, rivers, air, rain,
frosts, and the other agents concerned in the degradation
of the surface of the land (a in Fig. 88). If a new series of
FIG. 88. Unconformability.
deposits (b) were laid down upon the denuded edges of
these rocks, the bedding of the whole would not be con-
tinuous. The younger strata would rest successively upon
different parts of the older group, or, in other words, would
be unconformable. Such a relation or unconformability
implies a terrestrial disturbance, and usually also the lapse
of a long interval of time between the respective periods of
the older and younger rocks, during which denudation of
the older strata took place. It serves to mark one of the
breaks or gaps in geological history. Unconformabilities
differ much from each other in regard to the length of
242 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
interval which they denote. In some cases, the blank may
be of comparatively slight moment ; in others, it is so vast
as to include the greater part of the time represented by the
stratified rocks of the earth's crust.
By means of unconformabilities the different ages of
mountain-chains are determined. If, for example, a moun-
tain showed the structure represented in Fig. 88, its upheaval
must obviously have taken place between the deposition of
the two series of rocks. Suppose the series a to represent
Lower Silurian, and b Carboniferous rocks, the date of the
mountain would be between the Lower Silurian and Car-
boniferous periods. If, in another mountain, series b were
unconformably overlain by a younger series, say of Jurassic
age, this mountain would thereby be shown to have under-
gone a subsequent uplift in the long interval between the
Carboniferous and the Jurassic periods.
Summary. In this Lesson, some of the more character-
istic original features of sedimentary rocks have been con-
sidered. Of these features, one of the most distinctive is
the arrangement into layers or beds, each of which is the
record of a portion of geological history, the oldest being
below and the youngest above. The smallest subdivision
of these records is a lamina or thin leaf, such as those into
which shales may be split. A stratum or bed, which may
contain many laminae or none, is a thicker layer separable
with more or less ease from those below and above it.
Though strata lie on the whole parallel with each other,
they often show oblique current-bedding, especially in sand-
stones. Traces of shore-lines and of surfaces laid bare by
the retirement of the water in which they were deposited,
are found in sun-cracks, rain-pittings, and footprints. Not
infrequently, instead of being evenly spread out in layers,
xii.] SUMMARY. 243
the sedimentary material has been aggregated into variously
shaped concretions. Certain kinds of sedimentary rocks
are apt to occur together, such as clays and limestones, clay-
ironstones and shales, coals and fire-clays ; because the con-
ditions under which they were respectively deposited were
on the whole similar. As a rule, the finer the detritus, the
wider the area over which it is spread ; hence clays gener-
ally cover wider tracts than conglomerates. No inference
can safely be drawn from the relative thickness of strata as
to the length of time which they respectively represent ;
they must vary widely in this respect, and it is quite con-
ceivable that, in many cases, the interval of time between the
deposition of two successive beds of very different character
and composition may have been actually longer than the
period required for the deposition of the two beds. A thick
series of sedimentary deposits usually indicates that the sea-
bottom on which it was laid down was slowly sinking. In
subsiding, the later deposits spread beyond the limits of
the earlier ones, and thus present what is called an overlap.
Where they have been laid down continuously one upon
another they are said to be conformable ; where they have
been deposited on the disturbed and worn edges of an older
series they are unconformable.
CHAPTER XIII.
SEDIMENTARY ROCKS STRUCTURES SUPERINDUCED IN
THEM AFTER THEIR FORMATION.
AFTER their deposition sedimentary materials have under-
gone various changes before assuming the aspect which
they now wear.
CONSOLIDATION. The most obvious of these changes
is that, instead of consisting of loose materials, gravel, sand,
mud, and so on, they are now hard stone. This consolida-
tion has sometimes been the result of mere pressure. As
bed was piled over bed, those at the bottom would gradually
be more and more compressed by the increasing weight of
those that were laid down upon them, the water would be
squeezed out, and any tendency which the particles might
have to cohere would promote the consolidation of the mass.
Mud, for example, might in this way be converted into clay,
and clay in turn might be pressed into mudstone or shale.
But besides cohesion from the pressure of overlying masses,
sedimentary matter has often been bound together by some
kind of cement, either originally deposited with it or subse-
quently introduced by permeating water. Among natural
cements, the most common are silica, carbonate of lime,
and peroxide of iron. In a red sandstone, for example,
CHAP, xiii.] JOINTS. 245
the quartz grains may be observed to be coated over with
earthy iron peroxide, which serves to unite them together
into a more or less coherent stone. The effect of weather-
ing is not infrequently to remove the binding cement, and
thereby to allow the stone to return to its original condition
of loose sediment.
JOINTS. Next to their consolidation into stone, the most
common change which has affected sedimentary rocks is the
production in them of a series of divisional planes or fractures
termed Joints. Except in loose incoherent materials, this
structure is hardly ever absent. In any ordinary quarry of
sandstone, limestone, or other sedimentary rock, or along a
natural cliff of the same materials, a little attentive observa-
tion will show that the bare wall of rock forming the back
of the quarry or the face of the cliff has been determined
by one or more natural fissures in the stone, and that there
are other fissures running parallel with it through every out-
standing buttress of rock. Moreover, we may observe that
these vertical or highly inclined lines of fissure are cut across
by others, more or less nearly at a right angle, and that the
sides of the buttresses have been defined by these transverse
lines, just as the main face of rock has been formed by the
first set. Such lines of division are Joints. In close-grained
stone, they may be imperceptible until it is quarried or broken,
when they reveal themselves as sharply defined, nearly ver-
tical fractures, along which the stone splits. There are
usually at least two series of joints crossing each other at
right angles or obliquely, whereby a rock is divided into
quadrangular blocks. In the accompanying diagram (Fig.
89) a group of stratified rocks is seen to be traversed by two
sets of joints, one of which defines the faces that are in
shadow, the other those that are in light. By help of these
246 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
divisional planes, it is possible to obtain large blocks of stone
for building purposes. The art of the quarryman largely
FIG. 89. Joints in a stratified rock.
consists in taking advantage of these natural lines of fracture,
so as to obtain his materials with the least expenditure of
time and labour, and in large masses. In nature, also, the
existence of joints is a fact of the highest importance. Re-
ference has already been made to the way in which they
afford a passage for the descent of water from the surface.
It is in great measure along joints that the underground
circulation of water is conducted. At the surface too, where
rocks yield to the decomposing influence of the weather, it
is by their joints that they are chiefly split up. Along these
convenient planes of division, rain-water trickles and freezes ;
the walls of the joints are separated, and the space between
them is slowly widened, until in the end it opens into
yawning rents, and portions of a cliff are overbalanced and
fall, while detached pinnacles are here and there isolated.
The picturesqueness of the scenery of stratified rocks is, in
great measure, dependent upon the influence of joints in
xiii.] JOINTS, DIP. 247
promoting their dislocation and disintegration by air, rain,
and frost.
In many cases, joints may be due to contraction. A
mass of sand or mud, as it loses water and as its particles
are more firmly united to each other, gradually occupies
less room than at first. In consequence of the contraction
strains are set up in the stone, and relief from these is
eventually found in a system of cracks or fissures. In other
instances, joints have been produced by the compression or
torsion to which large masses of rock have been exposed
during movements of the earth's crust.
ORIGINAL HORIZONTALITY. As laid down upon the
margin or floor of the sea, on the bottoms of lakes, and on
the beds or alluvial plains of rivers, sedimentary accumula-
tions are in general nearly flat ; they slope gently seawards
from a shelving shore, and they gather at steeper angles in
slopes of debris at the foot of cliffs, and down the sides of
mountains. But, taken as a whole, and over wide areas,
their original position is not far removed from the horizontal.
If we turn, however, to the sedimentary rocks which, though
originally deposited for the most part over the sea-bottom,
now form so much of the dry land, we find them inclined
at all angles, and even sometimes standing on end. Such
situations, in which their deposition could never have taken
place, show that they have been disturbed. Not only have
they been upraised into dry land, but they have been tilted
unequally, some parts rising or sinking much more than
others.
DIP. The inclination of bedded rocks from the horizon
is called their Dip. The amount of dip is reckoned from the
plane of the horizon. A face of rock standing up vertically
above that plane is said to be at 90, while midway between
248 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
that position and horizontality it lies at an inclination of 45.
The angle of dip is accurately measured with an instrument
FIG. 90. Dip and Strike. The arrow shows the direction of dip ; the
line s s marks the strike.
called a clinometer, of which there are various forms. One
of the simplest kinds is a brass half -circle graduated into
90 on each side of the vertical, on which a pendulum is
hung as in Fig. 91. The instrument is held between the
eye and the angle to be measured, and the upper edge is
made to coincide with the line of the inclined rock. The
FIG. 91. Clinometer.
pendulum, remaining vertical, points to the angle of inclina-
tion from the horizon. A little practice, however, enables
an observer to estimate the amount of dip by the eye with
sufficient accuracy for most purposes. The direction of
dip is the point of the compass toward which a stratum is
inclined (shown by the arrow in Fig. 90), and is best ascer-
XIII.]
DIP, STRIKE, OUTCROP.
249
tained with a magnetic compass. But here, again, a little
experience in judging of the quarters of the sky without an
instrument will usually enable us to tell the direction of dip
with as much precision as may be required.
STRIKE. A line drawn at a right angle to the direction of
dip is called the Strike (s s in Figs. 90, 92). Where a series of
strata dips due north or due south the strike is east and west ;
but the direction of strike changes with that of the dip.
Suppose, for example, that certain strata dip due east, then
veer round by south-east to south, and so on by west and
north, back to east again. The strike following this change
would describe a circle. In fact, the beds would be included
in a basin-shaped or dome-shaped arrangement and the strike
would be the lip of the basin or rim of the truncated dome.
Though the dip may slightly vary from place to place, still,
if it remains in the same general direction along the line of
certain strata, their strike is, on the whole, uniform.
FIG. 92. Dip, Strike, and Outcrop.
OUTCROP. The actual edge presented by a stratum at
the surface of the ground is called its Outcrop. On a per-
250 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
fectly level surface, strike and outcrop must coincide ; but
as ground is seldom quite level they usually diverge from
each other, and do so the more in proportion to the low-
ness of angle of dip and the inequalities of the ground.
This may be illustrated by a diagram such as that given in
Fig. 92, which represents a portion of the edge of a table-
A /
/ / f / / / X
FIG. 93. Inclined strata shown to be parts of curves.
land, deeply trenched by two valleys that discharge their
waters into the plain below (P). The arrows point out
that the strata dip due N. at 5. On the level plain, the
outcrop and the strike (s s) of the beds are coincident and
run due E. and W. But as the ground rises towards the
high ground and the deep valleys, the outcrop (o o) is ob-
XIII.]
CURVATURE.
251
served to depart more and more from the strike till in some
places they are at right angles; yet, as the dip remains
the same, the strike is likewise unchanged, the sinuosities
of the outcrop being entirely due to the irregularities of the
surface of the ground.
CURVATURE. It requires no long observation to perceive
FIG. 94. Curved strata (anticlinal fold), near St. Abb's Head.
that in being tilted from their original more or less level
positions, stratified rocks have been thrown into curves.
Suppose, for instance, that in walking along a mile of coast-
line, where all the successive strata of a thick series are ex-
posed to view, we should observe such a section as is drawn
in Fig. 93. Beginning at A we find the beds tilted up at
angles of 70 which gradually lessen, till at B they have sunk
252 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
to 15. As there is no break in the series, it is evident that
the lines of bedding must be prolonged downward, and
must once have been continued upward in some such way
as is expressed by the dotted lines. The visible portion
which is here shaded must thus form part of a great curva-
ture of the rocks.
But the actual curvature may often be seen on coast-cliffs,
ravines, or hillsides. In Fig. 94, for example, a simple arch
is shown from the Berwickshire coast, wherein hard beds of
greywacke and shale have been folded. Again, in Fig. 95,
FIG, 95. Curved strata (synclinal fold), near Banff.
the reverse structure is exhibited, beds of grit and slate being
there curved into a trough. Where rocks dip away from a
central line of axis the structure is known as an Anticline ;
where, on the other hand, they dip towards an axis it is called
xiii.] PLICATION, SHEARING. 253
a Syndine. In Figs. 94 and 95 these two structures are pre-
sented on so small a scale as to be visible in a single section.
More usually, however, it is only by observing the upturned
edges of strata that anticlines and synclines can be detected.
The dark part of Fig. 96 represents all that can be actually
b a b a
FIG. 96. Anticlines (a a) and Synclines (b b}.
seen ; but the angles and direction of dip leave no doubt
that if we could restore the amount of rock which has here
been worn away from the surface of the land, the present
truncated ends of the strata would be prolonged upward in
some such way as is indicated by the dotted lines. By
observations of this truncation of strata at the present sur-
face of the land, some of the most interesting and important
evidence is obtained of the enormous extent to which the
land has been reduced by the removal of solid material
from its surface.
PLICATION, SHEARING. From such simple curvatures
as those depicted in the foregoing diagrams, we may advance
to more complex foldings, wherein the solid strata have been
doubled up and crumpled together, as if they had been mere
layers of carpet. So far is this plication sometimes carried,
that the lowest rocks are brought up and thrown over the
highest, the more yielding materials being squeezed into the
most intricate frillings and puckerings. It is in mountainous
regions, where the crust of the earth has been subjected to
the most intense corrugation, that these structures are best
seen. We can form some idea of the gigantic energy of the
254 STRUCTURES OF SEDIMENTARY ROCKS.
earth-movements that produced them, when we see a whole
mountain-range made up of solid limestones or sandstones
which have been bent, twisted, crumpled, and inverted, as we
might crush up sheets of paper.
So enormous has been the compression produced by
important movements of the earth's crust, that the solid
rocks have actually been squeezed out of shape or have
undergone a process of shearing. The amount of distortion
FIG. 97. Section of the Grosse Windgalle (10,482 feet), Canton Uri,
Switzerland, showing crumpled and inverted strata (after Heim).
may sometimes be measured by the extent to which shells
or other organic remains are pulled out in the direction of
movement. In Fig. 98 the proper shape of a trilobite
(Angelina Sedgwickii} is given, and alongside of it is a view
of the same organism which has been elongated by the dis-
tortion of the mass of rock in which it lies. Further results
of shearing will be immediately referred to in connection
with the cleavage and metamorphism of rocks,
CLEAVAGE. One of the most important structures de-
veloped by the great compression to which the rocks of the
XIII.]
CLEAVAGE.
255
earth's crust have been exposed is known as Cleavage. The
minute particles of rocks, being usually of irregular^shapes,
FIG. 98. Distortion of fossils by the shearing of rocks ; (a) a Trilobite
(Angelina Sedgwickii] distorted by shearing, the direction of move-
ment indicated by the arrows ; (b] the same fossil in its natural form.
have been compelled to arrange themselves with their long
axes perpendicular to the direction of pressure, or where
actual shearing has taken place, have been arranged in the
direction of movement. Hence, a fissile tendency has been
imparted to a rock, which will now split into leaves along
the planes of rearrangement of the particles. This super-
induced tendency to split into parallel leaves, irrespective of
what may have been the original structure of the rock, con-
stitutes cleavage. It is well developed in ordinary roofing-
slate. Though the leaves or plates into which a slate splits
resemble those in a shale, they have no necessary relation to
the layers of deposition but may cross them at any angle.
In Fig. 99, for instance, the original bedding is quite distinct
and shows that the strata have been folded by a force acting
from the right and left of the section the parallel highly
inclined lines traversing the folds of the bedding represent
the planes of cleavage. Where the material is of exceedingly
fine grain, such as fine consolidated mud, the original bed-
ding may be entirely effaced by the cleavage, and the rock
256 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
will only split along the cleavage-planes. Indeed, the finer
the grain of a rock, the more perfect may be its cleavage, so
FIG. 99. Curved and cleaved rocks. Coast of Wigtonshire. The fine
parallel oblique lines indicate the cleavage, which is finer in the dark
shales and coarser in the thicker sandy beds.
that where alternations of coarser and finer sediment have
been subjected to the same amount of compression, cleavage
may be perfect in the one and rudely developed in the other,
as is indicated in Fig. 99.
DISLOCATION. Another important structure produced in
rocks after their formation is Dislocation. Not only have they
been folded by the great movements to which the crust of the
earth has been subjected, but the strain upon them has often
been so great that they have snapped across. Such ruptures
of continuity present an infinite variety in the position of the
rocks on the two sides. Sometimes a mere fissure has been
caused, the rocks being simply cracked across, but remain-
XIIL] FAULTS. 257
ing otherwise unchanged in their relative situations. But
in the great majority of instances, one or both of the walls
a b c
FIG. 100. Examples of normal Faults.
of a fissure have moved-, producing what is termed a Fault.
Where the displacement has been small, a fault may appear
as if the strata had been sharply sliced through, shifted, and
firmly pressed together again (a in Fig. 100). Usually, how-
ever, they have not only been cut, but bent or crushed on one
or both sides (b) ; while not infrequently the line of fracture
is represented by a band of broken and crushed material
(Fault-rock, c). The fracture is seldom quite vertical ; almost
always it is inclined at angles varying up to 70 or more
from the vertical. In by far the largest number of faults,
the inclination of the plane of the fissure, or what is called
the Hade of the fault, is away from the side which has risen
or toward that which has sunk. In the examples given
in Fig. 100, a, b, this relation is expressed; but in nature
it often happens that the beds on two sides of a fault are
entirely different (Fig. 100, c\ and consequently that the
side of upthrow or downthrow cannot be determined by
the identification of the two severed positions of the same
bed. But if the hade of the fault can be seen, we may usu-
ally be confident that the strata on the upper or hanging
side belong to a higher part of the series than those on the
lower side. Faults that follow this rule (normal faults) are
by far the most frequent They occur universally, and are
258 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
probably for the most part caused by subsidence in the
earth's crust. In adjusting themselves to the new position
into which a- downward movement brings them, rocks
must often be subject to such strains that their limit of
elasticity is reached, and they break across, one portion
settling down farther than the part next to it. In a normal
fault, the same bed can never be cut twice by a vertical
line.
In mountainous districts, however, and generally where
the rocks of the earth's crust have been disrupted and
pushed over each other, what are termed reversed faults
occur. In these, the hade slopes in the direction of up-
FIG. 101. Sections to show the relations of Plications (a, b) to
reversed Faults (c}.
throw, and a vertical line may cut the same beds twice on
opposite sides of the fracture (Fig. 101). Such faults may
be observed more particularly where strata have been much
folded. A fold may be seen to have snapped asunder, the
whole being pushed over, and the upper side being driven
forward over the lower.
The amount of vertical displacement between the two
fractured ends of a bed is called the Throw of a fault. In
Fig. 102, for example, where bed a has been shifted from b to
xiii.] FAULTS. 259
d t a vertical line dropped from the end of the bed at b to
the level of the corresponding part of the bed at e will give
the amount of the subsidence of #', which is the throw.
Faults may be seen with a throw of less than an inch
FIG. 1 02. Throw of a Fault.
mere local cracks and trifling subsidences in a mass of
rock; in others the throw may be many thousand feet.
Large faults often bring rocks of entirely different characters
together, as for instance, shales against limestones or sand-
stones, or sedimentary against eruptive rocks. Consequently
they are not infrequently marked at the surface by the
difference between the form of ground characteristic of the
two kinds of rock. One side, perhaps, rises into a hilly or
undulating region, while the other side may be a plain.
Comparatively seldom does a fault make itself visible as a
line of ravine or valley. On the contrary, most faults cut
across valleys or only coincide with them here and there.
They run in straight or wavy lines which, where the amount
of displacement is great, may be traced for many miles.
The Scottish Highlands, for example, are bounded along
their southern margin by a great fault which places a thick
series of sandstones and conglomerates on end against the
flanks of the mountains. This fault may be traced across
the island from sea to sea a distance of fully 120 miles,
and by bringing two distinct kinds of rocks next each
other along a nearly straight line it has given rise to the
260 STRUCTURES OF SEDIMENTARY ROCKS. [CHAP.
boundary between Highland and Lowland scenery which,
in some places, is so singularly abrupt.
METAMORPHISM. The last structure, which will be
mentioned in this chapter as having been superinduced
upon rocks, is connected with the movements to which
reversed faults are due. So enormous has been the energy
with which these movements have been carried on, that not
only have the rocks been crumpled, ruptured, and pushed
over each other, but they have undergone such intense
shearing that their original structure has been partially or
wholly effaced. They have been so crushed that their com-
FIG. 103. Ordinary unaltered
red sandstone, Keeshorn,
Ross-shire (magnified).
FIG. 104. Sheared red sand-
stone forming now a mica-
ceous schist, Keeshorn, Ross-
shire (magnified).
ponent particles have been reduced, as it were, to powder,
and have assumed new crystalline arrangements along the
shearing-planes or surfaces of movement. A sandstone, for
example, which in its ordinary state shows, when magnified,
such a structure as is represented in Fig. 103, when it has
come within the influence of this crushing process has its
grains of quartz, felspar, and other materials squeezed against
each other in one general direction, while a good deal of
xni.] METAMORPHISM. 261
mica has been developed out of the crushed debris. This
change is intensified until the component grains are hardly
recognisable, and where the proportion of new mica has so
increased that the rock has become a mica-schist. This
alteration is known as (regional) metamorphism (see p. 266).
There are wide regions of the earth's surface where
schists of various kinds form the prevailing rock. Whether
they have all been produced by the shearing and alteration
of previously-formed rocks has not yet been determined, if,
indeed, it can ever be ascertained. But that a large number
of schists are truly altered or metamorphosed rocks admits
of no doubt. Sandstones, shales, limestones, quartzites,
diorites, syenites, granites, in short, any old form of rock
that has come within the crushing and shearing movements
here referred to has been converted into schist. The
gradation between the unaltered and the metamorphic con-
dition can often be clearly traced. Granite, by crushing,
passes into gneiss, diorite into hornblende-schist, sandstone
into quartz-schist or mica-schist, and so on. Even where
it is no longer possible to tell what the original nature of
the metamorphosed material may have been, there is usually
abundant evidence that the rock has undergone great com-
pression.
Summary. In this Lesson attention has been directed
to new structures produced in sedimentary rocks after their
formation. Beginning with the simplest and most universal
of these, we find that sediments have been consolidated
into stone, partly by pressure, and partly by some kind of
cement, such as silica or carbonate of lime. In the process
of consolidation and contraction, they have been traversed
by systems of joints, or have had these subsequently pro-
duced by the torsion accompanying movements of the crust.
262 STRUCTURES OF SEDIMENTARY ROCKS. [CH. xm.
Though at first nearly flat, they have by these movements
been thrown into various inclined positions, and more
especially into undulating folds, or more complicated plica-
tion and puckering. So great has been the compression
under which they have been moved, that a cleavage has
been developed in them. They have also been everywhere
more or less fractured, the dislocations being due either to
their gradual subsidence or to excessive plication. Their
most complete alteration is seen in metamorphism, where,
under the influence of intense shearing, their original struc-
ture has been more or less completely effaced, and a new
crystalline rearrangement has been developed in them, con-
verting them into schists.
CHAPTER XIV.
ERUPTIVE ROCKS AND MINERAL VEINS IN THE ARCHITEC-
TURE OF THE EARTH'S CRUST.
I
NOT only have sedimentary formations since their deposi-
tion been hardened, plicated, fractured, and sometimes
even turned into crystalline schists, but into the rents
opened in them new masses of mineral matter have been
introduced which, in many regions, have entirely changed
the structure of the crust below and the appearance of the
surface above. Broadly speaking, there are two ways in
which these new masses have been wedged into their places.
First of all, eruptive material in a molten, or at least in a
viscous or plastic condition, has been thrust upward into
the cool and consolidated crust of the earth ; and in the
next place, various ores and minerals have been deposited
from solution in cracks and fissures, which they have en-
tirely filled up. To each of these two kinds of later rocks
attention will be given in this chapter.
Eruptive Rocks.
The rise of eruptive matter thrust upwards from lower
depths within the planet is one of the causes by which the
structure of the crust has been most seriously affected. In
264 ERUPTIVE ROCKS. [CHAP.
chapter ix. reference was made to some of the features
connected with the protrusion of molten rocks in the pro-
duction of volcanoes, and more particularly to those sub-
terranean changes which, when all the outer and ordinary
tokens of a volcano have been swept away, remain as evi-
dence of former volcanic action, even in districts where
every symptom of volcanic activity has long vanished. We
have now to inquire, generally, in what forms eruptive matter
has been built into the earth's crust, and what changes it has
produced there, apart from those superficial manifestations
which are the visible signs of volcanic action.
When a mass of lava or other fluid or viscous material
is forced upwards from the heated interior of the earth
towards the surface, the form which it finally takes and in
which it cools and solidifies depends upon the shape of the
rent or cavity into which it has been thrust. We may
compare such a mass to a quantity of melted iron escaping
from a blast-furnace. The shape taken by the iron will, of
course, be fixed by that of the mould into which it is allowed
to run. The crust of the earth, as was pointed out in the
previous chapter, has undergone extensive movements,
whereby its rocks have been crumpled and broken. It
consequently presents in different parts very different
degrees of resistance to any force acting upon it from
below. Where much shattered it yields to the upward
pressure of eruptive materials, and these force a way along
its fissures or between the beds and joints of the strata.
According to the form of the mould in which they have
solidified, we may classify the eruptive rocks of the crust
into (i) bosses; (2) sheets; (3) veins and dykes; and (4)
necks.
BOSSES. These are circular, elliptical, or irregularly shaped
XIV.]
BOSSES.
265
eruptive masses which, while still in a liquid or viscous state,
have been ejected into irregular rents of the earth's crust
and have solidified there. They consist of various crystal-
line rocks, more especially granite, syenite, quartz-porphyry,
diorite, diabase, and basalt-rocks, and vary in width from a
few yards to several miles. Being generally harder than the
surrounding rocks, they commonly stand up as prominent
knobs, hills, or ridges. Their presence at the surface, how-
ever, is due, not to their original protrusion there as in a
FIG. 105. Outline and section of a Boss (a) traversing stratified
rocks (bb}.
volcanic cone, but to the removal of the overlying part of
the original crust under which they cooled and consolidated.
Every boss is thus a witness of the extensive wearing away
of the surface of the land (Fig. 105).
In some large bosses, there may have been a complex
system of fissures into which the eruptive material rose.
Forced upwards into these, the molten rock would no doubt
envelope separated masses of the crust, and might bear them
along with it in its ascent. We may even conceive it to
have melted down such enveloped masses. Pushing the
266 ERUPTIVE ROCKS. [CHAP.
rocks aside and thrusting itself into every available crack
in them, the eruptive mass would work its way across the
crust. Where it succeeded in opening a passage to the sur-
face, ordinary volcanic phenomena would take place, such as
disruption of the ground, ejection of stones and ashes, and
outflow of lava. But, no doubt, in a vast number of cases,
no such communication was ever effected. The eruptive
material paused in its upward passage and consolidated be-
low ground.
No rock affords more interesting bosses than granite.
Two features are there especially well developed the
marginal veins or dykes and the surrounding ring of meta-
morphism produced in the rocks through which the granite
has risen. Granite has invaded many different kinds of
rocks, and has effected various kinds of change in them.
Round its margin, large numbers of veins or dykes of granite
or quartz -porphyry often strike out from it into the sur-
rounding rocks. There can be no doubt that these are
portions of the granite material, squeezed into cracks that
opened in the crust around it during its ascent. More
important is the change that can be observed to have taken
place in the rocks immediately surrounding the boss. The
granite at the time of its protrusion was probably in a molten
or pasty condition and impregnated with hot water or steam
and vapours. For a distance varying up to two or three
miles, the rocks next to it have undergone alteration, the
nature and amount of which appear to have been in great
measure dependent on their chemical and mineralogical com-
position. This metamorphism consists partly in mere indura-
tion, but still more in the development of new minerals, or
a new crystalline structure, even out of non-crystalline sedi-
mentary materials. The very same rock, which is elsewhere
XIV.]
BOSSES, CONTACT-METAMORPHISM.
267
a dark limestone full of shells, corals, or other organic re-
mains, becomes a white crystalline marble next the granite,
with no trace of any organisms, and so unlike its usual con-
dition that no one would readily believe it to be the same
rock. Again, a dark shaly sandstone or greywacke traced
towards the granite begins .to show an increasing amount
of mica. New minerals likewise make their appearance,
particularly garnets, until the rock entirely loses its sedi-
FIG. 106. Ground-plan of Granite-boss with ring of Contact-Meta-
morphism ; (a) sandstones, shales, etc. , dipping at high angles in
the direction of the arrows ; (b) zone or ring within which these
rocks are metamorphosed ; (c) granite sending out veins into b.
mentary structure and becomes a garnetiferous mica-schist.
Shales and slates, as they approach the granite, likewise pre-
sent a remarkable development of fine mica-plates, and pass
into argillaceous schists or phyllites, with crystals of chiasto-
lite or other minerals developed in them. The alteration of
rocks round eruptive masses is called contact-metamorphism.
What the cause may be of this remarkable alteration has
not yet been satisfactorily made out. In some bosses, the
mere heat of the eruptive material was probably sufficient
to produce change. There must often have been also a
copious discharge of hot vapours and water, which would
268
ERUPTIVE ROCKS.
[CHAP.
powerfully affect the adjacent rocks. Silica and other sub-
stances might then be introduced, leading to induration
and new chemical rearrangements of the constituents. The
protrusion of enormous masses of granite may also have
given rise to mechanical movements in the earth's crust, like
those which have produced the shearing and schistose
structure, seen in regional metamorphism (pp. 255, 260).
SHEETS. Sometimes the easiest passage for the erupted
material from below has lain between the bedding of strata.
The molten rock, after ascending some fissure or pipe,
has found its farther progress barred, and has escaped
by forcing up the overlying beds and thrusting itself in
below them. On cooling and consolidating, it appears as
a sheet or bed intercalated between older rocks. This
structure is represented in Fig. 107. Any one examining
FIG. 107. Intrusive Sheet.
such a section on the ground, might naturally regard the
sheet s as a bed of lava erupted at the surface after the
formation of the strata a and before that of b. But various
features characteristic of intrusive or subsequently injected
sheets enable us to distinguish them from those which
have been poured out during the deposition of the strata
among which they lie. For example, intrusive sheets
break across the strata (as at d in Fig. 107) and send
XIV.]
SHEETS.
269
veins into them. They are commonly most close-grained
along their edges, where they have been most rapidly chilled
by contact with the cool rocks ; while, on the contrary, true
lava-streams erupted at the surface are generally most slaggy
and scoriform on their upper and under surfaces. Lastly,
they have generally hardened and otherwise altered the rocks
above and below them, sometimes baking or even fusing
them. Where these characters are present, we may con-
fidently infer that, though a sheet of crystalline rock, so far
as visible at the surface, may seem to be regularly inter-
stratified between sedimentary beds, as if it had been con-
temporaneously poured forth among them, it has nevertheless
been thrust in between them and may be of much younger
date.
On the other hand, a truly contemporaneous sheet or
group of sheets marking the actual outpouring of lava-streams
at the surface, during the deposition of the strata among
FIG. 108. Interstratified or contemporaneous Sheets.
which they now lie, may be recognised by equally distinctive
characters. Thus they do not break across nor send veins
into the overlying or underlying beds, while their upper and
under surfaces are usually their most open cellular por-
tions, though they are often more or less vesicular or amyg-
daloidal throughout. In Fig. 108 the beds marked i, 2, 3,
and 4 are sheets of different lavas interstratified contem-
poraneously in the series of sandstones, shales, limestones,
270
ERUPTIVE ROCKS.
[CHAP.
and other strata among which they lie. Fragments of them
are not infrequently to be detected in the overlying strata,
which are thus shown to be of later origin, and bands of tuff
are commonly associated with them, just as showers of ashes
accompany the lava-streams of living volcanoes. As an
illustration of the way in which the evidence of contem-
poraneous volcanic action may be gathered, the section given
in Fig. 109 may be taken supple-
mentary to the data given already
in chapter ix. At the bottom of
the section we stand on the slaggy
upper surface of a lava-stream (i)
which was poured out under water,
for directly above it comes a seam
of dark shale (2) representing fine
mud that was deposited from sus-
pension in water. That volcanic
explosions still continued after the
outflow of the lava, is indicated by
the abundant bits of slaggy lava
and volcanic detritus scattered
FIG. 109. Section to illus- through the shale, and that the
trate evidence of contem-
poraneous volcanic action.
scene of these operations was the
sea-floor is conclusively proved by
the numerous shells, crinoids, and other marine remains that
lie in some bands of the shale. The bottom must then have
been muddy and not so well suited as it afterwards became for
the support of life. Above the shale we find 2 feet of lime-
stone (3) which is entirely made up of fragments of marine
organisms. These creatures, when the water cleared, con-
tinued to flourish abundantly until their congregated remains
formed a bed of solid limestone. But from some change in
xiv.] HISTORY OF VOLCANIC ERUPTIONS. 271
the geography of the region, currents bearing dark mud
once more invaded this part of the sea and threw down the
material that now forms the band of shale (4). The absence
of organic remains in this band probably shows the inroad
of mud to have destroyed the life that had previously been
prolific. When this condition of things had been brought
about, renewed volcanic explosions took place in the neigh-
bourhood. First came showers of dust, ashes, and stones,
which fell over the sea, and are now represented by the band
of tuff (5). Then followed the outpouring of a stream of
lava (6), with its characteristic cellular structure. But this
did not quite exhaust the vigour of the volcano, for the band
of tuff (7) points to successive showers of dust and stones.
When the explosion ceased, the deposition of dark mud,
which had been interrupted by the volcanic episode, was
resumed, and the band of shale (8) was laid down. From
the fragments of ferns and other plants in this shale it is clear
that land was not far off. The sea had evidently been
gradually shallowing by the infilling of sediment and volcanic
materials, and at last, on the muddy flat, represented by the
layer of fire-clay (9), marshy vegetation sprang up into a
thick jungle like the mangrove-swamps of tropical shores
at the present day. But after growing long enough to form
the bed of matted vegetable matter now represented by the
coal-seam (10), the verdant jungle was invaded by the sea,
and sank under the muddy water that threw down upon its
submerged surface the grey shale (n). In this shale, we
detect interesting traces of the renewal of volcanic activity,
more especially in occasional large blocks of lava, which
have evidently been ejected by some volcanic explosion, as
in the example already cited on p. 137 (Fig. 38). A more
vigorous volcanic outburst poured out the stream of columnar
272 ERUPTIVE ROCKS. [CHAP.
lava (12) which buried the whole and forms the top of the
section.
VEINS AND DYKES. These have already been referred
to in chapter ix. as part of the evidence for volcanic action.
We have here to consider how they occur in connection
with the protrusion of eruptive material within the crust of
the earth. Where the material so erupted has solidified
in a vertical or nearly vertical fissure so as to form a wall-
like mass, it is called a dyke (d, Fig. 107). Otherwise the
portions of erupted rock that have consolidated in irregular
rents are known as veins.
Veins are of common occurrence round bosses of granite,
where they can be traced into the parent mass from which
they have proceeded. They may likewise be observed in
connection with intrusive sheets and bosses of basalt and
diorite, from which they ramify outwards into the surround-
ing rocks. Their occurrence there is one of the proofs of
the intrusive character and subsequent date of such sheets
(p. 268).
Dykes vary from less than a foot to 70 feet or upward
in breadth, and run in nearly straight courses sometimes for
many miles. They consist most usually of diabase, andesite,
basalt, or allied rock. Sometimes they have risen along
lines of fault ; but in hundreds of instances in Great Britain,
they do not appear to be connected with any faults, but actu-
ally cross some of the largest faults in the country without
being deflected. The remarkable way in which dykes have
risen through a complicated series of rocks and faults and
have preserved their courses is exemplified in Fig. no.
Like intrusive sheets, but in a less degree, dykes harden
or otherwise alter the rocks on either side of them ; they
likewise present a similar closeness of grain along their
XIV.]
DYKES, NECKS.
273
margins where the molten rock was most rapidly chilled by
coming in contact with the cold walls of the fissure. Some-
FlG. 1 10. Map of Dykes near Muirkirk, Ayrshire. I. Silurian rocks.
2. Lower Old Red Sandstone. 3. Carboniferous rocks. f,f,f-
Faults, d, d. Dykes.
times, indeed, their sides are coated with a thin crust of black
glass, as if they had been painted with tar. This glass repre-
sents the effect of rapid cooling. No doubt the whole rock
of the dyke, at the time when it rose from below and filled
up the space between the two walls of its opened fissure, was
a molten glass. The portions that were at once chilled by
contact with the walls adhered as a layer of glass. But
inside this layer, the molten rock had more time to cool.
In cooling, its various minerals crystallised and the present
crystalline structure was developed. But even yet, though
most of the rock is formed of crystalline minerals, portions
of the original glass may not infrequently be detected between
them when thin sections are placed under the microscope.
NECKS. These are the filled-up pipes or funnels of
former volcanic vents. Their connection with volcanic
action has been already alluded to in chapter ix. They
T
274
ERUPTIVE ROCKS.
[CHAP.
are circular or elliptical in ground-plan, and vary in diameter
from a few yards up to a mile or more. They consist of
some form of lava (quartz-porphyry, basalt, diorite, etc.) or
of the fragmentary materials that, after being ejected from
the volcanic chimney, fell back into it and consolidated there.
They occur more particularly in districts where beds of lava
and tuff are interstratified with other rocks. The necks
represent the vents from which these volcanic materials were
ejected. In Fig. in, for example, the beds of lava and tuff
FIG. in. Section of a volcanic neck. The clotted lines suggest
the original form of the volcano.
(b b] interstratified between the strata a a and c c have been
folded into an anticline. In the centre of the arch rises the
neck (n) which has probably been the chimney that sup-
plied these volcanic sheets, and which has been filled up
with coarse tuff, and traversed with veins of basalt (*). The
dotted lines, suggestive of the outline of the original volcano,
may serve to indicate the connection- between the neck and
its volcanic sheets, and also the effects of denudation.
Necks are frequently traversed by dykes (* in Fig. in),
as we know also to be the case with the craters of modern
volcanoes. The rocks surrounding a neck are sometimes
bent down round it, as if they had been dragged down by
the subsidence of the material filling up the vent ; they
xiv.] MINERAL VEINS. 275
are also frequently much hardened and baked. When we
reflect upon the great heat of molten lava and of the escap-
ing gases and vapours, we may well expect the walls of a
volcanic vent to bear witness to the effects of this heat.
Sandstones, for instance, have been indurated into quartzite,
and shales have been baked into a hardened clay or porce-
lain-like substance.
Mineral Veins.
Into the fissures opened in the earth's crust there have
been introduced various simple minerals and ores which,
solidifying there, have taken the form of Mineral Veins.
These materials are to be distinguished from the eruptive
veins and dykes above described. A true mineral vein
consists of one or more minerals filling up a fissure which
may be vertical, but is usually more or less inclined, and
may vary in width from less than an inch up to 150 feet or
more. The commonest minerals (or veinstones) found in
these veins are quartzite, calcite, barytes, and fluor-spar.
The metalliferous portions (or ores) are sometimes native
metals (gold and copper, for example), but more usually are
metallic oxides, silicates, carbonates, sulphides, chlorides, or
other combinations. These materials are commonly arranged
in parallel layers, and it may often be noticed that they
have been deposited in duplicate on each side of a vein. In
Fig. 112, for instance, we see that each wall (w w) is coated
with a band of quartz (i, i), followed successively by one
of blende (sulphide of zinc, 2, 2), galena (sulphide of lead,
3, 3), barytes (4, 4), and quartz (5, 5). The central portion
of the vein (6) is sometimes empty or may be filled up with
some veinstone or ore. Remarkable variations in breadth
characterise most mineral veins. Sometimes the two walls
276
MINERAL VEINS.
[CHAP.
come together and thereafter retire from each other far
enough to have allowed a thick mass of mineral matter to
FIG. 112. Section of a mineral vein.
be deposited between them. Great differences may
also be observed in the breadth of the several bands com-
posing a vein. One of these bands may swell out so as
to occupy the whole breadth of the vein and then rapidly
dwindle down. The ores are more especially liable to such
variations. A solid mass of ore may be found many feet
in breadth and of great value ; but when followed along
the course of the vein may die away into mere strings or
threads through the veinstones.
The duplication of the layers in mineral veins shows that
the deposition proceeded from the walls inwards to the
centre. In the diagram (Fig. 112) it is evident that the walls
were first coated with quartz. The next substance intro-
duced into the vein was sulphide of zinc, a layer of which
was deposited on the quartz. Then came sulphide of lead,
and lastly, quartz again. The way in which the quartz-
crystals project from the two sides shows that the space
xiv.] MINERAL VEINS. 277
between them was free, and, as above stated, it has some-
times remained unfilled up.
There appears to be now no reason to doubt that the
substances deposited in mineral veins were introduced dis-
solved in water. Not improbably heated waters rose in
the fissures, and as they cooled in their ascent, they coated
the walls with the minerals which they held in solution.
These minerals may have been abstracted from the sur-
rounding rocks by the permeating water ; or they may have
been carried up from some deeper source within the crust.
During the process of infilling, or after it was completed, a
fissure has sometimes reopened, and a new deposition of
veinstones or ores has taken place. Now and then, too,
land shells and pebbles are found far down in mineral veins,
showing that during the time when the layers of mineral
matter were being deposited, the fissures sometimes com-
municated with the surface.
Summary. In this chapter it has been shown that, in
many cases, the rents in the earth's crust have been filled up
with mineral matter introduced into them, either (I.) in the
molten state, or (II.) in solution in water. I. The forms
assumed by the masses of eruptive rock injected into the
crust of the earth have depended upon the shape of the
fissures into which the melted matter has been poured, as
the form of a cast-iron bar is regulated by that of the mould
into which the melted metal is allowed to run. Taking this
principle of arrangement, we find that eruptive rocks may
be grouped into (i) Bosses, or irregularly -shaped masses,
which have risen through irregular fissures, and now, owing
to the removal of the rock under which they solidified, form
hills or ridges. The eruptive material sends out veins into
the surrounding rocks which are sometimes considerably
278 MINERAL VEINS. [CHAP. xiv.
altered, forming a metamorphic ring round the eruptive rock.
(2) Sheets or masses which have been thrust between the
bedding-planes of strata. These resemble truly interstratified
beds, but the difference between the two kinds of structure
can be readily appreciated. Interstratified beds mark the
occurrence of volcanic phenomena at the surface during the
time of the formation of the strata among which they occur.
Intrusive sheets, on the other hand, are always subsequent
in date to the rocks between which they lie. (3) Veins and
dykes, consisting of eruptive rock which has been thrust
between the walls of irregular rents or straight fissures.
(4) Necks, or the filled-up pipes of former volcanic vents.
II. Mineral veins are masses of mineral matter which has
been deposited, probably from aqueous solution, between
the walls of fissures in the earth's crust, and consists of
bands of veinstones (quartz, calcite, barytes, etc.) and ores
(native metals, or oxides, sulphides, etc., of metals).
CHAPTER XV.
HOW FOSSILS HAVE BEEN ENTOMBED AND PRESERVED, AND
HOW THEY ARE USED IN INVESTIGATING THE STRUC-
TURE OF THE EARTH'S CRUST, AND IN STUDYING
GEOLOGICAL HISTORY.
IN an earlier part of this volume (chapter viii.) attention was
called to the various circumstances under which the remains
of plants and animals may be entombed and preserved in
sedimentary accumulations. When these remains have thus
been buried they are known as Fossils. The word " fossil,"
meaning literally "dug up," was originally applied to all
kinds of mineral substances taken out of the earth ; but it is
now exclusively used for the remains or traces of plants and
animals imbedded by natural causes in any kind of rock,
whether loose and incoherent, like blown sand, or solid,
like the most compact limestone. It includes not only the
actual remains of the organisms. The empty mould of a
shell which has decayed out of the stone that once enveloped
it, or the cast of the shell which has been entirely replaced
by inorganic sand, mud, calcite, silica, etc., are fossils. The
very impressions left by organisms, such as the burrow or
trail of a worm in hardened mud, and the footprints of birds
and quadrupeds upon what is now sandstone, are undoubted
280 NATURE AND USE OF FOSSILS. [CHAP.
fossils. In short, under this general term is included what-
ever bears traces of the form, structure, or presence of
organisms preserved in the sedimentary accumulations of
the surface, or in the rocks underneath.
In geological history fossils are of fundamental import-
ance. They enable us to investigate conditions of geography,
of climate, and of life in ancient times, when these con-
ditions were very different from those which now prevail
on the earth's surface. They likewise furnish the ground
on which the several epochs of geological history can be
determined, and on which the stages of that history in one
country can be compared with those in another. So valu-
able and varied is the evidence supplied by fossils to the
geologist, that he regards them as among the most precious
documents accessible to him for unravelling the past history
of the earth. Some knowledge of the structure and classi-
fication of plants and animals is essential for an intelligent
appreciation of the use of fossils in geological inquiry. To
aid the learner, a synopsis of the Vegetable and Animal
Kingdom is given in the Appendix, with especial reference
to the fossil forms; but it must be understood that for
adequate information on this subject recourse should be
had to text-books of Botany and Zoology.
It is obvious that all kinds of plants and animals have not
the same chances of being preserved as fossils. In the first
place, only those, as a rule, are likely to become fossils, whose
remains can be kept from decay and dissolution, by being
entombed in some kind of deposit. Hence land-animals
and plants, on the whole, have much less chance of pre-
servation than those living in the sea, because deposits
capable of receiving and securing their remains are excep-
tional on land, but are generally distributed over the floor
xv.] CONDITIONS FOR FOSSILISATION. 281
of the sea. We should expect, therefore, that among the
records of past time, traces of marine should largely pre-
ponderate over traces of terrestrial life. Now this is every-
where the case. We know relatively littl? of the assem-
blages of plants and animals which in successive epochs
have lived upon the dry land, but we have a comparatively
large amount of information regarding those which have
tenanted the sea. For this reason, marine fossils are more
valuable than terrestrial, in comparing the records of the
successive epochs of geological history in different parts of
the globe.
In the second place, from their own chemical composi-
tion and structure, plants and animals present extraordinary
differences in their aptitude for preservation as fossils.
Where they possess no hard parts, and are liable to speedy
decay, we can hardly expect tha.t they should leave behind
them any enduring relic of their existence. Hence a large
proportion, both of the vegetable and animal kingdoms, may
at once be excluded as inherently unlikely to occur in the
fossil condition. Of course, under exceptional circumstances,
traces of almost any organism may be preserved, and,
therefore, we should probably not be justified in saying
that by no chance might some recognisable vestige of it be
found fossil. Nothing seems more perishable than the tiny
gnats and other forms of insect life that fill the air on a
summer evening. Yet many of these short-lived flies have
been sealed up within the resin of trees (amber), and their
structure has been admirably preserved. Such exceptional
instances, however, only bring out more distinctly how large a
proportion of the living tribes of the land must utterly perish,
and leave no recognisable record of their ever having existed.
But, where there are hard parts in an organism, and
282 NATURE AND USE OF FOSSILS. [CHAP.
especially where, from their chemical composition, they can,
for some time, resist decay, they may, under favourable
conditions, be buried in sedimentary deposits, and may
remain for indefinite ages locked up there. It is obvious,
therefore, that animals possessing hard parts are much the
most likely to leave permanent relics of their presence, and
ought to occur most frequently as fossils. It is these animals
whose remains are preserved in peat-mosses, river-gravels,
lake-marls, and on the sea-floor at the present time. Yet, if
we were to judge of the extent of the whole existing animal
kingdom from these fragmentary remains alone, what an
utterly inadequate conception of it we should form ! So, too,
if we estimate the variety of the living creatures of past
time merely from the evidence of the fossils that have
chanced to be preserved among the rocks, we shall probably
form quite as erroneous a conclusion. There can be no
doubt that from the earliest time only an insignificant
fraction of the varied life of each period has been preserved
in the fossil state, as is unquestionably the case at the pre-
sent day.
The essential parts of the solid framework of plants con-
sist of the substances known as cellulose and vasculose,
which, when kept in dry air, or when waterlogged and
buried in stiff mud, may remain undecomposed for long
periods. The timber beams in the roofs and floors of old
buildings are evidence that, under favourable conditions,
wood may last for many centuries. Some plants eliminate
carbonate of lime from solution in water, and form with it
a solid substance which requires no further treatment to
enable it to endure for an indefinite period, when screened
from the action of water. Still more durable are the
remains of those plants which abstract silica and build it up
xv.] CONDITIONS FOR FOSSILISATION. 283
into their framework, such as the diatoms of which the
frustules become remarkably permanent fossils, in the
form of diatom-earth or tripoli-powder, which is made up of
them (p. in).
The hard parts of animals may be preserved with little
or no chemical change, and remain as durable relics. The
hard horny integuments of insects, arachnids, Crustacea,
and some other animals, are composed essentially of the
substance called chitin^ which can long resist decomposition,
and which may therefore be looked for in the sedimentary
deposits of the present time, as well as of former periods.
The chitin of some fossil scorpions, admirably preserved
among the Carboniferous rocks of Scotland, can hardly
be distinguished from that of the living scorpion. Many
of the lower forms of animal life secrete silica, and their
hard parts are consequently easily preserved, as in the case
of radiolaria and sponges. In the great majority of in-
stances, however, the hard parts of invertebrates consist
mainly of carbonate of lime, and are readily preserved among
sedimentary deposits. The skeletons of corals, the plates
of echinoderms, and the shells of molluscs, are examples of
the abundance of calcareous organisms, and the frequency
of their remains in the fossil state shows how well fitted they
are for preservation. Among vertebrates the hard parts
consist chiefly of phosphate of lime. In some forms
(ganoid fishes and crocodiles, for example) this substance
is partly disposed outside the body (exo-skeleton) in the
form of scales, scutes, or bony plates. But more usually it
is confined to the internal skeleton (endo-skeleton). It is
mainly by their bones and teeth that the higher vertebrates
can be recognised in the fossil state. Sometimes the excre-
ment has been preserved (coprolites), and may furnish in-
284 NATURE AND USE OF FOSSILS. [CIIAI>.
formation regarding the food of the animal, portions of
undigested scales, teeth, and bones being traceable in it.
FOSSILISATION. The process by which the remains of a
plant or animal are preserved in the fossil state is termed
Fossilisation. It varies greatly in details, but all these may
be reduced to three leading types.
1. Entire or partial preservation of the original sub-
stance. In rare instances, the entire animal or plant has
been preserved, of which the most remarkable examples
are those where carcases of the extinct mammoth have
been sealed up in the frozen mud and peat of Siberia, and
have thus been preserved in ice, every portion of the animal
substance being retained, and the flesh being fresh enough
to be devoured by living carnivores. Insects have been
preserved in the resin of trees, and may now be seen, em-
balmed like mummies, in amber. More usually, however,
a variable proportion of the organic matter has passed away,
and its more durable parts have been left, as in the car-
bonisation of plants (peat, lignite, coal) and the disappear-
ance of the organic matter from shells and bones, which
then become dry and brittle and adhere to the tongue.
2. Entire removal of the original substance and internal
structure, only the external form being preserved. When a
dead animal or plant has been entombed, the mineral
matter in which it lies hardens round it and takes a mould
of its form. This may be accomplished with great perfection
if the mineral is sufficiently fine-grained and solidifies before
the object within has time to decay. Carbonate of lime and
silica are specially well adapted for taking the moulds of
organisms, but fine mud, marl, and sand, are also effective.
The organism may then entirely decay, and its substance
may be gradually removed by percolating water, leaving a
XV.]
MODES OF FOSSILISATION.
285
hollow empty space or mould of its form. Such moulds are
of frequent occurrence among fossiliferous rocks, and are
specially characteristic of molluscs, the shells of which are
so abundant, and occur imbedded in so many different
kinds of material. Sometimes it is the external form of the
shell that has been taken, the shell itself having entirely
disappeared; in other cases a cast of the interior of the
shell has been preserved. How different these two repre-
sentations of the same shell may be is shown in Fig. 113,
FIG. 113. Common Cockle {Cardiitm edule)\ (a) side view of both
valves ; (/>) mould of the external form of one valve taken in plaster
of Paris ; (c) side view of cast in plaster of Paris of interior of the
united valves.
wherein a represents a side view of the common cockle,
while c is a cast of the interior of the shell in plaster of
Paris. The contrast between a mould of the outside and
inside of the same shell is shown by the difference between
b and c, which are both impressions taken in plaster.
After the decay and removal of the substance of the
enclosed organisms, the moulds may be filled up with
mineral matter, which is sometimes different from the sur-
rounding rock. The empty cavities have formed convenient
receptacles for any deposit which permeating water might
286 NATURE AND USE OF FOSSILS. [CHAP.
introduce. Hence we find casts of organisms in sand, clay,
ironstone, silica, limestone, pyrites, and other mineral sub-
stances. Of course, in such cases, though the external form
of the original organism is preserved, there is no trace of
internal structure. No single particle of the cast may ever
have formed part of the plant or animal.
3. Partial or entire petrifaction of organic structure by
molecular replacement. Plants and animals which have
undergone this change have had their substance gradually
removed and replaced, particle by particle, with mineral
matter. This transformation has been effected by percolat-
ing water containing mineral solutions, and has proceeded
so tranquilly, that sometimes not a delicate tissue in the
internal structure of a plant has been displaced, and yet so
rapidly, that the plant had not time to rot before the con-
version was completed. Accordingly, in true petrifactions,
that is, plants or animals of which the structure has been
more or less perfectly preserved in stone, the petrifying
material is always such as may have been deposited from
water. The most common substance employed by nature
in the process of petrifaction is carbonate of lime, which, as
we have seen, is almost always present in the water of springs
and rivers. Organic structures replaced by this substance
are said to be calcified. Frequently the carbonate of lime
has assumed, more or less completely, a crystalline structure
after its deposition, and in so doing has generally injured or
destroyed the organic structure which it originally replaced.
Where the calcareous matter of an organism has been
removed by percolating water, the fossil is said to be decalci-
fied. Another abundant petrifying medium in nature is
silica, which, in its soluble form, is generally diffused in
terrestrial waters, where humus acids or organic matter are
xv.] FOSSILS PROVE GEOGRAPHICAL CHANGES. 287
present in solution. The replacement of organic structures
by silica, called silitification, furnishes the most perfect form
of petrifaction. The interchange of mineral matter has been
so complete that even the finest microscopic structures have
been faithfully preserved. Silicified wood is an excellent
example of this perfect replacement. Sulphides, which are
often produced by the reducing action of decaying organic
matter upon sulphates, occur also as petrifying media, the
most common being the iron sulphide, usually in the less
stable form of marcasite, but sometimes as pyrite. Car-
bonate of iron likewise frequently replaces organic structures ;
the clay-ironstones of the Carboniferous system abound with
the remains of plants, shell-fishes, and other organisms which
have been converted into siderite (Fig. 62).
The chief value of fossils in geology is to be found
in the light which they cast upon former conditions of
geography and climate, and in the clue which they furnish
to the relative ages of different geological formations.
I. HOW FOSSILS INDICATE FORMER CHANGES IN GEO-
GRAPHY. Terrestrial plants and animals obviously point to
the existence of land. If their remains are found in strata
wherein most of the fossils are marine, they usually show that
the deposits were laid down upon the sea-floor not far from
land. But where they occur in the positions in which they
lived and died, they prove that their site was formerly a land-
surface. The stumps of trees remaining in their positions
of growth, with their roots branching out freely from them
in the clay or loam underneath, undoubtedly mark the posi-
tion of an ancient woodland. If, with these remains, there
are associated in the same strata wing-cases of beetles, bones
of birds and of land-animals, additional corroborative evi-
dence is thereby obtained as to the existence of the ancient
288 NATURE AND USE OF FOSSILS. [CHAP.
land. More usually, however, it is by the deposits left on
lake-bottoms that the land of former periods of geological
history is known. As already pointed out (chapter iv.), the
fine mud and marl of lakes receive and preserve abundant
relics of the vegetation and animal life of the surrounding
regions. As illustrations of lacustrine formations, from which
most of our knowledge of the contemporary terrestrial life is
obtained, reference may be made to the Molasse of Switzer-
land, the limestones and marls of the Limagne d'Auvergne,
and the vast depth of strata from which so rich an assem-
blage of plant and animal remains has been obtained in the
Western Territories of the United States (see chapter xxiv.)
Alternations of buried forests or peat -mosses, with lake
deposits, show how lakes have successively increased and
diminished in volume. The frequent occurrence of a bed of
lacustrine marl at the bottom of a peat -bog proves how
commonly shallow lakes have been filled up and displaced
by the growth of marshy vegetation.
Remains of marine plants and animals almost invariably
demonstrate that the locality in which they are found was
once covered by the sea. Exceptions to this rule are so few
as hardly to be worthy of special notice, as, for instance,
when molluscs, crustaceans, and other forms of marine life
are carried up by sea-birds to considerable elevations, where,
after their soft parts have been eaten, their hard shells and
crusts may be preserved in truly terrestrial deposits, or when
sea-shells, tossed up by breakers above the tide-line, are
swept inland by wind.
Rolled fragments of shells, mingled in well-rounded
gravel and sand, point to some former shore, where these
materials were ground down by beach-waves. Fine muddy
sediment, containing unbroken shells, echinoderms, crusta-
xv.] FOSSILS SHOW CHANGES OF CLIMATE. 289
ceans, and other relics of the sea, indicate deeper water
beyond the scour of waves, tides, and currents. Beds of
limestone, full of corals and crinoids, mark the site of a clear
sea, in which these organisms were allowed to flourish un-
disturbed for many generations. It may often be observed
that the fossils, which are abundant and large in a limestone,
become few in number and small in size in an overlying bed
of shale or clay; or that they wholly disappear in the argilla-
ceous rock. The meaning of this can hardly be mistaken.
The clear water in which the marine creatures were able to
build up the limestone was at last invaded by some current
carrying mud. Consequently, while the more delicate forms
perished, others continued to live on in diminished numbers
and dwarfed development, until at last the muddy sediment
settled down so thickly that the animals, whose hard parts
might have been preserved, were driven away from that area
of the sea-bottom.
2. HOW FOSSILS INDICATE FORMER CONDITIONS OF
CLIMATE. The remains of plants or animals characteristic
of tropical countries may be taken to bear witness to a
tropical climate at the time which they represent. If, for
example, a deposit were found containing leaves of palms
and bones of tigers, lions, and elephants, we should infer
that it was formed in some tropical country, such as the
warmer parts of Africa or Asia. On the other hand, were a
stratum to yield leaves of a small birch and willow, with
bones of reindeer, musk-ox, and lemming, we would regard
it as evidence of a cold climate. Such inferences, however,
should be based either upon the occurrence of the very same
species as are now living, and the characteristic climate of
which is known, or upon assemblages of plants or animals
which may be compared with corresponding assemblages
290 NATURE AND USE OF FOSSILS. [CHAP.
now living. We may be tolerably confident- that the existing
reindeer has always been restricted to a cold climate, and
that the living elephants have as characteristically been con-
fined to warm climates. But it would be rash to assume
that all deer prefer cold and all elephants choose heat. The
bones of an extinct variety of elephant and one of rhinoceros,
have long been known as occurring even up within the
Arctic regions, and when these remains were first found the
conclusion was naturally drawn that they proved the former
existence of a warm climate in the far north. But the sub-
sequent discovery of entire carcases of the animals covered
with a thick mat of woolly hair, showed that they were
adapted for life in a cold climate, and their occurrence in
association with the remains of animals which still live in
the Arctic regions, proved beyond doubt that the original
inference regarding them was erroneous. In drawing con-
clusions as to climate from fossil evidence, it is always
desirable to base them upon the concurrent testimony of as
large a variety of organisms as possible, and to remember
that they become less and less reliable in proportion as the
organisms on which they are founded depart from the species
now living.
3. HOW FOSSILS INDICATE GEOLOGICAL CHRONOLOGY.
As the result of careful observations all over the world, it
has been ascertained that in the youngest strata the organic-
remains are nearly or quite the same as species now living,
but that, as we proceed into older strata, the number of exist-
ing species diminishes, and the number of extinct species
increases, until at last no living species is to be found.
Moreover, the extinct species found in younger strata dis-
appear as we trace them back into older rocks, and their
places are taken by other extinct species. Every great series
xv.] FOSSILS AND GEOLOGICAL CHRONOLOGY. 291
of fossiliferous rocks is thus characterised by its own peculiar
assemblage of species. Not only do the species change;
the genera, too, disappear one by one as we follow them
into older rocks, until among the earliest strata only a few
of the living genera are represented. Whole families and
orders of animals which once flourished have utterly vanished
from the living world, and we only know of their existence
from the remains of them preserved among the rocks.
A certain definite order of succession has been observed
among the organic remains imbedded in the stratified rocks
of the earth's crust, and this order has been found to be
broadly alike all over the world. The fossils of the oldest
fossiliferous rocks of Europe, for instance, are like those of
the oldest fossiliferous rocks of Asia, Africa, America, and
Australasia, and those of each succeeding series of rocks
follow the same general sequence. It is obvious, therefore,
that fossils supply us with an invaluable means of fixing the
relative position of rocks in the series of geological forma-
tions. Whether or not the same type of fossils was always
contemporaneous over the whole planet cannot be deter-
mined ; but it generally occupied the same place in the pre-
cession of life. Hence stratified formations, which may
be quite unlike each other in regard to the nature of their
component materials, if they contain similar organic remains,
may be compared with each other, and classed under the
same name.
Fossils characteristic of particular subdivisions of the
series of geological formations are known as type-fossils, of
which the following are examples :
Lepidodendra and Sigillaria, characteristic of Old Red Sandstone
and Carboniferous rocks (p. 354).
Cycads, characteristic of Mesozoic rocks (pp. 305, 379).
292 NATURE AND USE OF FOSSILS. [CHAP.
Graptolites, characteristic of Silurian rocks (p. 323).
Trilobites ,, Cambrian to Carboniferous rocks (pp.
328, 344, 363).
Cystideans, characteristic of Silurian rocks (Fig. 121).
Blastoids ,, Carboniferous rocks (Fig. 150).
Hippurites ,, Cretaceous rocks (p. 413).
Orthoceratites ,, Palaeozoic rocks (Figs. 130, 157).
Ammonites ,, Mesozoic rocks (Figs. 176, 190).
Cephalaspid fishes ,, Silurian, Old Red Sandstone (p. 340).
Ichthyosaurus and Plesiosaurus Mesozoic rocks (Fig. 180).
Iguanodon Cretaceous rocks (Fig. 192).
Toothed birds Cretaceous rocks (p. 417).
Nummulites, Palaeotherium, Anoplotherium, Deinocerata, charac-
teristic of older Tertiary rocks (pp. 427-438).
Mastodon, Elephas, Equus, Cervus, Hycena, Apes, characteristic
of younger Tertiary and Recent rocks (pp. 439-478).
By attentive study and comparison, the fossiliferous
rocks in different countries have been subdivided into
sections, each characterised by its own facies or type or
organic remains. Consequently, beginning with the oldest
and proceeding upward to the youngest, we advance through
natural chronicles of the successive tribes of plants and
animals which have lived on the earth's surface. These
chronicles, consisting of sandstones, shales, limestones, and
the other kinds of stratified deposits, form what is called the
Geological Record. In order to establish their true sequence
in time, their order of superposition must first be deter-
mined ; that is, it is requisite to know which lie at the
bottom, and must have been formed first, and in what order
the others succeed them. When this fundamental question
has once been settled, then the fossils characteristic of each
group of strata serve as a guide for recognising that group
wherever it may be found.
While fossils enable us to divide the Geological Record
into chapters, they also show how strikingly imperfect this
xv.] THE GEOLOGICAL RECORD. 293
record is as a history of the plants and animals that have
lived on the surface of the earth, and of the revolutions which
that surface has undergone. We may be sure that the pro-
gress of life from its earliest appearance in lowly forms of
plant or animal has been continuous up to the present con-
dition of things. But in the Geological Record there occur
numerous gaps. The fossils of one group of rocks are suc-
ceeded by a more or less completely different series in the
next group. At one time it was supposed that such breaks
in the continuity of the record marked terrestrial convulsions
that caused the destruction of the plants and animals of the
time, and were followed by the creation of new tribes of
living things. But evidence has every year been augmenting
that no such general destruction and fresh creation ever took
place. The gaps in the record mark no real interruption
of the life of the globe. They are rather to be looked upon
as chapters that have been torn out of the annals, or which
never were written. We have already learnt in chapter viii.
how many chances there must be against the preservation of
anything like a complete record of the life of the globe at
any particular time. It is also clear that even where the
chronicle may have been comparatively full, it is exposed to
many dangers afterwards. The rocks containing it may be
hidden beneath the sea, or raised up into land and entirely
worn away, or entombed beneath volcanic ejections, or so
crushed and crumpled as to become no longer legible.
Taking fossils as a guide, geologists have partitioned the
fossiliferous rocks into what are called stratigraphical sub-
divisions as follows : A bed, or limited number of beds, in
which one or more distinctive species of fossils occur, is called
a zone or horizon, and may be named after its most typical
fossil. Thus in the Lias, the zone in which the ammonite
294
NATURE AND USE OF FOSSILS.
[CHAP.
known as Ammonites Jamesoni occurs, is spoken of as the
"zone of Ammonites Jamesoni" or "famesom-zone." Two
or more zones, united by the occurrence in them of a number
of the same characteristic species or genera, form what are
known as Beds or an Assise. Two or more of such beds or
assises may be termed a Group or Stage. Where the number
of assises in a stage is large they may be subdivided into Sub-
stages or Sub-groups. The stage or group will then consist
of several sub -stages, and each sub -stage or sub-group of
several assises. A number of groups or stages is combined
into a Series, Section, or Formation, and a number of series,
sections, or formations constitute a System. A number of
systems are connected together to form each of the great
divisions of the Geological Record. This classification will
be best understood if placed in tabular form, as in the sub-
joined subdivisions, which occur in the Cretaceous System. 1
Stratigraphical Components.
Descriptive Names.
Examples from the Cre-
taceous System.
A stratum, layer, ^
seam, or bed, or a
number of such
minor subdivisions, r =
Zone or horizon . .
Zone of Peclcn aspcr.
characterised by
some distinctive
fossil
Two or more zones =
Beds or an assise
Warminster beds.
C Group or stage,
Cenomanian stage,
Two or more sets of con-
which may be
comprising the
nected beds or assises =
subdivided into
Rothomagian and
sub-groups or
Carentonian sub-
I sub-stages
stages.
Two or more groups or
Series, section, or
Neocomian forma-
stages
formation
tion.
Several related forma- ) _
System
Cretaceous System.
tions i
1 For an account of the Cretaceous System, see chapter xxiii
xv.J STRATIGRAPHICAL NOMENCLATURE. 295
The names by which the larger subdivision of the Geo-
logical Record are known have been adopted at various times
and on no regular system. Some of them are purely litho-
logical ; that is, they refer to the mere mineral nature of the
strata, apart altogether from their fossils, such as Coal-
measures, Chalk, Greensand, Oolite. These names belong
to the early years of the progress of geology, before the
nature and value of organic remains had been definitely
realised. Other epithets have been suggested by localities
where the strata occur, as London Clay, Oxford Clay, Moun-
tain Limestone. The more recent names for the larger
divisions have, in general, been chosen from districts where
the strata are typically developed, or where they were first
critically studied, e.g. Silurian, Devonian, Permian, Jurassic.
In some cases, the larger subdivisions have received names
from some distinguishing feature in their fossil contents, as
Eocene, Miocene, Pliocene. 1 But it is mainly to the minor
sections that the characters of the fossil contents have sup-
plied names.
The designation of any particular group of strata has
gradually come to acquire a chronological meaning. Thus
we speak of the Oolites or Oolitic formations of England,
and include under these terms a thick series of limestones,
clays, sandstones, and other strata, replete with organic
remains, and containing the records of a long interval of
geological time. But we also speak of the Oolitic period
a phrase which, in the strict grammatical use of the word, is
of course incorrect, but which conveniently designates the
period of geological time during which the great series of
Oolites was deposited, and when the abundant life of which
they contain the remains flourished on the surface of the
1 For the meanings of these names see chapter xxiv.
296 NATURE AND USE OF FOSSILS. [CHAP.
earth. This chronological meaning has indeed come to be
the more usual sense in which the names of the major
subdivisions of the Geological Record are generally em-
ployed. Such adjectives as Devonian and Jurassic do not
so much suggest to the mind of the geologist Devonshire
and the Jura Mountains, from which they were taken, nor
even the rocks to which they are applied, as the great sections
of the earth's history of which these rocks contain the
memorials. He compares the Jurassic or Devonian rocks
of one country with those of another, studies the organic
remains contained in them, and then obtains materials for
forming some conception of what were the conditions of
geography and climate, and what was the general character
of the vegetable and animal life of the globe, during the
periods which he classes as Jurassic and Devonian.
Summary. Fossils are the remains or traces of plants
and animals which have been imbedded in the rocks of the
earth's crust. From the exceptional nature of the circum-
stances in which these remains have been entombed and
preserved, only a comparatively small proportion of the
various tribes of plants and animals living at any time upon
the earth is likely to be fossilised. Those organisms which
contain hard parts are best fitted for becoming fossils. The
original substance of the organism may, in rare cases, be pre-
served ; more usually the organic matter is partially or wholly
removed. Sometimes a mere cast of the plant or animal in
amorphous mineral matter retains the outward form without
any trace of the internal structure. In other instances, true
petrifaction has taken place, the organic structure being
reproduced in calcite, silica, or other mineral by molecular
replacement.
Fossils are of the utmost value in geology, inasmuch as
xv.] SUMMARY. 297
they indicate (i) former changes in geography, such as the
existence of ancient land -surfaces, lakes, and rivers, the
former extension of the sea over what is now dry land, and
changes in the currents of the ocean ; (2) former conditions
of climate, such as an Arctic state of things as far south as
Central France, where bones of reindeer and other Arctic
animals have been found ; (3) the chronological sequence
of geological formations, and, consequently, the succession of
events in geological history, each great group of strata being
characterised by its distinctive fossils. This is the most
important use of fossils. Having ascertained the order of
superposition of fossiliferous rocks, that is, the order in
which they were successively deposited, and having found
what are the characteristic fossils of each subdivision, we
obtain a guide by which to identify the various rock-groups
from district to district, and from country to country. By
means of the evidence of fossils the stratified rocks of the
Geological Record have been divided into sections and sub-
sections, to which names are applied that have now come to
designate not merely the rocks and their fossils, but the
period of geological time during which these rocks were
accumulated and these fossils actually lived.
PART IV.
THE GEOLOGICAL RECORD OF THE
HISTORY OF THE EARTH.
CHAPTER XVI.
THE EARLIEST CONDITIONS OF THE GLOBE THE
ARCH/EAN PERIODS.
THE foregoing chapters have dealt chiefly with the materials
of which the crust of the earth consists, with the processes
whereby these materials are produced or modified, and with
the methods pursued by geologists in making their study of
these materials and processes subservient to the elucidation
of the history of the earth. The soils, rocks, and minerals
beneath our feet, like the inscriptions and sculptures of a
long-lost race of people, are in themselves full of interest,
apart from the story which they chronicle ; but it is when
they are made to reveal the history of land and sea, and of
life upon the earth, that they are put to their noblest use.
The investigation of the various processes whereby geological
changes are carried on at the present day is undoubtedly
full of fascination for the student of nature ; yet he is con-
scious that it gains enormously in interest when he reflects
CHAP, xvi.] GEOLOGICAL HISTORY. 299
that in watching the geological operations of the present
day he is brought face to face with the same instruments
whereby the very framework of the continents has been
piled up and sculptured into the present outlines of moun-
tain, valley, and plain.
The highest aim of the geologist is to trace the history
of the earth. All his researches, remote though they may
seem from this aim, are linked together in the one great
task of unravelling the successive mutations through which
each area of the earth's surface has passed, and of discover-
ing what successive races of plants and animals have
appeared upon the globe. The investigation of facts and
processes, to which the previous pages have been devoted,
must accordingly be regarded as in one sense introductory
to the highest branch of geological inquiry. We have now
to apply the methods and principles already discussed to
the elucidation of the history of our planet and its in-
habitants. Within the limits of this volume only a mere
outline of what has been ascertained regarding this history
can be given. I shall arrange in chronological order the
main phases through which the globe seems to have passed,
and present such a general summary of the more important
facts regarding each of them as may, I hope, convey an
adequate outline of what is at present known regarding the
successive periods of geological history.
As the primitive stages of mankind upon the earth and
the early progress of every race fade into the obscurities of
mythology and archaeology, so the story of the primeval
condition of our globe is lost in the dim light of remote ages,
regarding which almost all that is known or can be surmised
is furnished by the calculations and speculations of the astron-
omer. If the earth's history could only be traced out from
300 EARLIEST CONDITIONS OF THE GLOBE. [CHAP.
evidence supplied by the planet itself, it could be followed
no further back than the oldest portions of the earth now
accessible to us. Yet there can be no doubt that the planet
must have had a long history before the appearance of any
of the solid portions now to be seen. That such was the
case is made almost certain by the traces of a gradual evolu-
tion or development which astronomers have been led to
recognise among the heavenly bodies. Our earth being
only one of a number of planets revolving round the sun,
the earliest stages of its separate existence must be studied
in reference to the whole planetary system of which it forms
a part. Thus, in compiling the earliest chapter of the history
of the earth, the geologist turns for evidence to the researches
of the astronomer among stars and nebulae.
In recent years, more precise methods of inquiry, and,
in particular, the application of the spectroscope to the study
of the stars, have gone far to confirm the speculation known
as the Nebular Hypothesis. According to this view, the
orderly related series of heavenly bodies, which we call the
Solar System, existed at one time, enormously remote from
the present, as a Nebula that is, a cloudy mass of matter,
like one of those nebulous, faintly luminous clouds which
can be seen in the heavens. This nebula probably extended
at least as far as the outermost planetary member of the
system is now removed from the sun. It may have .con-
sisted entirely of incandescent gases or vapours, or of clouds
of stones in rapid movement, like the stones that from time
to time fall through our atmosphere as meteorites, and reach
the surface of the earth. The collision of these stones mov-
ing with planetary velocity would dissipate them into vapour,
as is perhaps the case in the faint luminous tails of comets.
At all events, the materials of the nebula began to condense,
xvi.] NEBULAR HYPOTHESIS. 301
and in so doing, threw off, or left behind, successive rings
(like those around the planet Saturn), which, in obedience to
the rotation of the parent nebula, began to rotate in one
general plane around the gradually shrinking nucleus. As
the process of condensation proceeded, these rings broke up,
and their fragments rushed together with such force as not
improbably to generate heat enough to dissipate them again
into vapour. They eventually condensed into planets,
sometimes with a further formation of rings, or with a dis-
ruption of these secondary rings, and the consequent forma-
tion of moons or satellites round the planets. The outer
planets would thus be the oldest, and, on the whole, the
coolest and least dense. Towards the centre of the nebula
the heaviest elements might be expected to condense, and
there the high temperature would longest continue. The
sun is the remaining intensely hot nucleus of the original
nebula, from which heat is still radiated to the furthest part
of the system.
When a planetary ring broke up, and by the heat thereby
generated was probably reduced to the state of vapour, its
materials, as they cooled, would tend to arrange themselves
in accordance with their respective densities, the heaviest in
the centre, and the lightest outside. In process of time, as
cooling and contraction advanced, the outer layers might
grow. quite cold, while the inner nucleus of the planet might
still be intensely hot. Such, in brief, is the well-known Nebu-
lar Hypothesis.
Now the present condition of our earth is very much
what, according to this hypothesis or theory, it might be
expected to be. On the outside comes the lightest layer or
shell in the form of an Atmosphere, consisting of gases and
vapours. Below this gaseous envelope which entirely sur-
302 EARLIEST CONDITIONS OF THE GLOBE. [CHAP.
rounds the globe lies an inner envelope of water, the ocean,
which covers about two -thirds of the earth's surface, and is
likewise composed of gases. Underneath this watery cover-
ing, and rising above it in dry land, rests the solid part of
the globe, which, so far as accessible to us, is composed of
rocks twice to thrice the weight of pure water. But obser-
vations with the pendulum at various heights above the sea
show that the attraction of the earth as a whole indicates
that the globe probably has a density about five and a half
times that of water. Hence we may infer that its inner
nucleus not improbably consists of heavy materials, and may
be metallic. There is thus evidence of an arrangement of
the planet's materials in successive spherical shells, the
lightest or least dense being on the outside, and the heaviest
or densest in the centre.
Again, the outside of the earth is now quite cool ; but
abundant proof exists that at no great distance below the
surface the temperature is high. Volcanoes, hot springs,
and artificial borings all over the world testify to the abun-
dant store of heat within the earth. Probably at a depth of
not more than 20 miles from the surface the temperature
is as high as the melting-point of any ordinary rock at the
surface. By far the largest part of the planet, therefore,
is hotter than molten iron. We need have no hesitation
in admitting it to be highly probable that the earth was
originally in the state of incandescent vapour, and that it
has ever since that time been cooling and contracting. Its
present shape affords strong presumption in favour of the
opinion that the globe was once in a plastic condition. The
flattening at the poles and bulging at the equator, or what
is called the oblately spheroidal figure of the planet, is just
the shape which a plastic mass would have assumed in
xvi.] PRIMEVAL SEA AND ATMOSPHERE. 303
obedience to the influence of the movement of rotation,
imparted to it when detached from the parent nebula.
At present a complete rotation is performed by the earth
in twenty-four hours. But calculations have been made
with the result of showing that originally the rate of rotation
was much greater. Fifty-seven millions of years ago it was
about four times faster, the length of the day being only six
and three-quarter hours. The moon at that time was only
about 35,000 miles distant from the earth, instead of 239,000
miles as at present. Since these early times the rate of rota-
tion has gradually been diminishing, and the figure of the
earth has been slowly tending to become more spherical,
by sinking in the equatorial and rising in the polar regions.
Of the first hard crust that formed upon the surface of
the earth no trace has yet been found. Indeed, there is
reason to suppose that this original crust would break up
and sink into the molten mass beneath, and that not until
after many such formations and submergences did a crust
establish itself of sufficient strength to form a permanent
solid surface. Even though solid, the surface may still
have been at a glowing red-heat, like so much molten iron.
Over this burning nucleus lay the original atmosphere,
consisting not merely of the gases in the present atmo-
sphere, but of the hot vapours which subsequently con-
densed into the ocean, or were absorbed into the crust.
It was a hot, vaporous envelope, under the pressure of which
the first layers of water that condensed from it may have
had the temperature of molten lead. As the steam
passed into water, it would carry down with it the gaseous
chlorides of sodium, magnesium, and other vapours in the
original atmosphere, so that the first ocean was probably
not only hot, but intensely saline.
304 EARLIEST CONDITIONS OF THE GLOBE. [CHAP.
Regarding these early ages in the earth's history we can
only surmise, for no direct record of them has been pre-
served. They are sometimes spoken of as pre-geological ;
but geology really embraces the whole history of the planet,
no matter from what sources the evidence may be obtained.
Deposits from this original hot saline ocean have been
supposed to be recognisable in the very oldest crystalline
schists ; but for this supposition there does not appear to
be any good ground. The early history of our planet like
that of man himself is lost in the dimness of antiquity, and
we can only speculate about it on more or less plausible
suppositions.
When we come to the solid framework of the earth we
stand on firmer footing in the investigation of geological
history. The terrestrial crust, or that portion of the globe
which is accessible to human observation, has been found
to consist of successive layers of rock, which, though far
from constant in their occurrence, and though often broken
and crumpled by subsequent disturbance, have been recog-
nised over a large part of the globe. They contain the
earth's own chronicle of its history, which has already been
referred to as the Geological Record, and the subdivision
of which into larger and minor sections, according mainly
to the evidence of fossils, was explained in the preceding
chapter.
Had the successive layers of rock that constitute the
Geological Record remained in their original positions,
only the uppermost, and therefore most recent of them
would have been visible, and nothing more could have
been learnt regarding the underlying layers, except in so far
as it might have been possible to explore them by boring
into them. But the deepest mines do not reach greater
xvi.] DIVISIONS OF GEOLOGICAL RECORD. 35
depths than between 3000 and 4000 feet from the surface.
Owing, however, to the way in which the crust of the earth
has been plicated and fractured, portions of the bottom
layers have been pushed up to the surface, and those that
lay above them have been thrown into vertical or inclined
positions, so that we can walk over their upturned edges
and examine them, bed by bed. Instead of being restricted
to merely the uppermost few hundred feet of the crust, we
are enabled to examine many thousand feet of its rocks.
The total mean thickness of the accessible fossiliferous
rocks of Europe has been estimated at 75,000 feet or
upwards of 14 miles. This vast depth of rock has been
laid bare to observation by successive disturbances of the
crust.
The main divisions of the Geological Record and, we
may also say, of geological time, are five: (i) Archaean,
embracing the periods of the earliest rocks, wherein no
traces of organic life occur; (2) Palaeozoic (ancient life) or
Primary, including the long succession of ages during which
the earliest types of life existed ; (3) Mesozoic (middle life)
or Secondary, comprising a series of periods when more
advanced types of life flourished ; (4) Cainozoic (recent life)
or Tertiary, embracing the ages when the existing types of
life appeared ; but excluding man ; and (5) Quaternary or
Post-tertiary and Recent, including the time since man ap-
peared upon the earth.
Each of these main sections is further subdivided into
systems or periods, and each system into formations as
already explained. Arranged in their order of sequence,
the various divisions of the Geological Record may be
placed as in the accompanying Table,
306 EARLIEST CONDITIONS OF THE GLOBE. [CHAP.
THE GEOLOGICAL RECORD,
or, Order of Succession of the Stratified Formations of the Earth's Crust.
rt _
Recent and Prehistoric.
Pleistocene or Glacial.
Pliocene.
Miocene.
Oligocene.
Eocene.
o
Cretaceous.
Danian.
Senonian.
Turonian.
Cenomanian.
Gault.
Neocomian.
Jurassic.
Purbeckian.
Portlandian.
Kimmeridgian.
Corallian.
Oxfordian.
Bathonian.
Bajocian.
Liassic.
Triassic.
Rhaetic.
Keuper or Upper Trias.
Muschelkalk.
Bunter or Lower Trias.
2
Permian.
Upper Red Sandstones, clays, and gypsum.
Magnesian Limestone (Zechstein).
Marl vSlate (Kupferschiefer).
Lower Red Sandstones, breccias, etc. (Rothliegende).
xvi.] ARCHAEAN PERIODS. 307
Carboniferous.
Coal Measures.
Millstone Grit.
Carboniferous Limestone series.
Devonian and Old Red Sandstone.
n . ( Upper Cypridina and Goniatite beds.
^ -I Middle Stringocephalus (Eifel) Limestone.
I Lower Spirifer Sandstone, etc.
Old r d f Upper Yellow an( l Rec l Sandstones, with Holop-
o if) tychius, Pterichthys major, etc.
T | Lower Sandstones,flagstones, and conglomerates,
! with Cephalaspis, Coccosteus, Astcrohpis, etc.
Silurian.
IH / Ludlow group.
od
Wenlock group.
V Upper Llandovery group.
./
Cavadoc and Bala group.
I Lower Llandovery group.
9 | Llandeilo group.
1 Arenig group.
'rt . ( Upper Tremadoc Slates.
1 -| I Lingula Flags.
S j Lower Menevian group.
S M ! Harlech and Longmynd group.
o v
ArcliL.janTPre-Cambrian).
THE ARCHAEAN PERIODS.
Owing to the revolutions which the crust of the earth
has undergone, there have been pushed up to the surface,
from underneath the oldest fossiliferous strata, certain very
ancient crystalline rocks which form what is termed the
Archaean system. As already mentioned, these rocks
have by some geologists been supposed to be a part of the
primeval crust of the planet, which solidified from fusion.
3S ARCHAEAN. [CHAP.
By others they have been thought to have been formed in
the boiling ocean, which first condensed upon the still hot
surface of the globe. In truth, we are still profoundly
ignorant as to the conditions under which they arose. We
have hardly any means of ascertaining in what order they
were formed. We know no method of determining
whether those of one region belong to the same period as
those of another. Nor can we always be sure that what
have been called Archaean rocks may not belong to a much
later part of Geological Record, their peculiar crystalline
structure having been superinduced upon them by some of
those subterranean movements described in chapter xiii.
Of Archaean rocks the most abundant is gneiss, passing
on the one hand into granite, and on the other into
micaceous and argillaceous schists, with interstratified
bands of various hornblendic, pyroxenic, and garnetiferous
rocks, limestone, dolomite, serpentine, quartzite, graphite,
haematite, magnetite, etc. These various materials are
more or less distinctly bedded. But the beds are for the
most part inconstant, swelling out into thick zones, and
then rapidly diminishing and dying out. This bedding
somewhat resembles that of sedimentary rocks, and the
manner in which the limestone and graphite occur, recalls
the way in which limestone and coal are found in the
fossiliferous formations. The inference has accordingly
been drawn that the Archaean crystalline bands were really
deposited as chemical precipitates or mechanical sediments
on the floor of the primeval ocean, and have since been
more or less crystallised and disturbed. But from what has
been brought forward in chapter xiii., regarding the totally-
new structures which have been developed in rocks by
subterranean movement, it is evident that a bedded arrange-
xvi.] GNEISSES AND SCHISTS. 309
ment and a crystalline texture, like those of the Archaean
gneisses and schists, have sometimes been induced in rocks
by excessive crumpling, fracture, and shearing. How far,
therefore, the apparent bedding of Archaean rocks is their
original condition, or is the result of subsequent disturbance,
is a question that cannot yet be answered.
The alternations of gneiss and other crystalline masses
form bands which are usually placed on end or at high angles,
and are often intensely crumpled and puckered, having evi-
dently undergone enormous crushing (Fig. 114). Attempts
FIG. 114. Fragment of crumpled Schist.
have been made to subdivide them into groups or series, ac-
cording to their apparent order of succession and lithological
characters. But such subdivisions, even where practicable,
are probably only of local value. As a rule, those members
of the system which, if the succession of beds may be trusted,
are the lowest and oldest, present coarser crystalline characters
than those which seem to be higher and later. They often con-
sist of massive granitic gneiss, with abundant veins and bands
of the coarsely crystalline variety of granite, known as peg-
matite. The apparently higher rocks are less coarsely crystal-
line gneiss, and often mica-schists and other schistose masses.
310 ARCHAEAN. [CHAP.
No unquestionable relic of organic existence has been
met with among Archaean rocks. Some of the Archsean
limestones of Canada have yielded a peculiar mixture of
serpentine and calcite, with a structure which is regarded
by some able naturalists as that of a reef-building foraminifer.
It occurs in masses, and is supposed by these writers to
have grown in large, thick sheets or reefs over the sea-
bottom. By other observers, however, this supposed
organism (to which the name of Eozoon has been given) is
regarded as merely a mineral segregation, and various un-
doubted mineral structures are pointed to in illustration
and confirmation of this view. The rocks in which Eozoon
occurs have been so greatly mineralised by the processes of
metamorphism, that any original organic structure in them
could hardly be expected to have escaped destruction.
Though the structure in Eozoon is in some respects peculiar,
it nevertheless so much resembles some recognised mineral
arrangements, that its claim to be regarded as an organism
has not been satisfactorily established.
Archsean rocks cover a large area in Europe. In the
British Islands, they are principally developed among the
Hebrides and along the north-west coasts of the Scottish
Highlands, where they give rise to a singular type of
scenery. Over much of that region they form hummocky
bosses of naked rock, with tarns and peat-bogs lying in the
hollows, seldom rising into mountains, but forming the
platform which supports a singular group of red sandstone
mountains. Here and there, they mount up into solitary
hills or groups of hills. The highest point they reach on
the mainland is at Stack, near Loch Laxford, which is 2364
feet above the sea. But in the Island of Harris they sweep
upwards into rugged mountainous ground, of which the
xvi.] LAURENTIAN, HURONIAN. 311
highest summits rise more than 2600 feet out of the Atlantic,
and are visible far and wide as a notable landmark.
On the continent of Europe, Archaean rocks have their
greatest extension in Scandinavia, where they evidently be-
long to the same ancient land as that of which the Hebrides
and Scottish Highlands are fragments. They range through
Finland far into Russia, appearing in the centre of the chain
of the Ural Mountains. They form likewise the nucleus
of the Carpathians and the Alps, and appear in detached
areas in Bavaria, Bohemia, France, and the Pyrenees. They
are estimated to occupy an area of more than 2,000,000 of
square miles in the more northerly part of North America,
stretching from the Arctic regions southwards to the great
lakes. In this vast region they have been subdivided into
an older series, termed Laurentian, and a younger series,
called Huronian. It thus appears that both in the Old
and New World, the Archaean rocks are chiefly exposed in
the northern tracts of the continents. The areas which
they there overspread were probably land at a very early
geological period, and it was mainly from the waste of this
land that the original materials were derived, out of which
the enormous masses of stratified rocks were formed.
In the southern hemisphere, also, ancient gneisses and
other schists, referred to the Archaean system, rise from
under the oldest fossiliferous formations. In Australia and
in New Zealand they cover large tracts of country, and
appear in the heart of the mountain ranges. It thus
appears that all over the world the oldest known rocks are
gneisses and similar or allied crystalline masses, having a
remarkable uniformity of character.
CHAPTER XVII.
THE PAL/EOZOIC PERIODS SILURIAN.
THE portion of geological history which treats of those ages
in which the earliest known types of plants and animals
lived is termed Palaeozoic. Of the first appearance of
organic life upon our planet we know nothing. Whether
plants or animals came first, and in 
