The World Before the Deluge
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Title: The World Before the Deluge
Author: Louis Figuier
Editor: Henry W. Bristow
Release date: May 18, 2012 [eBook #39723]
Language: English
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[Illustration: THE FIRST MAN.]
THE
WORLD BEFORE THE DELUGE.
BY
_LOUIS FIGUIER_.
NEWLY EDITED AND REVISED
BY
H. W. BRISTOW, F.R.S., F.G.S.,
_Of the Geological Survey of Great Britain; Hon. Fellow of King’s
College, London._
With 235 Illustrations.
CASSELL, PETTER, & GALPIN,
LONDON, PARIS, AND NEW YORK.
CONTENTS.
PAGE
GENERAL CONSIDERATIONS 1
CONSIDERATION OF FOSSILS 4
CHEMICAL AND NEBULAR HYPOTHESES OF THE GLOBE 15
MODIFICATIONS OF THE EARTH’S SURFACE 26
ERUPTIVE ROCKS 30
PLUTONIC ERUPTIONS 31
Granite 31
Syenite 34
Protogine 35
Porphyry 37
Serpentine 38
VOLCANIC ROCKS 39
Trachytic Formations 39
Basaltic Formations 44
Volcanic or Lava Formations 51
METAMORPHIC ROCKS 71
General Metamorphism 74
THE BEGINNING 80
PRIMARY EPOCH 99
CAMBRIAN PERIOD 101
SILURIAN PERIOD 102
Lower Silurian Period 104
Upper Silurian Period 110
OLD RED SANDSTONE AND DEVONIAN PERIOD 119
CARBONIFEROUS PERIOD 130
Carboniferous Limestone 140
Coal Measures 150
Formation of Coal 159
PERMIAN PERIOD 170
Permian Rocks 177
SECONDARY EPOCH 185
TRIASSIC, OR NEW RED PERIOD 185
New Red Sandstone 187
Muschelkalk 188
Keuper Period 199
RHÆTIC (PENARTH) PERIOD 207
JURASSIC PERIOD 211
Liassic Period 211
Oolitic Sub-Period 243
Lower Oolite Fauna 244
---- ---- Rocks 249
Middle Oolite 255
Upper Oolite 265
CRETACEOUS PERIOD 275
Lower Cretaceous Period 286
Upper Cretaceous Period 300
TERTIARY EPOCH 312
Eocene Period 315
Miocene Period 336
Pliocene Period 357
QUATERNARY EPOCH 378
POST-PLIOCENE 378
EUROPEAN DELUGES 422
GLACIAL PERIOD 435
CREATION OF MAN 464
ASIATIC DELUGE 480
EPILOGUE 489
TABLE AND DIAGRAM OF BRITISH SEDIMENTARY AND FOSSILIFEROUS
STRATA 493
FULL-PAGE ILLUSTRATIONS.
FRONTISPIECE--THE FIRST MAN.
PAGE
I. De Sancy Peak, Mont Dore 42
II. Basaltic Mountain of La Coupe d’Ayzac 46
III. Extinct Volcanoes of Le Puy 52
IV. Mud Volcano of Turbaco 62
V. Great Geyser of Iceland 66
VI. The Earth in a gaseous state circulating in space 82
VII. Condensation and rainfall 94
VIII. Ideal Landscape of the Silurian Period 104
IX. Ideal Landscape of the Devonian Period 121
X. Ideal view of marine life in the Carboniferous
Period 147
XI. Ideal view of a marshy forest in the Coal Period 156
XII. Ideal Landscape of the Permian Period 172
XIII. Ideal Landscape of the Muschelkalk Period 191
XIV. Ideal Landscape of the Saliferous or Keuper Period 198
XV. Ideal Scene of the Lias Period with Ichthyosaurus
and Plesiosaurus 231
XVI. Ideal Landscape of the Liassic Period 241
XVII. Ideal Landscape of the Lower Oolite Period 254
XVIII. Ideal Landscape of the Middle Oolite Period 258
XIX. Apiocrinites rotundus and Encrinus liliiformis 261
XX. Ideal Landscape of the Upper Oolite Period 267
XXI. Ideal Scene of the Lower Cretaceous Period 296
XXII. Ideal Landscape of the Cretaceous Period 307
XXIII. Ideal Landscape of the Eocene Period 328
XXIV. Ideal Landscape of the Miocene Period 352
XXV. Ideal Landscape of the Pliocene Period 375
XXVI. Skeleton of the Mammoth in the St. Petersburg
Museum 394
XXVII. Skeleton of Megatherium 403
XXVIII. Ideal View of the Quaternary Epoch--Europe 416
XXIX. Ideal Landscape of the Quaternary Epoch--America 419
XXX. Deluge of the North of Europe 425
XXXI. Glaciers of Switzerland 445
XXXII. Appearance of Man 468
XXXIII. Asiatic Deluge 483
DIAGRAM AT END--Ideal Section of the Earth’s Crust, showing the
order of superposition or chronological succession of
the principal groups of strata.
PREFACE.
The object of “The World before the Deluge” is to trace the progressive
steps by which the earth has reached its present state, from that
condition of chaos when it “was without form and void, and darkness was
upon the face of the deep,” and to describe the various convulsions and
transformations through which it has successively passed. In the words
of the poet--
“Where rolls the deep, there grew the tree;
O Earth, what changes hast thou seen!
There, where the long street roars, hath been
The silence of the central sea.”
It has been thought desirable that the present edition of the work
should undergo a thorough revision by a practical geologist, a task
which Mr. H. W. Bristow has performed. Mr. Bristow has however confined
himself to such alterations as were necessary to secure accuracy in the
statement of facts, and such additions as were necessary to represent
more precisely the existing state of scientific opinion. Many points
which are more or less inferential and therefore matters of individual
opinion, and especially those on which M. Figuier bases his
speculations, have been left in their original form, in preference to
making modifications which would wholly change the character of the
book. In a work whose purpose is to give the general reader a summarised
account of the results at which science has arrived, and of the method
of reasoning regarding the facts on which these generalisations rest, it
would be out of place, as well as ineffective, to obscure general
statements with those limitations which caution imposes on the
scientific investigator.
In the original work the Author had naturally enough drawn most of his
facts from French localities; in the translation these are mostly
preserved, but others drawn from British Geology have been added, either
from the translator’s own knowledge, or from the works of well-known
British writers. It was considered desirable, for similar reasons, to
enlarge upon the opinions of British geologists, to whom the French work
scarcely does justice, considering the extent to which the science is
indebted to them for its elucidation.
In the original work the chapter on Eruptive Rocks comes at the end of
the work, but, as the work proceeded, so many unexplained allusions to
that chapter were found that it seemed more logical, and more in
accordance with chronological order, if the expression may be used, to
place that chapter at the beginning.
A new edition of the French work having appeared in the early part of
1866, to which the Author contributed a chapter on Metamorphic Rocks, a
translation of it is appended to the chapter on Eruptive Rocks.
A chapter on the Rhætic (or Penarth) beds has been inserted (amongst
much other original matter), the stratigraphical importance of that
series having been recognised since the publication of the First
Edition.
In the present Edition the text has been again thoroughly revised by Mr.
Bristow, and many important additions made, the result of the recent
investigations of himself and his colleagues of the Geological Survey.
THE
WORLD BEFORE THE DELUGE.
GENERAL CONSIDERATIONS.
The observer who glances over a rich and fertile plain, watered by
rivers and streams which have, during a long series of ages, pursued the
same uniform and tranquil course; the traveller who contemplates the
walls and monuments of a great city, the first founding of which is lost
in the night of ages, testifying, apparently, to the unchangeableness of
things and places; the naturalist who examines a mountain or other
locality, and finds the hills and valleys and other accidents of the
soil in the very spot and condition in which they are described by
history and tradition--none of these observers would at first suspect
that any serious change had ever occurred to disturb the surface of the
globe. Nevertheless, the earth has not always presented the calm aspect
of stability which it now exhibits; it has had its convulsions, and its
physical revolutions, whose story we are about to trace. The earth, like
the body of an animal, is wasted, as the philosophical Hutton tells us,
at the same time that it is repaired. It has a state of growth and
augmentation; it has another state, which is that of diminution and
decay: it is destroyed in one part to be renewed in another; and the
operations by which the renewal is accomplished are as evident to the
scientific eye as those by which it is destroyed. A thousand causes,
aqueous, igneous, and atmospheric, are continually at work modifying the
external form of the earth, wearing down the older portions of its
surface, and reconstructing newer out of the older; so that in many
parts of the world denudation has taken place to the extent of many
thousand feet. Buried in the depths of the soil, for example, in one of
those vast excavations which the intrepidity of the miner has dug in
search of coal or other minerals, there are numerous phenomena which
strike the mind of the inquirer, and carry their own conclusions with
them. A striking increase of temperature in these subterranean places
is one of the most remarkable of these. It is found that the temperature
of the earth rises one degree for every sixty or seventy feet of descent
from its surface. Again: if the mine be examined vertically, it is found
to consist of a series of layers or beds, sometimes horizontal, but more
frequently inclined, upright, or contorted and undulating--even folded
back upon themselves. Then, instances are numerous where horizontal and
parallel beds have been penetrated, and traversed vertically or
obliquely by veins of ores or minerals totally different in their
appearance and nature from the surrounding rocks. All these undulations
and varying inclinations of strata are indications that some powerful
cause, some violent mechanical action, has intervened to produce them.
Finally, if the interior of the beds be examined more minutely--if,
armed with the miner’s pick and hammer, the rock is carefully broken
up--it is not impossible that the very first efforts at mining may be
rewarded by the discovery of some fossilised organic form no longer
found in the living state. The remains of plants and animals belonging
to the earlier ages of the world, are, in fact, very common; entire
strata are sometimes formed of them; and in some localities the rocks
can scarcely be disturbed without yielding fragments of bones and
shells, or the impressions of fossilised animals and vegetables--the
buried remains of extinct creations.
These bones--these remains of animals or vegetables which the hammer of
the geologist has torn from the rock--belong possibly to some organism
which no longer any where exists: it may not be identical with any
animal or plant living in our times: but it is evident that these
beings, whose remains are now so deeply buried, have not always been so
covered; they once lived on the surface of the earth as plants and
animals do in our days, for their organisation is essentially the same.
The beds in which they now repose must, then, in older times have formed
the surface of the earth; and the presence of these fossils proves that
the earth has suffered great mutations at some former period of its
history.
Geology explains to us the various transformations which the earth has
passed through before it arrived at its present condition. We can
determine, with its help, the comparative epoch to which any beds
belong, as well as the order in which others have been superimposed upon
them. Considering that the stratigraphical crust of the earth with which
the geologist has to deal may be some ten miles thick, and that it has
been deposited in distinct layers in a definite order of succession, the
dates or epochs of each formation may well be approached with hesitation
and caution.
Dr. Hutton, the earliest of our philosophical geologists, eloquently
observes, in his “Theory of the Earth,” that the solid earth is
everywhere wasted at the surface. The summits of the mountains are
necessarily degraded. The solid and weighty materials of these mountains
have everywhere been carried through the valleys by the force of running
water. The soil which is produced in the destruction of the solid earth
is gradually transported by the moving waters, and is as constantly
supplying vegetation with its necessary aid. This drifted soil is at
last deposited upon some coast, where it forms a fertile country. But
the billows of the ocean again agitate the loose material upon the
shore, wearing away the coast with endless repetitions of this act of
power and imparted force; the solid portion of our earth, thus sapped to
its foundations, is carried away into the deep and sunk again at the
bottom of the sea whence it had originated, and from which sooner or
later it will again make its appearance. We are thus led to see a
circulation of destruction and renewal in the matter of which the globe
is formed, and a system of beautiful economy in the works of Nature.
Again, discriminating between the ordinary and scientific observer, the
same writer remarks, that it is not given to common observation to see
the operation of physical causes. The shepherd thinks the mountain on
which he feeds his flock has always been there. The inhabitant of the
valley cultivates the soil as his fathers did before him, and thinks the
soil coeval with the valley or the mountain. But the scientific observer
looks into the chain of physical events, sees the great changes that
have been made, and foresees others that must follow from the continued
operation of like natural causes. For, as Pythagoras taught 2,350 years
ago, “the minerals and the rocks, the islands and the continents, the
rivers and the seas, and all organic Nature, are perpetually changing;
there is nothing stationary on earth.” To note these changes--to
decipher the records of this system of waste and reconstruction, to
trace the physical history of the earth--is the province of GEOLOGY,
which, the latest of all modern sciences, is that which has been
modified most profoundly and most rapidly. In short, resting as it does
on observation, it has been modified and transformed according to every
series of facts recorded; but while many of the facts of geology admit
of easy and obvious demonstration, it is far otherwise with the
inferences which have been based upon them, which are mostly
hypothetical, and in many instances from their very nature incapable of
proof. Its applications are numerous and varied, projecting new and
useful lights upon many other sciences. Here we ask of it the teachings
which serve to explain the origin of the globe--the evidence it
furnishes of the progressive formation of the different rocks and
mineral masses of which the earth is composed--the description and
restoration of the several species of animals and vegetables which have
existed, have died and become extinct, and which form, in the language
of naturalists, the _Fauna_ and _Flora_ of the ancient world.
* * * * *
In order to explain the origin of the earth, and the cause of its
various revolutions, modern geologists invoke three orders of facts, or
fundamental considerations:
I. The hypothesis of the original incandescence of the globe.
II. The consideration of fossils.
III. The successive deposition of the sedimentary rocks.
As a corollary to these, the hypothesis of the upheaval of the earth’s
crust follows--upheavals having produced local revolutions. The result
of these upheavals has been to superimpose new materials upon the older
rocks, introducing extraneous rocks called _Eruptive_, beneath, upon,
and amongst preceding deposits, in such a manner as to change their
nature in divers ways. Whence is derived a third class of rocks called
_Metamorphic_ or altered _rocks_, our knowledge of which is of
comparatively recent date.
FOSSILS.
The name of _Fossil_ (from _fossilis_, dug up) is given to all organised
bodies, animal or vegetable, buried naturally in the terrestrial strata,
and more or less petrified, that is, converted into stone. Fossils of
the older formations are remains of organisms which, so far as species
is concerned, are quite extinct; and only those of recent formations
belong to genera living in our days. These fossil remains have neither
the beauty nor the elegance of most living species, being mutilated,
discoloured, and often almost shapeless; they are, therefore,
interesting only in the eyes of the observer who would interrogate them,
and who seeks to reconstruct, with their assistance, the Fauna and Flora
of past ages. Nevertheless, the light they throw upon the past history
of the earth is of the most satisfactory description, and the science of
fossils, or palæontology, is now an important branch of geological
inquiry. Fossil shells, in the more recent deposits, are found scarcely
altered; in some cases only an impression of the external form is
left--sometimes an entire cast of the shell, exterior and interior. In
other cases the shell has left a perfect impression of its form in the
surrounding mud, and has then been dissolved and washed away, leaving
only its mould. This mould, again, has sometimes been filled up by
calcareous spar, silica, or pyrites, and an exact cast of the original
shell has thus been obtained. Petrified wood is also of very common
occurrence.
These remains of an earlier creation had long been known to the curious,
and classed as _freaks of Nature_, for so we find them described in the
works of the ancient philosophers who wrote on natural history, and in
the few treatises on the subject which the Middle Ages have bequeathed
to us. Fossil bones, especially those of elephants, were known to the
ancients, giving rise to all sorts of legends and fabulous histories:
the tradition which attributed to Achilles, to Ajax, and to other heroes
of the Trojan war, a height of twenty feet, is attributable, no doubt,
to the discovery of the bones of elephants near their tombs. In the time
of Pericles we are assured that in the tomb of Ajax a _patella_, or
knee-bone of that hero, was found, which was as large as a dinner-plate.
This was probably only the patella of a fossil elephant.
The uses to which fossils are applied by the geologist are--First, to
ascertain the relative age of the formations in which they occur;
secondly, the conditions under which these were deposited. The age of
the formation is determined by a comparison of the fossils it contains
with others of ascertained date; the conditions under which the rocks
were deposited, whether marine, lacustrine, or terrestrial, are readily
inferred from the nature of the fossils. The great artist, Leonardo da
Vinci, was the first to comprehend the real meaning of fossils, and
Bernard Palissy had the glory of being the first modern writer to
proclaim the true character of the fossilised remains which are met
with, in such numbers, in certain formations, both in France and Italy,
particularly in those of Touraine, where they had come more especially
under his notice. In his work on “Waters and Fountains,” published in
1580, he maintains that the _figured stones_, as fossils were then
called, were the remains of organised beings preserved at the bottom of
the sea. But the existence of marine shells upon the summits of
mountains had already arrested the attention of ancient authors. Witness
Ovid, who in Book XV. of the “Metamorphoses” tells us he had seen land
formed at the expense of the sea, and marine shells lying dead far from
the ocean; and more than that, an ancient anchor had been found on the
very summit of a mountain.
“Vidi factas ex æquore terras,
Et procul a pelago conchæ jacuere marinæ,
Et vetus inventa est in montibus anchora summis.”
Ov., _Met._, Book xv.
The Danish geologist Steno, who published his principal works in Italy
about the middle of the seventeenth century, had deeply studied the
fossil shells discovered in that country. The Italian painter Scilla
produced in 1670 a Latin treatise on the fossils of Calabria, in which
he established the organic nature of fossil shells.
The eighteenth century gave birth to two very opposite theories as to
the origin of our globe--namely, the _Plutonian_ or igneous, and the
_Neptunian_ or aqueous theory. The Italian geologists gave a marked
impulse to the study of fossils, and the name of Vallisneri[1] may be
cited as the author to whom science is indebted for the earliest account
of the marine deposits of Italy, and of the most characteristic organic
remains which they contain. Lazzaro Moro[2] continued the studies of
Vallisneri, and the monk Gemerelli reduced to a complete system the
ideas of these two geologists, endeavouring to explain all the phenomena
as Vallisneri had wished, “without violence, without fiction, without
miracles.” Marselli and Donati both studied in a very scientific manner
the fossil shells of Italy, and in particular those of the Adriatic,
recognising the fact that they affected in their beds a regular and
constant order of superposition.[3]
[1] Dei corpi marini, &c., 1721.
[2] Sui crostaccei ed altri corpi marini che sè trovano sui monti,
1740.
[3] Consult Lyell’s “Principles of Geology” and the sixth edition of
the “Elements,” with much new matter, for further information
relative to the study of fossils during the last two centuries.
In France the celebrated Buffon gave, by his eloquent writings, great
popularity to the notions of the Italian naturalists concerning the
origin of fossil remains. In his admirable “Époques de la Nature” he
sought to prove that the shells found in great quantities buried in the
soil, and even on the tops of mountains, belonged, in reality, to
species not living in our days. But this idea was too novel not to find
objectors: it counted among its adversaries the bold philosopher who
might have been expected to adopt it with most ardour. Voltaire
attacked, with his jesting and biting criticism, the doctrines of the
illustrious innovator. Buffon insisted, reasonably enough, that the
presence of shells on the summit of the Alps was a proof that the sea
had at one time occupied that position. But Voltaire asserted that the
shells found on the Alps and Apennines had been thrown there by pilgrims
returning from Rome. Buffon might have replied to his opponent, by
pointing out whole mountains formed by the accumulation of these shells.
He might have sent him to the Pyrenees, where shells of marine origin
cover immense areas to a height of 6,600 feet above the present
sea-level. But his genius was averse to controversy; and the philosopher
of Ferney himself put an end to a discussion in which, perhaps, he would
not have had the best of the argument. “I have no wish,” he wrote, “to
embroil myself with Monsieur Buffon about shells.”
It was reserved for the genius of George Cuvier to draw from the study
of fossils the most wonderful results: it is the study of these remains,
in short, which, in conjunction with mineralogy, constitutes in these
days positive geology. “It is to fossils,” says the great Cuvier, “that
we owe the discovery of the true theory of the earth; without them we
should not have dreamed, perhaps, that the globe was formed at
successive epochs, and by a series of different operations. They alone,
in short, tell us with certainty that the globe has not always had the
same envelope; we cannot resist the conviction that they must have lived
on the surface of the earth before being buried in its depths. It is
only by analogy that we have extended to the primary formations the
direct conclusions which fossils furnish us with in respect to the
secondary formations; and if we had only unfossiliferous rocks to
examine, no one could maintain that the earth was not formed all at
once.”[4]
[4] “Ossements Fossiles” (4to), vol. i., p. 29.
The method adopted by Cuvier for the reconstruction and restoration of
the fossil animals found in the plaster-quarries of Montmartre, at the
gates of Paris, has served as a model for all succeeding naturalists;
let us listen, then, to his exposition of the vast problem whose
solution he proposed to himself. “In my work on fossil bones,” he says,
“I propose to ascertain to what animals the osseous fragments belong; it
is seeking to traverse a road on which we have as yet only ventured a
few steps. An antiquary of a new kind, it seemed to me necessary to
learn both to restore these monuments of past revolutions, and to
decipher their meaning. I had to gather and bring together in their
primitive order the fragments of which they are composed; to reconstruct
the ancient beings to which these fragments belonged; to reproduce them
in their proportions and with their characteristics; to compare them,
finally, with others now living on the surface of the globe: an art at
present little known, and which supposes a science scarcely touched upon
as yet, namely, that of the laws which preside over the co-existence of
the forms of the several parts in organised beings. I must, then,
prepare myself for these researches by others, still more extended, upon
existing animals. A general review of actual creation could alone give a
character of demonstration to my account of these ancient inhabitants
of the world; but it ought, at the same time, to give me a great
collection of laws, and of relations not less demonstrable, thus forming
a body of new laws to which the whole animal kingdom could not fail to
find itself subject.”[5]
[5] “Ossements Fossiles” (4to), vol. i., pp. 1, 2.
“When the sight of a few bones inspired me, more than twenty years ago,
with the idea of applying the general laws of comparative anatomy to the
reconstruction and determination of fossil species; when I began to
perceive that these species were not quite perfectly represented by
those of our days, which resembled them the most--I no longer doubted
that I trod upon a soil filled with spoils more extraordinary than any I
had yet seen, and that I was destined to bring to light entire races
unknown to the present world, and which had been buried for incalculable
ages at great depths in the earth.
“I had not yet given any attention to the published notices of these
bones, by naturalists who made no pretension to the recognition of their
species. To M. Vaurin, however, I owe the first intimation of the
existence of these bones, with which the gypsum-quarries swarm. Some
specimens which he brought me one day struck me with astonishment; I
learned, with all the interest the discovery could inspire me with, that
this industrious and zealous collector had already furnished some of
them to other collectors. Received by these amateurs with politeness, I
found in their collections much to confirm my hopes and heighten my
curiosity. From that time I searched in all the quarries with great care
for other bones, offering such rewards to the workmen as might awaken
their attention. I soon got together more than had ever been previously
collected, and after a few years I had nothing to desire in the shape of
materials. But it was otherwise with their arrangement, and with the
reconstruction of the skeleton, which could alone lead to any just idea
of the species.
“From the first moment of discovery I perceived that, in these remains,
the species were numerous. Soon afterwards I saw that they belonged to
many genera, and that the species of the different genera were nearly
the same size, so that size was likely rather to hinder than aid me.
Mine was the case of a man to whom had been given at random the
mutilated and imperfect remains of some hundreds of skeletons belonging
to twenty sorts of animals; it was necessary that each bone should find
itself alongside that to which it ought to be connected: it was almost
like a small resurrection, and I had not at my disposal the
all-powerful trumpet; but I had the immutable laws prescribed to living
beings as my guide; and at the voice of the anatomist each bone and each
part of a bone took its place. I have not expressions with which to
describe the pleasure I experienced in finding that, as soon as I
discovered the character of a bone, all the consequences of the
character, more or less foreseen, developed themselves in succession:
the feet were found conformable to what the teeth announced; the teeth
to that announced by the feet; the bones of the legs, of the thighs, all
those which ought to reunite these two extreme parts, were found to
agree as I expected; in a word, each species was reproduced, so to
speak, from only one of its elements.”[6]
[6] “Ossements Fossiles,” vol. iv. (4to), p. 32.
While the Baron Cuvier was thus zealously prosecuting his inquiries in
France, assisted by many eminent fellow-labourers, what was the state of
geological science in the British Islands? About that same time, Dr.
William Smith, better known as “the father of English geology,” was
preparing, unaided, the first geological map of this country. Dr. Smith
was a native of Wiltshire, and a canal engineer in Somersetshire; his
pursuits, therefore, brought him in the midst of these hieroglyphics of
Nature. It was his practice, when travelling professionally, during many
years to consult masons, miners, wagoners, and agriculturists. He
examined the soil; and in the course of his inquiries he came to the
conclusion that the earth was not all of the same age; that the rocks
were arranged in layers, or strata, superimposed on each other in a
certain definite order, and that the strata, when of the same age, could
be identified by means of their organic remains. In 1794 he formed the
plan of his geological map, showing the superposition of the various
beds; for a quarter of a century did he pursue his self-allotted task,
which was at last completed, and in 1801 was published, being the first
attempt to construct a stratigraphical map.
Taking the men in the order of the objects of their investigation,
rather than in chronological order, brings before us the patient and
sagacious investigator to whom we are indebted for our knowledge of the
Silurian system. For many years a vast assemblage of broken and
contorted beds had been observed on the borders of North Wales,
stretching away to the east as far as Worcestershire, and to the south
into Gloucester, now rising into mountains, now sinking into valleys.
The ablest geologists considered them as a mere labyrinth of ruins,
whose order of succession and distinctive organic remains were entirely
unknown, “But a man came,” as M. Esquiros eloquently writes, “who threw
light upon this sublime confusion of elements.” Sir Roderick Impey
Murchison, then a young President of the Geological Society, had his
attention directed, as he himself informs us, to some of these beds on
the banks of the Wye. After seven years of unremitting labour, he was
rewarded by success. He established the fact that these sedimentary
rocks, penetrated here and there by eruptive masses of igneous origin,
formed a unique system, to which he gave the name of _Silurian_, because
the rocks which he considered the most typical of the whole were most
fully developed, charged with peculiar organic remains, in the land of
the ancient Silures, who so bravely opposed the Roman invaders of their
country. Many investigators have followed in Sir Roderick’s steps, but
few men have so nobly earned the honours and fame with which his name is
associated.
The success which attended Sir R. Murchison’s investigations soon
attracted the attention of other geologists. Professor Sedgwick examined
the older slaty strata, and succeeded in proving the position of the
Cambrian rocks to be at the base of the Silurian. Still it was reserved
for Sir William Logan, the Director of the Canadian Geological Survey,
to establish the fact that immense masses of gneissic formation lay at
the base of the Cambrian; and, by subsequent investigations, Sir
Roderick Murchison satisfied himself that this formation was not
confined to Canada, but was identical with the rocks termed by him
Fundamental Gneiss, which exist in enormous masses on the west coast of
Scotland, and which he proved to be the oldest stratified rocks in the
British Isles. Subsequently he demonstrated the existence of these same
Laurentian rocks in Bohemia and Bavaria, far beneath the Silurian rocks
of Barrande.
While Murchison and Sedgwick were prosecuting their inquiries into the
Silurian rocks, Hugh Miller and many others had their attention occupied
with the Old Red Sandstone--the Devonian of Sedgwick and
Murchison--which immediately overlies them. After a youth passed in
wandering among the woods and rocks of his native Cromarty, the day came
when Miller found himself twenty years of age, and, for the time, a
workman in a quarry. A hard fate he thought it at the time, but to him
it was the road to fame and success in life. The quarry in which he
laboured was at the bottom of a bay formed by the mouth of a river
opening to the south, a clear current of water on one side, as he
vividly described it, and a thick wood on the other. In this silent
spot, in the remote Highlands, a curious fossil fish of the Old Red
Sandstone was revealed to him; its appearance struck him with
astonishment; a fellow-workman named a spot where many such monuments of
a former world were scattered about; he visited the place, and became a
geologist and the historian of the “Old Red.” And what strange fantastic
forms did it afterwards fall to his lot to describe! “The figures on a
China vase or Egyptian obelisk,” he says, “differ less from the real
representation of the objects than the fossil fishes of the ‘Old Red’
differ from the living forms which now swim in our seas.”
The _Carboniferous Limestone_, which underlies the coal, the
_Coal-measures_ themselves, the _New Red Sandstone_, the _Lias_, and the
_Chalk_, have in their turn found their historians; but it would be
foreign to our object to dwell further here on these particular branches
of the subject.
Some few of the fossilised beings referred to resemble species still
found living, but the greater part belong to species which have become
altogether extinct. These fossil remains may constitute natural
families, none of the genera of which have survived. Such is the
_Pterodactyle_ among Pterosaurian reptiles; the _Ammonite_ among
Mollusca; the _Ichthyosaurus_ and the _Plesiosaurus_ among the
Enaliosaurian reptiles. At other times there are only extinct genera,
belonging to families of which there are still some genera now living,
as the genus _Palæoniscus_ among fishes. Finally, in Tertiary deposits,
we meet with some extinct species belonging to genera of our existing
fauna: the _Mammoth_, for example, of the youngest Tertiary deposits, is
an extinct species of the genus elephant.
Some fossils are terrestrial, like the gigantic Irish stag, _Cervus
Megaceros_, the snail or _Helix_; fluviatile or lacustrine, like the
_Planorbis_, the _Lymnæa_, the _Physa_, and the _Unio_; marine, or
inhabiting the sea exclusively, as the Cowry (_Cypræa_), and the Oyster,
(_Ostrea_).
Fossils are sometimes preserved in their natural state, or are but very
slightly changed. Such is the state of some of the bones extracted from
the more recent caves; such, also, is the condition of the insects found
enclosed in the fossil resins in which they have been preserved from
decomposition; and certain shells, found in recent and even in old
formations, such as the Jurassic and Cretaceous strata--in some of which
the shells retain their colours, as well as their brilliant pearly
lustre or nacre. At Trouville, in Normandy, in the Kimeridge strata,
magnificent _Ammonites_ are found in the clay and marl, all brilliant
with the colours of mother-of-pearl. In the Cretaceous beds at
Machéroménil, some species of _Ancyloceras_ and _Hamites_ are found
still covered with a nacre, displaying brilliant reflections of blue,
green, and red, and retaining an admirable lustre. At Glos, near
Liseaux, in the Coral Rag, not only the _Ammonites_, but the _Trigoniæ_
and _Aviculæ_ have preserved all their brilliant nacre. Sometimes these
remains are much changed, the organic matter having entirely
disappeared; it sometimes happens also, though rarely, that they become
petrified, that is to say, the external form is preserved, but the
original organic elements have wholly disappeared, and have been
replaced by foreign mineral substances--generally by silica or by
carbonate of lime.
[Illustration: Fig. 1.--Labyrinthodon pachygnathus and footmarks.]
Geology also enables us to draw very important conclusions from certain
fossil remains whose true nature was long misunderstood, and which,
under the name of _coprolites_, had given rise to much controversial
discussion. Coprolites are the petrified excrements of extinct fossil
animals. The study of these singular remains has thrown unexpected light
on the habits and physiological organisation of some of the great
antediluvian animals. Their examination has revealed the scales and
teeth of fishes, thus enabling us to determine the kind of food in which
the animals of the ancient world indulged: for example, the coprolites
of the great marine reptile which bears the name of _Ichthyosaurus_
contain the bones of other animals, together with the remains of the
vertebræ, or of the phalanges (paddle-bones) of other Ichthyosauri;
showing that this animal habitually fed on the flesh of its own species,
as many fishes, especially the more voracious ones, do in our days.
The imprints left upon mud or sand, which time has hardened and
transformed into sandstone, furnish to the geologist another series of
valuable indications. The reptiles of the ancient world, the turtles in
particular, have left upon the sands, which time has transformed into
blocks of stone, impressions which evidently represent the exact moulds
of the feet of those animals. These impressions have, sometimes, been
sufficient for naturalists to determine to what species the animal
belonged which thus left its impress on the wet ground. Some of these
exhibit tracks to which we shall have occasion to refer; others present
traces of the footprints of the great reptile known as the
_Labyrinthodon_ or _Cheirotherium_, whose footmarks slightly resemble
the impression made by the human hand (Fig. 1). Another well-known
impression, which has been left upon the sandstone of Corncockle Moor,
in Dumfriesshire, is supposed to be the impress of the foot of some
great fossil Turtle.
[Illustration: Fig. 2.--Impressions of rain-drops.]
We may be permitted to offer a short remark on this subject. The
historian and antiquary may traverse the battle-fields of the Greeks and
Romans, and search in vain for traces of those conquerors, whose armies
ravaged the world. Time, which has overthrown the monuments of their
victories, has also effaced the marks of their footsteps; and of the
many millions of men whose invasions have spread desolation throughout
Europe, not even a trace of a footprint is left. Those reptiles, on the
other hand, which crawled thousands of ages ago on the surface of our
planet when it was still in its infancy, have impressed on the soil
indelible proofs of their existence. Hannibal and his legions, the
barbarians and their savage hordes, have passed over the land without
leaving a material mark of their passage; while the poor turtle, which
dragged itself along the silent shores of the primitive seas, has
bequeathed to learned posterity the image and impression of a part of
its body. These imprints may be perceived as distinctly on the rocks, as
the traces left on moist sand or in newly-fallen snow by some animal
walking under our own eyes. What grave reflections should be awakened
within us at the sight of these blocks of hardened earth, which thus
carry back our thoughts to the early ages of the world! and how
insignificant seem the discoveries of the archæologist who throws
himself into ecstacies before some piece of Greek or Etruscan pottery,
when compared with these veritable antiquities of the earth!
The palæontologist (from παλαιος “ancient,” οντος “being,” λογος
“discourse”), who occupies himself with the study of animated beings
which have lived on the earth, takes careful account also of the sort of
moulds left by organised bodies in the fine sediment which has enveloped
them after death. Many organic beings have left no trace of their
existence in Nature, except their impressions, which we find perfectly
preserved in the sandstone and limestone, in marl or clay, and in the
coal-measures; and these moulds are sufficient to tell us the kind to
which the living animals belonged. We shall, no doubt, astonish our
readers when we tell them that there are blocks of sandstone with
distinct impressions of drops of rain which had fallen upon sea-shores
of the ancient world. The impressions of these rain-drops, made upon the
sands, were preserved by desiccation; and these same sands, being
transformed by subsequent hardening into solid and coherent sandstones,
their impressions have been thus preserved to the present day. Fig. 2
represents impressions of this kind upon the sandstone of Connecticut
river in America, which have been reproduced from the block itself by
photography. In a depression of the granitic rocks of Massachusetts and
Connecticut, the red sandstone occupies an area of a hundred and fifty
miles in length from north to south, and from five to ten miles in
breadth. “On some shales of the finest texture,” says Sir Charles Lyell,
“impressions of rain-drops may be seen, and casts of them in the
argillaceous sandstones.” The same impressions occur in the recent red
mud of the Bay of Fundy. In addition to these, the undulations left by
the passage of the waters of the sea, over the sands of the primitive
world, are preserved by the same physical agency. Traces of undulations
of this kind have been found in the neighbourhood of Boulogne-sur-Mer,
and elsewhere. Similar phenomena occur in a still more striking manner
in some sandstone-quarries worked at Chalindrey (Haute-Marne). The
strata there present traces of the same kind over a large area, and
along with them impressions of the excrements of marine worms. One may
almost imagine oneself to be standing on the sea-shore while the tide is
ebbing.
CHEMICAL AND NEBULAR HYPOTHESES OF THE GLOBE.
Among the innumerable hypotheses which human ingenuity has framed to
explain the phenomena which surround the globe, the two which have found
most ready acceptance have been termed respectively the CHEMICAL, and
the NEBULAR or mechanical hypothesis. By the first the solid crust is
supposed to have contained abundance of potassium, sodium, calcium,
magnesium, and other metallic elements. The percolating waters, coming
in contact with these substances, produce combinations resulting in the
conversion of the metals into their oxides--potash, soda, lime, and
magnesia--all of which enter largely into the composition of volcanic
rocks. The second hypothesis involves the idea of an original
incandescent mass of vapour, succeeded by a great and still existing
central fire.
This idea of a great central fire is a very ancient hypothesis: admitted
by Descartes, developed by Leibnitz, and advocated by Buffon, it is
supposed to account for many phenomena otherwise inexplicable; and it is
confirmed by a crowd of facts, and adopted, or at least not opposed, by
the leading authorities of the age. Dr. Buckland makes it the basis of
his Bridgewater treatise. Herschel, Hind, Murchison, Lyell, Phillips,
and other leading English astronomers and geologists give a cautious
adhesion to the doctrine. The following are some of the principal
arguments adduced in support of the hypothesis, for, in the nature of
the proofs it admits of, it can be no more.
When we descend into the interior of a mine, it is found that the
temperature rises in an appreciable manner, and that it increases with
the depth below the surface.
The high temperature of the waters in Artesian wells when these are very
deep, testifies to a great heat of the interior of the earth.
The thermal waters which issue from the earth--of which the temperature
sometimes rises to 100° Centigrade and upwards--as, for instance, the
Geysers of Iceland--furnish another proof in support of the hypothesis.
Modern volcanoes are said to be a visible demonstration of the existence
of central heat. The heated gases, the liquid lava, the flames which
escape from their craters, all tend to prove sufficiently that the
interior of the globe has a temperature prodigiously elevated as
compared with that at its surface.
The disengagement of gases and burning vapours through the accidental
fissures in the crust, which accompany earthquakes, still further tends
to establish the existence of a great heat in the interior of the globe.
We have already said that the temperature of the globe increases about
one degree for every sixty or seventy feet of depth beneath its surface.
The correctness of this observation has been verified in a great number
of instances--indeed, to the greatest depth to which man has penetrated,
and been able to make use of the thermometer. Now, as we know exactly
the length of the radius of the terrestrial sphere, it has been
calculated from this progression of temperature, supposing it to be
regular and uniform, that the centre of the globe ought to have at the
present time a mean temperature of 195,000° Centigrade. No matter could
preserve its solid state at this excessive temperature; it follows,
then, that the centre of the globe, and all parts near the centre, must
be in a permanent state of fluidity.
The works of Werner, of Hutton, of Leopold von Buch, of Humboldt, of
Cordier, W. Hopkins, Buckland, and some other English philosophers, have
reduced this hypothesis to a theory, on which has been based, to a
considerable extent, the whole science of modern geology; although,
properly speaking, and in the popular acceptation of the term, that
science only deals with the solid crust of the earth.
The nebular theory thus embraces the whole solar system, and, by
analogy, the universe. It assumes that the SUN was originally a mass of
incandescent matter, that vast body being brought into a state of
evolution by the action of laws to which the Creator, in His divine
wisdom, has subjected all matter. In consequence of its immense
expansion and attenuation, the exterior zone of vapour, expanding beyond
the sphere of attraction, is supposed to have been thrown off by
centrifugal force. This zone of vapour, which may be supposed at one
time to have resembled the rings of Saturn, would in time break up into
several masses, and these masses coalescing into globes, would (by the
greater power of attraction which they would assume as consolidated
bodies) revolve round the sun, and, from mechanical considerations,
would also revolve with a rotary motion on their own axes.
This doctrine is applied to all the planets, and assumes each to have
been in a state of incandescent vapour, with a central incandescent
nucleus. As the cooling went on, each of these bodies may be supposed to
have thrown off similar masses of vapour, which, by the operation of the
same laws, would assume the rotary state, and, as satellites, revolve
round the parent planet. Such, in brief, was the grand conception of
Laplace; and surely it detracts nothing from our notions of the
omnipotence of the Creator that it initiates the creation step by step,
and under the laws to which matter is subjected, rather than by the
direct fiat of the Almighty. The hypothesis assumes that as the vaporous
mass cooled by the radiation of heat into space, the particles of matter
would approximate and solidify.
That the figure of the earth is such as a very large mass of matter in a
state of fluidity would assume from a state of rotation, seems to be
admitted, thus corroborating the speculations of Leibnitz, that the
earth is to be looked on as a heated fluid globe, cooled, and still
cooling at the surface, by radiation of its superfluous heat into space.
Mr. W. Hopkins[7] has put forth some strong but simple reasons in
support of a different theory; although he does not attempt to solve the
problem, but leaves the reader to form his own conclusions. As far as we
have been able to follow his reasoning we gather from it that:--
[7] See _Phil. Transactions_, 1839-40-42; also, _Quarterly Journal of
the Geological Society_, vol. viii., p. 56.
If the earth were a fluid mass cooled by radiation, the cooled parts
would, by the laws of circulating fluids, descend towards the centre,
and be replaced on the surface by matter at a higher temperature.
The consolidation of such a mass would, therefore, be accompanied by a
struggle for superiority between pressure and temperature, both of which
would be at their maximum at the centre of the mass.
At the surface, it would be a question of rapidity of cooling, by
radiation, as compared with the internal condition--for comparing which
relations we are without data; but on the result of which depends
whether such a body would most rapidly solidify at the surface by
radiation, or at the centre by pressure.
The effect of the first would be solidification at the surface, followed
by condensation at the centre through pressure. There would thus be two
masses, a spherical fluid nucleus, and a spherical shell or envelope,
with a large zone of semi-fluid, pasty matter between, continually
changing its temperature as its outer or inner surface became converted
to the solid state.
If pressure, on the other hand, gained the victory, the centre would
solidify before the circulation of the heated matter had ceased; and the
solidifying process would proceed through a large portion of the globe,
and even approach the surface before that would become solid. In other
words, solidification would proceed from the centre until the
diminishing power of pressure was balanced by radiation, when the
gradual abstraction of heat would allow the particles to approximate and
become solid.
The terrestrial sphere may thus be a solid indurated mass at the centre,
with a solid stony crust at the surface, and a shifting viscous, but
daily-decreasing, mass between the two; a supposition which the
diminished and diminishing frequency and magnitude of volcanic and other
eruptive convulsions seem to render not improbable.
It is not to be supposed that amongst the various hypotheses of which
the cosmogony of the world has been the object, a literal acceptation of
the scriptural account finds no defenders among men of science. “Why,”
asks one of these writers,[8] after some scornful remarks upon the
geologists and their science--“why an omnipotent Creator should have
called into being a gaseous-granite nebulous world, only to have to cool
it down again, consisting as it does of an endless variety of
substances, should even have been supposed to be originally constituted
of the matter of granite alone, for nothing else was provided by the
theory, nobody can rationally explain. How the earth’s centre now could
be liquid fire with its surface solid and cold and its seas not boiling
caldrons, has never been attempted to be accounted for. How educated
gentlemen, engaged in scientific investigations, ever came to accept
such a monstrously stupid mass of absurdities as deductions of
‘science,’ and put them in comparison with the rational account of the
creation given by Moses, is more difficult to understand than even this
vague theory itself, which it is impossible to describe.
[8] “Fresh Springs of Truth.” R. Griffin and Co.
“Of the first creation of the chaotic world,” the same writer goes on to
say, “or the material elements, before they were shaped into their
present forms, we can scarce have the most vague conception. All our
experience relates to their existing conditions. But knowing somewhat of
the variety of the constituent elements and their distinct properties,
by which they manifest their existence to us, we cannot conceive of
their creation without presupposing a Divine wisdom, and--if I may say
so, with all reverence, and only to suit our human notions--a Divine
ingenuity,” and he follows for six days the operations as described by
Moses, with a running comment. When light is created, the conception of
the work becomes simpler to our minds. Its least manifestation would
suffice at once to dispel darkness, and yet how marvellous is the light!
In the second day’s work the firmament of heaven is opened; the expanse
of the air between the heavens and the earth, dividing the waters above
from the waters below, is the work recorded as performed. Not till the
third day commence the first geological operations. The waters of the
earth are gathered together into seas, and the dry land is made to
appear. It is now that we can imagine that the formation of the primary
strata commenced, while by some of the internal forces of matter the
earth was elevated and stood above the waters.
Immediately the dry land is raised above and separated from the waters
the fiat goes forth, “Let the earth _bring forth_ grass, and herb and
tree;” vegetable life begins to exist, and the world is first decorated
with its beauteous flora, with all its exquisite variety of forms and
brilliancy of colouring, with which not even Solomon in all his glory
can compare. In like manner, on the sixth day the earth is commanded to
bring forth land-animals--the living creature “after his kind,” cattle
and creeping thing, and beast of the earth, “after his kind;” and last
of all, but on the same day, man is created, and made the chief and
monarch of God’s other living creatures--for that is “man’s place in
Nature.” “Let us now see,” he continues, “how this history came to be
discredited by the opposition of a falsely so-called ‘science’ of
geology, that, while spared by our theologians, has since pulled itself
to pieces. The first step in the false inductions geology made arose
from the rash deduction, that the order in which the fossil remains of
organic being were found deposited in the various strata necessarily
determined the order of their creation; and the next error arose from
blindly rushing to rash conclusions, and hasty generalisation from a
very limited number of facts, and the most imperfect investigations.
There were also (and, indeed, are still) some wild dogmatisms as to the
time necessary to produce certain geologic formations; but the
absurdities of science culminated when it adopted from Laplace the
irrational and unintelligible theory of a _natural_ origin for the world
from a nebula of gaseous granite, intensely hot, and supposed to be
gradually cooled while gyrating senselessly in space.”
In this paper the writer does not attempt to deal with the various
phenomena of volcanoes, earthquakes, hot springs, and other matters
which are usually considered as proofs of great internal heat. Mr. Evan
Hopkins, C.E., F.G.S., is more precise if less eloquent. He shows that,
in tropical countries, plains of gravel may in a day be converted into
lagoons and marshes; that by the fall of an avalanche rivers have been
blocked up, which, bursting their banks, have covered many square miles
of fertile country with several feet of mud, sand, and gravel. “Two
thousand four hundred years ago,” he says, “Nineveh flourished in all
its grandeur, yet it is now buried in oblivion, and its site overwhelmed
with sand. Look at old Tyre, once the queen of cities and mistress of
the sea. She was in all her pride two thousand four hundred and forty
years ago. We now see but a bare rock in the sea, on which fishermen
spread their nets! A thousand years ago, according to Icelandic
histories, Greenland was a fertile land in the south, and supported a
large population. Iceland at that period was covered with forests of
birch and fir, and the inhabitants cultivated barley and other grain. We
may, therefore, conclude, with these facts before us, that there is no
necessity to assign myriads of ages to terrestrial changes, as assumed
by geologists, as they can be accounted for by means of alterations
effected during a few thousand years, for the surface of the earth is
ever changing.
“Grant geological speculators,” Mr. Hopkins continues, “a few millions
of centuries, with a command over the agencies of Nature to be brought
into operation when and how they please, and they think they can form a
world with every variety of rock and vegetation, and even transform a
worm into a man! Yet the wisest of our philosophers would be puzzled if
called upon to explain why fluids become spheres, as dew-drops; why
carbonate of lime acquires in solidifying from a liquid the figure of an
obtuse rhomboihedron, silica of a six-sided prism; and why oxygen and
hydrogen gases produce both _fire_ and _water_. And what do they gain,”
he proceeds to ask, “by carrying back the history of the world to these
myriads of centuries? Do they, by the extension of the period to
infinity, explain how the ‘_Original_’ materials were created? But,” he
adds, “geologists are by no means agreed in their assumed geological
periods! The so-called glacial period has been computed by some to be
equal to about eighty-three thousand years, and by others at even as
much as twelve hundred and eighty millions of years! Were we to ask for
a _demonstrative proof_ of any given deposit being more than four or
five thousand years old, they could not give it. Where is Babylon, the
glory of the kingdoms? Look at Thebes, and behold its colossal columns,
statues, temples, obelisks, and palaces desolated; and yet those great
cities flourished within the last three thousand years. Even Pompeii and
Herculaneum were all but lost to history! What,” he asks after these
brief allusions to the past--“what, as a matter of fact, have geologists
discovered, as regards the great terrestrial changes, more than was
known to Pythagoras and the ancient philosophers who taught, two
thousand three hundred and fifty years ago, ‘that the surface of the
earth was ever changing--solid land converted into sea, sea changed into
dry land, marine shells lying far distant from the deep, valleys
excavated by running water, and floods washing down hills into the
sea?’”
In reference to the argument of the vast antiquity of the earth, founded
on elevation of coasts at a given rate of upheaval, he adduces many
facts to show that upheavals of equal extent have occurred almost within
the memory of man. Two hundred and fifty years ago Sir Francis Drake,
with his fleet, sailed into Albemarle Sound through Roanoke Outlet,
which is now a sand-bank above the reach of the highest tides. Only
seventy years ago it was navigable by vessels drawing twelve feet of
water. The whole American coast, both on the Atlantic and Pacific, have
undergone great changes within the last hundred years. The coast of
South America has, in some places, been upheaved twenty feet in the last
century; in others, a few hundred miles distant, it has been depressed
to an equal extent. A transverse section from Rio Santa Cruz to the base
of the Cordilleras, and another in the Rio Negro, in Patagonia, showed
that the whole sedimentary series is of recent origin. Scattered over
the whole at various heights above the sea, from thirteen hundred feet
downwards, are found recent shells of _littoral_ species of the
neighbouring coast--denoting upheavals which might have been effected
during the last three thousand years.
Coming nearer home, he shows that in 1538 the whole coast of Pozzuoli,
near Naples, was raised twenty feet in a single night. Then, with regard
to more compact crystalline or semi-crystalline rocks, no reliable
opinion can be formed on mere inspection. Two blocks of marble may
appear precisely alike, though formed at different periods. A crystal of
carbonate of lime, formed in a few years, would be found quite perfect,
and as compact as a crystal formed during many centuries. Nothing can be
deduced from the process of petrifaction and crystallisation, unless
they enclose relics of a known period. At San Filippo, a solid mass of
limestone thirty feet thick has been formed in about twenty years. A
hard stratum of travertine a foot thick is obtained, from these thermal
springs, in the course of four months. Nor can geologists demonstrate
that the Amiens deposits, in which the flint-implements occur, are more
than three or four thousand years old.
The causes of these changes and mutations are referred by some persons
to floods, or to pre-Adamite convulsions, whereas the cause is in
constant operation; they are due to an invisible and subtle power which
pervades the air, the ocean, and the rocks below--in which all are
wrapped and permeated--which is universally present, namely,
magnetism--a power always in operation, always in a state of activity
and tension. It has an attractive power towards the surface of the
earth, as well as a directive action from pole to pole. “It is, indeed,”
he adds, emphatically, “the _terrestrial gravitation_. Magnetic needles
freely suspended show its meridional or directive polar force, and that
the force converges at two opposite parts, which are bounded by the
Antarctic and Arctic circles.”
This polar force, like a stream, is constantly moving from pole to pole;
and experiment proves that this movement is from the South Pole to the
North. “Hence the various terrestrial substances, solids and fluids,
through which this subtle and universal power permeates, are controlled,
propelled, and modified over the entire surface of our globe, commencing
at the south and dissolving at the north. Thus, all terrestrial matter
moves towards the Arctic region, and finally disappears by dissolution
and absorption, to be renewed again and again in the Antarctic Sea to
the end of time.”
In order to prove that the north polar basin is the receptacle of the
final dissolution of all terrestrial substances, Mr. Hopkins quotes the
Gulf Stream. Bottles, tropical plants, and wrecks cast into the sea in
the South Atlantic, are carried to Greenland in a comparatively short
time. The great _tidal_ waves commence at the fountain-head in the
Antarctic circle, impinge against the south coast of Tierra del Fuego,
New Zealand, and Tasmania, and are then propelled northward in a series
of undulations. The South Atlantic stream, after doubling the Cape of
Good Hope, moves towards the Guinea coast, bends towards the Caribbean
Sea, producing the trade winds; again leaves Florida as the Gulf Stream,
and washes the coasts of Greenland and Norway, and finally reaches the
north polar basin.
Again the great polar force shows itself in the arrangement of the
mineral structure below. In all the primary rocks in every quarter of
the globe where they have been examined, its action is recognised in
giving to the crystalline masses--granites and their laminated
elongations--a polar grain and vertical cleavage. “Had it been possible
to see our globe stripped of its sedimentary deposits and its oceanic
covering, we should see it like a gigantic melon, with a uniform grain
extending from pole to pole.” This structure appears to give polarity to
earthquakes--thermal waters and earthquakes--which are all traceable in
the direction of the polar grain or cleavage from north to south.
In England, for instance, thermal and saline springs are traceable from
Bath, through Cheltenham, to Dudley. In Central France, mineral springs
occur in lines, more or less, north and south. All the known
salt-springs in South America occur in meridional bands. Springs of
chloride of sodium in the Eastern Cordilleras stretch from Pinceima to
the Llanoes de Meta, a distance of 200 miles. The most productive
metalliferous deposits are found in meridional bands. The watery
volcanoes in South America are generally situated along the lines of the
meridional splits and the secondary eruptive pores on the transverse
fractures. The sudden ruptures arising locally from increasing tension
of the polar force, and the rapid expansion of the generated gases,
produce a vibratory jar in the rocky structure below, which being
propagated along the planes of the polar cleavage, gives rise to great
superficial oscillations, and thus causes earthquakes and subterranean
thunder for thousands of miles, from south to north.
In 1797, the district round the volcano of Tunguraqua in Quito, during
one of the great meridional shocks, experienced an undulating movement,
which lasted upwards of four minutes, and this was propagated to the
shores of the Caribbean Sea.
All these movements demonstrated, according to Mr. Hopkins, that the
land as well as the ocean moves from the south pole and north pole, and
that the magnetic power has a tendency to proceed from pole to pole in a
_spiral_ path from south-east to north-west, a movement which produces
an apparent change in the equinoxes, or the outer section of the plane
of the ecliptic with the equator, a phenomenon known to astronomers as
the precession of the equinoxes.
Such is a very brief summary of the arguments by which Mr. Evan Hopkins
maintains the literal correctness of the Mosaic account of the creation,
and attempts to show that all the facts discovered by geologists may
have occurred in the ages included in the Mosaic chronology.
That the mysterious power of terrestrial magnetism can perform all that
he claims for it, we can perhaps admit. But how does this explain the
succession of Silurian, Old Red Sandstone, Carboniferous and other
strata, up to the Tertiary deposits, with their fossils, each differing
in character from those of the preceding series? That these were
successive creations admits of no doubt, and while it is undeniable that
the fiat of the Creator could readily produce all these phenomena, it
may reasonably be asked if it is probable that all these myriads of
organic beings, whose remains serve as records of their existence, were
created only to be immediately destroyed.
Again, does not the author of the “Principles of Terrestrial Physics”
prove too much? He admits that 3,000 years ago the climate of England
was tropical: he does not deny that a subsequent period of intense cold
intervened, 2,550 years ago. He admits historical records, and 2,350
years ago Pythagoras constructed his cosmography of the world, which has
never been seriously impugned; and yet he has no suspicion that
countries so near to his own had changed their climates first from
tropical to glacial, and back again to a temperate zone. It is not
reasonable to believe this parable.
The school of philosophy generally considered to be the most advanced in
modern science has yet another view of cosmogony, of which we venture to
give a brief outline. Space is infinite, says the exponent of this
system,[9] for wherever in imagination we erect a boundary, we are
compelled to think of space as existing beyond it. The starry heavens
proclaim that it is not entirely void; but the question remains, are the
vast regions which surround the stars, and across which light is
propagated, absolutely empty? No. Modern science, while it rejects the
notion of the luminiferous particles of the old philosophy, has cogent
proofs of the existence of a luminiferous ether with definite mechanical
properties. It is infinitely more attenuated, but more solid than gas.
It resembles jelly rather than air, and if not co-extensive with space,
it extends as far as the most distant star the telescope reveals to us;
it is the vehicle of their light in fact; it takes up their molecular
tremors and conveys them with inconceivable rapidity to our organs of
vision. The splendour of the firmament at night is due to this
vibration. If this ether has a boundary, masses of ponderable matter may
exist beyond it, but they could emit no light. Dark suns may burn there,
metals may be heated to fusion in invisible furnaces, planets may be
molten amid intense darkness; for the loss of heat being simply the
abstraction of molecular motion by the ether, where this medium is
absent no cooling could take place.
[9] Professor Tyndall in _Fortnightly Review_.
This, however, does not concern us; as far as our knowledge of space
extends, we are to conceive of it as the holder of this luminiferous
ether, through which the fixed stars are interspersed at enormous
distances apart. Associated with our planet we have a group of dark
planetary masses revolving at various distances around it, each rotating
on its axis; and, connected with them, their moons. Was space furnished
at once, by the fiat of Omnipotence, with these burning orbs? The man of
science should give no answer to this question: but he has better
materials to guide him than anybody else, and can clearly show that the
present state of things _may_ be derivative. He can perhaps assign
reasons which render it probable that it _is derivative_. The law of
gravitation enunciated by Newton is, that every particle of matter in
the universe attracts every other particle with a force which diminishes
as the square of the distance increases. Under this law a stone falls to
the ground, and heat is produced by the shock; meteors plunge into the
atmosphere and become incandescent; showers of such doubtless fall
incessantly upon the sun, and were it stopped in its orbit, the earth
would rush towards the sun, developing heat in the collision (according
to the calculations of MM. Joule, Mayer, Helmholtz, and Thomson), equal
to the combustion of five thousand worlds of solid coal. In the
attraction of gravity, therefore, acting upon this luminous matter, we
have a source of heat more powerful than could be derived from any
terrestrial combustion.
To the above conception of space we must add that of its being in a
continual state of tremor. The sources of vibration are the ponderable
masses of the universe. Our own planet is an aggregate of solids,
liquids, and gases. On closer examination, these are found to be
composed of still more elementary parts: the water of our rivers is
formed by the union, in definite proportions, of two gases, oxygen and
hydrogen. So, likewise, our chalk hills are formed by a combination of
carbon, oxygen, and calcium; elements which in definite proportions form
chalk. The flint found within that chalk is compounded of oxygen and
silicon, and our ordinary clay is for the most part formed by a union
of silicon, oxygen, and aluminum. By far the greater part of the earthy
crust is thus compounded of a few elementary substances.
Such is Professor Tyndall’s view of the universe, rising incidentally
out of his theory of heat, his main object being to elucidate his theory
of heat and light.
MODIFICATIONS OF THE SURFACE OF THE GLOBE.
As a consequence of the hypothesis of central heat, it is admitted that
our planet has been agitated by a series of local disturbances; that is
to say, by ruptures of its solid crust occurring at more or less distant
intervals. These partial revolutions at the surface are supposed to have
been produced, as we shall have occasion to explain, by upheavals or
depressions of the solid crust, resulting from the fluidity of the
central parts, and by the cooling down of the external crust of the
globe.
Almost all bodies, in passing from a liquid to a solid state, are
diminished in size in the process. In molten metals which resume the
solid state by cooling, this diminution amounts to about a tenth of
their volume; but the decrease in size is not equal throughout the whole
mass. Hence, as a result of the solidification of the internal parts of
the globe, the outer envelope would be too large; and would no longer
fit the inner sphere, which had contracted in cooling. Cracks and
hollows occur under such circumstances, even in small masses, and the
effect of converting such a vast body as the earth from a liquid, or
rather molten condition, to a solid state, may be imagined. As the
interior became solid and concrete by cooling, furrows, corrugations,
and depressions in the external crust of the globe would occur, causing
great inequalities in its surface; producing, in short, what are now
called _chains of mountains_.
At other times, in lieu of furrows and irregularities, the solid crust
has become ruptured, producing fissures and fractures in the outer
envelope, sometimes of immense extent. The liquid substances contained
in the interior of the globe, with or without the action of the gases
they enclose, escape through these openings; and, accumulating on the
surface, become, on cooling and consolidating, _mountains_ of various
heights.
It would also happen, and always from the same cause, namely, from the
internal contraction caused by the unequal cooling of the globe, that
minor fissures would be formed in the earth’s crust; incandescent liquid
matter would be afterwards injected into these fissures, filling them
up, and forming in the rocky crust those long narrow lines of foreign
substances which we call _dykes_.
Finally, it would occasionally happen, that in place of molten matter,
such as granite or metalliferous compounds, escaping through these
fractures and fissures in the globe, actual rivers of boiling water,
abundantly charged with various mineral salts (that is to say, with
silicates, and with calcareous and magnesian compounds), would also
escape, since the elements of water would be abundant in the
incandescent mass. Added to these the chemical and mechanical action of
the atmosphere, of rain, rivers, and the sea, have all a tendency to
destroy the hardest rocks. The mineral salts and other foreign
substances, entering into combination with those already present in the
waters of the sea, and separating at a subsequent period from these
waters, would be thrown down, and thus constitute extensive
deposits--that is to say, _sedimentary formations_. These became, on
consolidation, the _sedimentary rocks_.
The furrows, corrugations, and fractures in the terrestrial crust, which
so changed the aspect of the surface, and for the time displaced the
sea-basins, would be followed by periods of calm. During these periods,
the débris, torn by the movement of the waters from certain points of
the land, would be transported to other parts of the globe by the
oceanic currents. These accumulated heterogeneous materials, when
deposited at a later period, would ultimately constitute
formations--that is, _transported or drifted rocks_.
We have ventured to explain some of the theories by which it is sought
to explain the cosmography of the world. But our readers must understand
that all such speculations are, of necessity, purely hypothetical.
In conformity with the preceding considerations we shall divide the
mineral substances of which the earth is composed into three general
groups, under the following heads:--
1. _Eruptive Rocks._--Crystalline, like the second, but formed at all
geological periods by the irruption or intrusion of the liquid matter
occupying the interior of our globe through all the pre-existing rocks.
2. _Crystalline Rocks._--That portion of the terrestrial crust which was
primarily liquid, owing to the heat of the globe, but which solidified
at the period of its first cooling down; forming the masses known as
Fundamental Gneiss, and Laurentian, &c.
3. _Sedimentary Rocks._--Consisting of various mineral substances
deposited by the water of the sea, such as silica, the carbonates of
lime and magnesia, &c.
The mineral masses which constitute the _sedimentary rocks_ form beds,
or _strata_, having among themselves a constant order of superposition
which indicates their relative age. The mineral structure of these beds,
and the remains of the organised beings they contain, impress on them
characters which enable us to distinguish each bed from that which
precedes and follows it.
It does not follow, however, that all these beds are met with, regularly
superimposed, over the whole surface of the globe; under such
circumstances geology would be a very simple science, only requiring the
use of the eyes. In consequence of the frequent eruptions of granite,
porphyry, serpentine, trachyte, basalt, and lava, these beds are often
broken, cut off, and replaced by others.
_Denudation_ has been another fruitful source of change. Professor
Ramsay[10] shows, in the “Memoirs of the Geological Survey,” that beds
once existed above a great part of the Mendip Hills to the extent of at
least 6,000 feet, which have been removed by the denuding agency of the
sea; while in South Wales and the adjacent country, a series of
Palaeozoic rocks, eleven thousand feet in thickness, has been removed by
the action of water. In fact, every foot of the earth now forming the
dry land is supposed to have been at one time under water--to have
emerged, and to have been again submerged, and subjected to the
destructive action of the ocean. At certain points a whole series of
sedimentary deposits, and often several of them, have been removed by
this cause, known by geologists as _Denudation_. The regular series of
rock formations are, in fact, rarely found in unbroken order. It is only
by combining the collected observations of the geologists of all
countries, that we are enabled to arrange, according to their relative
ages, the several beds composing the solid terrestrial crust as they
occur in the following Table, which proceeds from the surface towards
the centre, in descending order:--
[10] “Memoirs of the Geological Survey of Great Britain,” vol. i., p.
297.
ORDER OF STRATIFICATION.
Quaternary Epoch Modern Period.
{ Pliocene Period.
Tertiary Epoch { Miocene Period.
{ Eocene Period.
{ Cretaceous Rocks.
Secondary Epoch { Jurassic Rocks.
{ Triassic Rocks.
{ Permian Rocks.
Primary Epoch { Carboniferous Rocks.
{ Devonian Rocks.
{ Silurian Rocks.
Metamorphic Series { Cambrian Rocks.
{ Fundamental Gneiss, or Laurentian.
Under these heads we propose to examine the successive transformations
to which the earth has been subjected in reaching its present condition;
in other words, we propose, both from an historical and descriptive
point of view, to take a survey of the several _epochs_ which can be
distinguished in the gradual formation of the earth, corresponding with
the formation of the great groups of rocks enumerated in the preceding
table. We shall describe the living creatures which have peopled the
earth at each of these epochs, and which have disappeared, from causes
which we shall also endeavour to trace. We shall describe the plants
belonging to each great phase in the history of the globe. At the same
time, we shall not pass over entirely in silence the rocks deposited by
the waters, or thrown up by eruption during these periods; we propose,
also, to give a summary of the mineralogical characters and of the
fossils characteristic of, or peculiar to each formation. What we
propose, in short, is to give a history of the formation of the globe,
and a description of the principal rocks which actually compose it; and
to take also a rapid glance at the several generations of animals and
plants which have succeeded and replaced each other on the earth, from
the very beginning of organic life up to the time of man’s appearance.
ERUPTIVE ROCKS.
Nothing is more difficult than to write a chronological history of the
revolutions and changes to which the earth has been subjected during the
ages which preceded the historic times. The phenomena which have
concurred to fashion its enormous mass, and to give to it its present
form and structure, are so numerous, so varied, and sometimes so nearly
simultaneous in their action, that the records defy the powers of
observation to separate them. The deposition of the sedimentary rocks
has been subject to interruption during all ages of the world. Violent
igneous eruptions have penetrated the sedimentary beds, elevating them
in some places, depressing them in others, and in all cases disturbing
their order of superposition, and ejecting masses of crystalline rocks
from the incandescent centre to the surface. Amidst these perturbations,
sometimes stretching over a vast extent of country, anything like a
rigorous chronological record becomes impossible, for the phenomena are
so continuous and complex that it is no longer possible to distinguish
the fundamental from the accidental and secondary causes.
In order to render the subject somewhat clearer, the great facts
relative to the progressive formation of the terrestrial globe are
divided into epochs, during which the sedimentary rocks were formed in
due order in the seas of the ancient world, the mud and sand in which
were deposited day by day. Again, even where the line of demarcation is
clearest between one formation and another, it must not be supposed
there is any sharply defined line of separation between them. On the
contrary, one system gradually merges into that which succeeds it. The
rocks and fossils of the one gradually disappear, to be succeeded by
those of the overlying series in the regular order of succession. The
newly-made strata became the cemetery of the myriads of beings which
lived and died in the bosom of the ocean. The rocks thus deposited were
called _Neptunian_ by the older geologists.
But while the seas of each epoch were thus building up, grain by grain,
and bed by bed, the new formation out of the ruins of the older, other
influences were at work, sometimes, to all appearance, impeding
sometimes advancing, the great work. The _Plutonic rocks_--the _igneous
or eruptive rocks_ of modern geology, as we have seen above, were the
great disturbing agents, and these disturbances occur in every age of
the earth’s history. We shall have occasion to speak of these eruptive
formations while describing the phenomena of the several epochs. But it
is thought that the narrative will be made clearer and more instructive
by grouping this class of phenomena into one chapter, which we place at
the commencement, inasmuch as the constant reference to the eruptive
rocks will thus be rendered more intelligible. To these are now added
the section “Metamorphic Rocks,” from the fifth edition of the French
work.
The rocks which issued from the centre of the earth in a state of fusion
are found associated or interstratified with masses of every epoch, more
especially with those of the more ancient strata. The formations which
these rocks have originated possess great interest; first, because they
enter into the composition of the terrestrial crust; secondly, because
they have impressed on its surface, in the course of their eruption,
some of the characteristics of its configuration and structure; finally,
because, by their means, the metals which are the objects of human
industry have been brought nearer to the surface. According to the order
of their appearance, or as nearly so as can be ascertained, we shall
class the eruptive rocks in two groups:--
I. The _Volcanic Rocks_, of comparatively recent origin, which have
given rise to a succession of trachytes, basalts, and modern lavas.
These, being of looser texture, are presumed to have cooled more rapidly
than the Plutonic rocks, and at or near the surface.
II. The _Plutonic Rocks_, of older date, which are exemplified in the
various kinds of granites, the syenites, the protogines, porphyries, &c.
These differ from the volcanic rocks in their more compact crystalline
structure, in the absence of tufa, as well as of pores and cavities;
from which it is inferred that they were formed at considerable depths
in the earth, and that they have cooled and crystallised slowly under
great pressure.
PLUTONIC ERUPTIONS.
The great eruptions of _ancient granite_ are supposed to have occurred
during the primary epoch, and chiefly in the carboniferous period. They
present themselves sometimes in considerable masses, for the earth’s
crust being still thin and permeable, it was prepared as it were for
absorbing the granite masses. In consequence of its weak cohesion, the
primitive crust of the globe would be rent and penetrated in all
directions, as represented in the following section of Cape Wrath, in
Sutherlandshire, in which the veins of granite ramify in a very
irregular manner across the gneiss and hornblende-schist, there
associated with it. (Fig. 3.)
[Illustration: Fig. 3.--Veins of granite traversing the gneiss of Cape
Wrath.]
Granite, when it is sound, furnishes a fine building-stone, but we must
not suppose that it deserves that character of extreme hardness with
which the poets have gratuitously gifted it. Its granular texture
renders it unfit for road-stone, where it gets crushed too quickly to
dust. With his hammer the geologist easily shapes his specimens; and in
the Russian War, at the bombardment of Bomarsund, the shot from our
ships demonstrated that ramparts of granite could be as easily
demolished as those constructed of limestone.
The component minerals of granite are felspar, quartz, and mica, in
varying proportions; felspar being generally the predominant ingredient,
and quartz more plentiful than mica--the whole being united into a
confusedly granular or crystalline mass. Occasionally it passes
insensibly from fine to coarse-grained granite, and the finer grained is
even sometimes found embedded in the more coarsely granular variety:
sometimes it assumes a porphyritic texture. Porphyritic granite is a
variety of granite, the components of which--quartz, felspar, and
mica--are set in a non-crystallised paste, uniting the mass in a manner
which will be familiar to many of our readers who may have seen the
granite of the Land’s End, in Cornwall. Alongside these orthoclase
crystals, quartz is implanted, usually in grains of irregular shape,
more rarely crystallised, and seldom in the form of perfect crystals. To
these ingredients are added thin scales or small hexagonal plates and
crystals of white, brown, black, or greenish-coloured mica. Finally, the
name of _quartziferous porphyry_ is reserved for those varieties which
present crystals of quartz; the other varieties are simply called
_porphyritic granite_. _True_ porphyry presents a paste essentially
composed of compact felspar, in which the crystals of orthoclase--that
is, felspar with a potash base--are abundantly disseminated, and
sometimes with great regularity.
Granite is supposed to have been “formed at considerable depths in the
earth, where it has cooled and crystallised slowly under great pressure,
where the contained gases could not expand.”[11] “The influence,” says
Lyell, “of subterranean heat may extend downwards from the crater of
every active volcano to a great depth below, perhaps several miles or
leagues, and the effects which are produced deep in the bowels of the
earth may, or rather must, be distinct; so that volcanic and plutonic
rocks, each different in texture, and sometimes even in composition, may
originate simultaneously, the one at the surface, the other far beneath
it.” Other views, however, of its origin are not unknown to science:
Professor Ramsay and some other geologists consider granite to be
metamorphic. “For my own part,” says the Professor, “I believe that in
one sense it is an igneous rock; that is to say, that it has been
completely fused. But in another sense it is a metamorphic rock, partly
because it is impossible in many cases to draw any definite line between
gneiss and granite, for they pass into each other by insensible
gradations; and granite frequently _occupies the space that ought to be
filled with gneiss_, were it not that the gneiss has been entirely
fused. I believe therefore that granite and its allies are simply the
effect of the extreme of metamorphism, brought about by great heat with
presence of water. In other words, when the metamorphism has been so
great that all traces of the semi-crystalline laminated structure have
disappeared, a more perfect crystallisation has taken place.”[12] It is
obvious that the very result on which the Professor founds his theory,
namely, the difficulty “in many cases,” of drawing a line between the
granite and the gneiss, would be produced by the sudden injection of the
fluid minerals into gneiss, composed of the same materials. Moreover, it
is only in some cases that the difficulty exists; in many others the
line of separation is definable enough.[13]
[11] Lyell’s “Elements of Geology,” p. 694.
[12] “Physical Geology and Geography of Great Britain,” by A. C.
Ramsay, p. 38, 2nd ed.
[13] At the same time it may be safely assumed (as Professor Ramsay
believes to be the case) that granite in most cases is a
metamorphic rock; yet are there many instances in which it may
with greater truth be considered as a true plutonic rock.
The granitic rock called _Syenite_, in which a part of the mica is
replaced by hornblende or amphibole, has to all appearance been erupted
to the surface subsequently to the granite, and very often alongside of
it. Thus the two extremities of the Vosges, towards Belfort and
Strasburg, are eminently syenitic, while the intermediate part, towards
Colmar, is as markedly granitic. In the Lyonnais, the southern region is
granitic; the northern region, from Arbresle, is in great part syenitic.
Syenite also makes its appearance in the Limousin.
Syenite, into which rose-coloured felspar often enters, forms a
beautiful rock, because the green or nearly black hornblende heightens,
by contrast, the effect of its colour. This rock is a valuable adjunct
for architectural ornament; it is that out of which the ancient
Egyptians shaped many of their monumental columns, sphinxes, and
sarcophagi; the most perfect type of it is found in Egypt, not far from
the city of Syene, from which it derives its name. The obelisk of Luxor
now in Paris, several of the Egyptian obelisks in Rome, and the
celebrated sphinxes, of which copies may be seen in front of the
Egyptian Court at the Crystal Palace, the pedestal of the statue of
Peter the Great at St. Petersburg, and the facing of the sub-basement of
the column in the Place Vendôme in Paris, are of this stone, of which
there are quarries in the neighbourhood of Plancher-les-Mines in the
Vosges.
Syenite disintegrates more readily than granite, and it contains
indurated nodular concretions, which often remain in the form of large
spherical balls, in the midst of the débris resulting from
disintegration of the mass. It remains to be added that syenitic masses
are often very variable as regards their composition; the hornblende is
sometimes wanting, in which case we can only recognise an ancient
granite. In other instances the hornblende predominates to such a
degree, that a large or small-grained _diorite_, or greenstone, results.
The geologist should be prepared to observe these transitions, which are
apt to lead him into error if passed over without being noticed.
_Protogine_ is a talcose granite, composed of felspar, quartz, and talc
or _chlorite_, or decomposed mica, which take the place of the usual
mica. Excessively variable in its texture, protogine passes from the
most perfect granitic aspect to that of a porphyry, in such a manner as
to present continual subjects of uncertainty, rendering it very
difficult to determine its geological age. Nevertheless, it is supposed
to have come to the surface before and during the coal-period; in short,
at Creusot, protogine covers the coal-fields so completely, that it is
necessary to sink the pits through the protogine, in order to penetrate
to the coal, and the rock has so manifestly acted on the coal-measure
strata, as to have contorted and metamorphosed them. Something analogous
to this manifests itself near Mont Blanc, where the colossal mass which
predominates in that chain, and the peaks which belong to it, consist of
protogine. But as no such action can be perceived in the overlying rocks
of the Triassic period, it may be assumed that at the time of the
deposition of the New Red Sandstone the protoginous eruptions had
ceased.
It is necessary to add, however, that if the protogine rises in such
bold pinnacles round Mont Blanc, the circumstance only applies to the
more elevated parts of the mountain, and is influenced by the excessive
rigour of the seasons, which demolishes and continually wears away all
the parts of the rock which have been decomposed by atmospheric agency.
Where protogine occurs in milder climates--around Creusot, and at
Pierre-sur-Autre, in the Forez chain, for instance--the mountains show
none of the scarped and bristling peaks exhibited in the chain of Mont
Blanc. Only single isolated masses occasionally form _rocking-stones_,
so called because, resting with a convex base upon a pedestal also
convex, but in a contrary way, it is easy to move these naturally
balanced blocks, although from their vast size it would require very
considerable force to displace them. This tendency to fashion themselves
into rounded or ellipsoidal forms belongs, also, to other granitic
rocks, and even to some of the variegated sandstones. The rocking-stones
have often given rise to legends and popular myths.
The great eruptions of granite, protogine, and porphyry took place,
according to M. Fournet, during the carboniferous period, for
porphyritic pebbles are found in the conglomerates of the Coal-measure
period. “The granite of Dartmoor, in Devonshire,” says Lyell,[14] “was
formerly supposed to be one of the most ancient of the plutonic rocks,
but it is now ascertained to be posterior in date to the culm-measures
of that county, which from their position, and as containing true
coal-plants, are regarded by Professor Sedgwick and Sir R. Murchison as
members of the true Carboniferous series. This granite, like the
syenitic granite of Christiana, has broken through the stratified
formations without much changing their strike. Hence, on the north-west
side of Dartmoor, the successive members of the Culm-measures abut
against the granite, and become metamorphic as they approach. The
granite of Cornwall is probably of the same date, and therefore as
modern as the Carboniferous strata, if not newer.”
[14] “Elements of Geology,” p. 716, 6th edition.
The _ancient granites_ show themselves in France in the Vosges, in
Auvergne, at Espinouse in Languedoc, at Plan-de-la-Tour in Provence, in
the chain of the Cévennes, at Mont Pilat near Lyons, and in the southern
part of the Lyonnaise chain. They rarely impart boldness or grandeur to
the landscape, as might be expected from their crystallised texture and
hardness; for having been exposed to the effects of atmospheric changes
from the earliest times of the earth’s consolidation, the rocks have
become greatly worn away and rounded in the outlines of their masses. It
is only when recent dislocations have broken them up that they assume a
picturesque character.
The Christiania granite alluded to above was at one time thought to have
belonged to the Silurian period. But, in 1813, Von Buch announced that
the strata in question consisted of limestones containing orthoceratites
and trilobites; the shales and limestone being only penetrated by
granite-veins, and altered for a considerable distance from the point of
contact.[15] The same granite is found to penetrate the ancient gneiss
of the country on which the fossiliferous beds rest--unconformably, as
the geologists say; that is, they rest on the edges of the gneiss, from
which other stratified deposits had been washed away, leaving the gneiss
denuded before the sedimentary beds were deposited. “Between the origin,
therefore, of the gneiss and the granite,”[16] says Lyell, “there
intervened, first, the period when the strata of gneiss were denuded;
secondly, the period of the deposition of the Silurian deposits. Yet
the granite produced after this long interval is often so intimately
blended with the ancient gneiss at the point of the junction, that it is
impossible to draw any other than an arbitrary line of separation
between them; and where this is not the case, tortuous veins of granite
pass freely through gneiss, ending sometimes in threads, as if the older
rock had offered no resistance to their passage.” From this example Sir
Charles concludes that it is impossible to conjecture whether certain
granites, which send veins into gneiss and other metamorphic rocks, have
been so injected while the gneiss was scarcely solidified, or at some
time during the Secondary or Tertiary period. As it is, no single mass
of granite can be pointed out more ancient than the oldest known
fossiliferous deposits; no Lower Cambrian stratum is known to rest
immediately on granite; no pebbles of granite are found in the
conglomerates of the Lower Cambrian. On the contrary, granite is usually
found, as in the case of Dartmoor, in immediate contact with primary
formations, with every sign of elevation subsequent to their deposition.
Porphyritic pebbles are found in the Coal-measures; porpyhries continue
during the Triassic period; since, in some parts of Germany, veins of
porphyry are found traversing the New Red Sandstone, or _grès bigarré_
of French geologists. Syenites have especially reacted upon the Silurian
deposits and other old sedimentary rocks, up to those of the Lower
Carboniferous period.
[15] “Elements of Geology,” p. 717.
[16] Ibid, p. 718.
The term porphyry is usually applied to a rock with a paste or base of
compact felspar, in which felspathic crystals of various sizes assume
their natural form. The variety of their mineralogical characters, the
admirable polish which can be given to them, and which renders them
eminently useful for ornamentation, give to the porphyries an artistic
and industrial importance, which would be greatly enhanced if the
difficulty of working such a hard material did not render the price so
high.
The porphyries possess various degrees of hardness and compactness. When
a fine dark-red colour--which contrasts well with the white of the
felspar--is combined with hardness, a magnificent stone is the result,
susceptible of taking a polish, and fit for any kind of ornamental work;
for the decoration of buildings, for the construction of vases, columns,
&c. The red Egyptian porphyry, called _Rosso antico_, was particularly
sought after by the ancients, who made sepulchres, baths, and obelisks
of it. The grandest known mass of this kind of porphyry is the Obelisk
of Sextus V. at Rome. In the Museum of the Louvre, in Paris, some
magnificent basins and statues, made of the same stone, may also be
seen.
In spite of its compact texture porphyry disintegrates, like other
rocks, when exposed to air and water. One of the sphinxes transported
from Egypt to Paris, being accidentally placed under a gutter of the
Louvre, soon began to exhibit signs of exfoliation, notwithstanding it
had remained sound for ages under the climate of Egypt. In this country,
and even in France, where the climate is much drier, the porphyries
frequently decompose so as to become scarcely recognisable. They crop
out in various parts of France, but are only abundant in the
north-eastern part of the central plateau, and in some parts of the
south. They form mountains of a conical form, presenting, nearly always,
considerable depressions on their flanks. In the Vosges they attain a
height of from three to four thousand feet.
The _Serpentine_ rocks are a sort of compact _talc_, which owe their
soapy texture and greasy feel to silicate of magnesia. Their softness
permits of their being turned in a lathe and fashioned into vessels of
various forms. Even stoves are constructed of this substance, which
bears heat well. The serpentine quarried on the banks of Lake Como,
which bears the name of pierre ollaire, or pot-stone, is excellently
adapted for this purpose. Serpentine shows itself in the Vosges, in the
Limousin, in the Lyonnais, and in the Var; it occupies an immense tract
in the Alps, as well as in the Apennines. Mona marble is an example of
serpentine; and the Lizard Point, Cornwall, is a mass of it. A portion
of the stratified rocks of Tuscany, and also those of the Island of
Elba, have been upheaved and overturned by eruptions of it.
As for the British Islands, plutonic rocks are extensively developed in
Scotland, where the Cambrian and Silurian rocks, often of gneissic
character--associated here and there with great bosses of granite and
syenite--form by far the greater part of the region known as the
Highlands. In the Isle of Arran a circular mass of coarse-grained
granite protrudes through the schists of the northern part of the
island; while, in the southern part, a finer-grained granite and veins
of porphyry and coarse-grained granite have broken through the
stratified rocks.[17] In Devonshire and Cornwall there are four great
bosses of granite; in the southern parts of Cornwall the mineral axis is
defined by a line drawn through the centre of the several bosses from
south-west to north-east; but in the north of Cornwall, and extending
into Devonshire, it strikes nearly east and west. The great granite
mass in Cornwall lies on the moors north of St. Austell, and indicates
the existence of more than one disturbing force. “There was an elevating
force,” says Professor Sedgwick,[18] “protruding from the St. Austell
granite; and, if I interpret the phenomena correctly, there was a
contemporaneous elevating force acting from the south; and between these
two forces, the beds, now spread over the surface from the St. Austell
granite to the Dodman and Narehead, were broken, contorted, and placed
in their present disturbed position. Some great disturbing forces,” he
observes, “have modified the symmetry of this part of Cornwall,
affecting,” he believes, “the whole transverse section of the country
from the headlands near Fowey to those south of Padstow.” This great
granite-axis was upheaved in a line commencing at the west end of
Cornwall, rising through the slate-rocks of the older Devonian group,
continuing in association with them as far as the boss north of St.
Austell, producing much confusion in the stratified masses; the
granite-mass between St. Clear and Camelford rose between the deposition
of the Petherwin and that of the Plymouth group; lastly, the Dartmoor
granite rose, partially moving the adjacent slates in such a manner that
its north end abuts against and tilts up the base of the Culm-trough,
mineralising the great Culm-limestone, while on the south it does the
same to the base of the Plymouth slates. These facts prove that the
granite of Dartmoor, which was formerly thought to be the most ancient
of the Plutonic rocks, is of a date subsequent to the Culm-measures of
Devonshire, which are now regarded as forming part of the true
carboniferous series.
[17] “Geology of the Island of Arran,” by Andrew C. Ramsay. “Geology
of Arran and Clydesdale,” by James Bryce.
[18] See _Quarterly Journal of Geological Society_, vol. viii., pp. 9
and 10.
VOLCANIC ROCKS.
Considered as a whole, the volcanic rocks may be grouped into three
distinct formations, which we shall notice in the following order, which
is that of their relative antiquity, namely:--1. _Trachytic_; 2.
_Basaltic_; 3. _Volcanic or Lava formations_.
[Illustration: Fig. 4.--A peak of the Cantal chain.]
TRACHYTIC FORMATIONS.
_Trachyte_ (derived from τραχυς, rough), having a coarse, cellular
appearance, and a rough and gritty feel, belongs to the class of
volcanic rocks. The eruptions of trachyte seem to have commenced towards
the middle of the Tertiary period, and to have continued up to its
close. The trachytes present considerable analogy in their composition
to the felspathic porphyries, but their mineralogical characters are
different. Their texture is porous; they form a white, grey, black,
sometimes yellowish matrix, in which, as a rule, felspar predominates,
together with disseminated crystals of felspar, some hornblende or
augite, and dark-coloured mica. In its external appearance trachyte is
very variable. It forms the three most elevated mountain ranges of
Central France; the groups of Cantal and Mont Dore, and the chain of the
Velay (Puy-de-Dôme).[19]
[19] For full information in reference to the rocks and geology of
this part of France, the reader is referred to the masterly work
on “The Geology and Extinct Volcanoes of Central France,” by G.
Poulett Scrope, 2nd edition, 1858.
[Illustration: I.--Peak of Sancy in the Mont Dore group, Auvergne.]
The igneous group of Cantal may be described as a series of lofty
summits, ranged around a large cavity, which was at one period probably
a volcanic crater, the circular base of which occupies an area of nearly
fifteen leagues in diameter. The strictly trachytic portion of the group
rises in the centre, and is composed of high mountains, throwing off
spurs, which gradually decrease in height, and terminate in plateaux
more or less inclined. These central mountains attain a height varying
between 4,500 and 5,500 feet above the level of the sea. A scaly or
schistose variety of trachyte, called _phonolite_, or clinkstone (from
the ringing metallic sound it emits when struck with the hammer), with
an unusual proportion of felspar, or, according to Gmelin, composed of
felspar and zeolite, forms the steep trachytic escarpments at the
centre, which enclose the principal valleys; their abrupt peaks giving a
remarkably picturesque appearance to the landscape. In the engraving on
p. 40 (Fig. 4) the slaty, laminated character of the clinkstone is well
represented in one of the phonolitic peaks of the Cantal group. The
group at Mont Dore consists of seven or eight rocky summits, occupying a
circuit of about five leagues in diameter. The massive trachytic rock,
of which this mountainous mass is chiefly formed, has an average
thickness of 1,200 to 2,600 feet; comprehending over that range
prodigious layers of scoriæ, pumiceous conglomerates, and detritus,
interstratified with beds of trachyte and basalt, bearing the signs of
an igneous origin, tufa forming the base; and between them occur layers
of lignite, or imperfectly mineralised woody fibre, the whole being
superimposed on a primitive plateau of about 3,250 feet in height.
Scored and furrowed out by deep valleys, the viscous mass was gradually
upheaved, until in the needle-like Pic de Sancy (PLATE I.), a pyramidal
rock of porphyritic trachyte, which is the loftiest point of Mont Dore,
it attains the height of 6,258 feet. The Pic de Sancy, represented on
page 40 (Fig. 4), gives an excellent idea of the general appearance of
the trachytic mountains of Mont Dore.
Upon the same plateau with Mont Dore, and about seven miles north of its
last slopes, the trachytic formation is repeated in four rounded
domes--those of Puy-de-Dôme, Sarcouï, Clierzou, and Le Grand Suchet. The
Puy-de-Dôme, one of the most remarkable volcanic domes in Auvergne,
presents another fine and very striking example of an eruptive trachytic
rock. The rock here assumes a peculiar mineral character, which has
caused the name of _domite_ to be given to it.
The chain of the Velay forms a zone, composed of independent plateaux
and peaks, which forms upon the horizon a long and strangely
intersected ridge. The bareness of the mountains, their forms--pointed
or rounded, sometimes terminating in scarped plateaux--give to the whole
landscape an appearance at once picturesque and characteristic. The peak
of Le Mezen, which rises 5,820 feet above the sea, forms the culminating
point of the chain. The phonolites of which it consists have been
erupted from fissures which present themselves at a great number of
points, ranging from north-north-west to south-south-east.
On the banks of the Rhine and in Hungary the trachytic formation
presents itself in features identical with those which indicate it in
France. In America it is principally represented by some immense cones,
superposed in the chain of the Andes; the colossal Chimborazo being one
of those trachytic cones.
[Illustration: II.--Mountain and basaltic crater of La Coupe d’Ayzac, in
the Vivarais.]
[Illustration: Fig. 5.--Theoretical view of a basaltic plateau.]
[Illustration: Fig. 6.--Basalt in prismatic columns.]
BASALTIC FORMATIONS.
Basaltic eruptions seem to have occurred during the Secondary and
Tertiary periods. Basalt, according to Dr. Daubeny,[20] in its more
strict sense, “is composed of an intimate mixture of augite with a
zeolitic mineral, which appears to have been formed out of labradorite
(felspar of Labrador), by the addition of water--the presence of water
being in all _zeolites_ the cause of that bubbling-up under the
blow-pipe to which they owe their appellation.” M. Delesse and other
mineralogists are of opinion that the idea of augite being the
prevailing mineral in basalt, must be abandoned; and that although its
presence gives the rock its distinctive character, as compared with
trachytic and most other trap rocks, still the principal element in
their composition is felspar. Basalt, a lava consisting essentially of
augite, labradorite (or nepheline) and magnetic iron-ore is the rock
which predominates in this formation. It contains a smaller quantity of
silica than the trachyte, and a larger proportion of lime and magnesia.
Hence, independent of the iron in its composition, it is heavier in
proportion, as it contains more or less silica. Some varieties of basalt
contain very large quantities of olivine, a mineral of an olive-green
colour, with a chemical composition differing but slightly from
serpentine. Both basalts and trachyte contain more soda and less silica
in their composition than granites; some of the basalts are highly
fusible, the alkaline matter and lime in their composition acting as a
flux to the silica. There are examples of basalt existing in
well-defined flows, which still adhere to craters visible at the present
day, and with regard to the igneous origin of which there can be no
doubt. One of the most striking examples of a basaltic cone is furnished
by the mountain or crater of La Coupe d’Ayzac, in the Vivarais, in the
south of France. PLATE II., on the opposite page, gives an accurate
representation of this curious basaltic flow. The remnants of the stream
of liquefied basalt which once flowed down the flank of the hill may
still be seen adhering in vast masses to the granite rocks on both sides
of a narrow valley where the river Volant has cut across the lava and
left a pavement or causeway, forming an assemblage of upright prismatic
columns, fitted together with geometrical symmetry; the whole resting on
a base of gneiss. Basaltic eruptions sometimes form a plateau, as
represented in Fig. 5, where the process of formation is shown
theoretically and in a manner which renders further explanation
unnecessary. Many of these basaltic table-lands form plateaux of very
considerable extent and thickness; others form fragments of the same,
more or less dislocated; others, again, present themselves in isolated
knolls, far removed from similar formations. In short, basaltic rocks
present themselves in veins or dykes, more or less, in most countries,
of which Central France and the banks of the Rhine offer many striking
examples. These veins present very evident proofs that the matter has
been introduced from below, and in a manner which could only result from
injection from the interior to the exterior of the earth. Such are the
proofs presented by the basaltic veins of Villeneuve-de-Berg, which
terminate in slender filaments, sometimes bifurcated, which gradually
lose themselves in the rock which they traverse. In several parts of the
north of Ireland, chalk-formations with flints are traversed by basaltic
dykes, the chalk being converted into granular marble near the basalt,
the change sometimes extending eight or ten feet from the wall of the
dyke, and being greatest near the surface of contact. In the Island of
Rathlin, the walls of basalt traverse the chalk in three veins or dykes;
the central one a foot thick, that on the right twenty feet, and on the
left thirty-three feet thick, and all, according to Buckland and
Conybeare, within the breadth of ninety feet.
[20] “Volcanoes,” 2nd ed.
[Illustration: Fig. 7.--Basaltic Causeway, on the banks of the river
Volant, in the Ardèche.]
One of the most striking characteristics of basalt is the prismatic and
columnar structure which it often assumes; the lava being homogeneous
and of very fine grain, the laws which determine the direction of the
fissures or divisional planes consolidated from a molten to a solid
state, become here very manifest--these are always at right angles to
the surfaces of the rock through which the heat of the fused mass
escaped. The basaltic rocks have been at all times remarkable for this
picturesque arrangement of their parts. They usually present columns of
regular prisms, having generally six, often five, and sometimes four,
seven, or even three sides, whose disposition is always perpendicular to
the cooling surfaces. These are often divided transversely, as in Fig.
6, at nearly equal distances, like the joints of a wall, composed of
regularly arranged, equal-sided pieces adhering together, and frequently
extending over a more or less considerable space. The name of Giant’s
Causeway has been given, from time immemorial, to these curious columnar
structures of basalt. In France, in the Vivarais and in the Velay, there
are many such basaltic causeways. That of which Fig. 7 is a sketch lies
on the banks of the river Volant, where it flows into the Ardèche.
Ireland has always been celebrated for its Giant’s Causeway, which
extends over the whole of the northern part of Antrim, covering all the
pre-existing strata of Chalk, Greensand, and Permian formations; the
prismatic columns extend for miles along the cliffs, projecting into the
sea at the point specially designated the Giant’s Causeway.
These columnar formations vary considerably in length and diameter.
McCulloch mentions some in Skye, which “are about four hundred feet
high; others in Morven not exceeding an inch (vol. ii. p. 137). In
diameter those of Ailsa Craig measure nine feet, and those of Morven an
inch or less.” Fingal’s Cave, in the Isle of Staffa, is renowned among
basaltic rocks, although it was scarcely known on the mainland a century
ago, when Sir Joseph Banks heard of it accidentally, and was the first
to visit and describe it. Fingal’s Cave has been hollowed out, by the
sea, through a gallery of immense prismatic columns of trap, which are
continually beaten by the waves. The columns are usually upright, but
sometimes they are curved and slightly inclined. Fig. 8 is a view of the
basaltic grotto of Staffa.
Grottoes are sometimes formed by basaltic eruptions on land, followed by
their separation into regular columns. The Grotto of Cheeses, at
Bertrich-Baden, between Trèves and Coblentz, is a remarkable example of
this kind, being so called because its columns are formed of round, and
usually flattened, stones placed one above the other in such a manner as
to resemble a pile of cheeses.
[Illustration: Fig. 8.--Basaltic cavern of Staffa--exterior.]
If we consider that in basalt-flows the lower part is compact, and often
divided into prismatic columns, while the upper part is porous,
cellular, scoriaceous, and irregularly divided--that the points of
separation on which they rest are small beds presenting fragments of the
porous stony concretions known under the name of _Lapilli_--that the
lower portions of these masses present a multitude of points which
penetrate the rocks on which they repose, thereby denoting that some
fluid matter had moulded itself into its crevices--that the neighbouring
rocks are often calcined to a considerable thickness, and the included
vegetable remains carbonised--no doubt can exist as to the igneous
origin of basaltic rocks. When it reached the surface through certain
openings, the fluid basalt spread itself, flowing, as it were, over the
horizontal surface of the ground; for if it had flowed upon inclined
surfaces it could not have preserved the uniform surface and constant
thickness which it generally exhibits.
[Illustration: III.--Extinct volcanoes forming the Puy-de-Dôme Chain.]
VOLCANIC OR LAVA FORMATIONS.
The _lava_ formations comprehend both extinct and active volcanoes. “The
term,” says Lyell, “has a somewhat vague signification, having been
applied to all melted matter observed to flow in streams from volcanic
vents. When this matter consolidates in the open air, the upper part is
usually scoriaceous, and the mass becomes more and more stony as we
descend, or in proportion as it has consolidated more slowly and under
greater pressure.”[21]
[21] “Elements of Geology,” p. 596.
The formation of extinct volcanoes is represented in France by the
volcanoes situated in the ancient provinces of Auvergne, Velay, and the
Vivarais, but principally by nearly seventy volcanic cones of various
sizes and of the height of from 500 to 1,000 feet, composed of loose
scoriæ, lava, and pozzuolana, arranged upon a granitic table-land, about
twelve miles wide, which overlooks the town of Clermont-Ferrand, and
which seem to have been produced along a longitudinal fracture in the
earth’s crust, running in a direction from north to south. It is a range
of volcanic hills, the “chain of _Puys_” nearly twenty miles in length,
by two in breadth. By its cellular and porous structure, which is also
granular and crystalline, the felspathic or pyroxenic lava which flowed
from these volcanoes is readily distinguishable from the analogous lavas
which belong to the basaltic or trachytic formations. Their surface is
irregular, and bristles with asperities, formed by heaped-up angular
blocks.
The volcanoes of the chain of _Puys_, represented on opposite page (PL.
III.) are so perfectly preserved, their lava is so frequently superposed
on sheets of basalt, and presents a composition and texture so distinct,
that there is no difficulty in establishing the fact that they are
posterior to the basaltic formation, and of very recent age.
Nevertheless, they do not appear to belong to the historic ages, for no
tradition attests their eruption. Lyell places these eruptions in the
Lower Miocene period, and their greatest activity in the Upper Miocene
and Pliocene eras. “Extinct quadrupeds of those eras,” he says,
“belonging to the genera mastodon, rhinoceros, and others, were buried
in ashes and beds of alluvial sand and gravel, which owe their
preservation to overspreading sheets of lava.”[22]
[22] Ibid, p. 677.
[Illustration: Fig. 9.--Section of a volcano in action.]
All volcanic phenomena can be explained by the theory we have already
indicated, of fractures in the solid crust of the globe resulting from
its cooling. The various phenomena which existing volcanoes present to
us are, as Humboldt has said, “the result of every action exercised by
the interior of a planet on its external crust.”[23] We designate as
volcanoes all conduits which establish a permanent communication between
the interior of the earth and its surface--a conduit which gives passage
at intervals to eruptions of _lava_, and in Fig. 9 we have represented,
in an ideal section, the geological mode of action of volcanic
eruptions. The volcanoes on the surface of the globe, known to be in an
occasional state of activity, number about three hundred, and these may
be divided into two classes: the _isolated_ or _central_, and the
_linear_ or those volcanoes which belong to a _series_.[24]
[23] “Cosmos,” vol. i., p. 25. Bohn.
[24] “Cosmos,” vol. i., p. 237.
The first are active volcanoes, around which there may be established
many secondary active mouths of eruption, always in connection with some
principal crater. The second are disposed like the chimneys of furnaces,
along fissures extending over considerable distances. Twenty, thirty,
and even a greater number of volcanic cones may rise above one such rent
in the earth’s crust, the direction of which will be indicated by their
linear course. The Peak of Teneriffe is an instance of a central
volcano; the long rampart-like chain of the Andes, presents, from the
south of Chili to the north-west coast of America, one of the grandest
instances of a continental volcanic chain; the remarkable range of
volcanoes in the province of Quito belong to the latter class. Darwin
relates that on the 19th of March, 1835, the attention of a sentry was
called to something like a large star which gradually increased in size
till about three o’clock, when it presented a very magnificent
spectacle. “By the aid of a glass, dark objects, in constant succession,
were seen in the midst of a great glare of red light, to be thrown up
and to fall down. The light was sufficient to cast on the water a long
bright reflection--it was the volcano of Osorno in action.” Mr. Darwin
was afterwards assured that Aconcagua, in Chili, 480 miles to the north,
was in action on the same night, and that the great eruption of
Coseguina (2,700 miles north of Aconcagua), accompanied by an earthquake
felt over 1,000 miles, also occurred within six hours of this same time;
and yet Coseguina had been dormant for six-and-twenty years, and
Aconcagua most rarely shows any signs of action.[25] It is also stated
by Professor Dove that in the year 1835 the ashes discharged from the
mountain of Coseguina were carried 700 miles, and that the roaring noise
of the eruption was heard at San Salvador, a distance of 1,000 miles.
[25] Darwin’s “Journal,” p. 291, 2nd edition.
In the sea the _series_ of volcanoes show themselves in groups of
islands disposed in longitudinal series.
Among these may be ranged the volcanic series of Sunda, which, according
to the accounts of the matter ejected and the violence of the eruptions,
seem to be among the most remarkable on the globe; the series of the
Moluccas and of the Philippines; those of Japan; of the Marianne
Islands; of Chili; of the double series of volcanic summits near Quito,
those of the Antilles, Guatemala, and Mexico.
Among the central, or isolated volcanoes, we may class those of the
Lipari Islands, which have _Stromboli_, in permanent activity, for
their centre; _Etna_, _Vesuvius_, the volcanoes of the _Azores_, of the
_Canaries_, of the _Cape de Verde_, of the _Galapagos_ Islands, the
_Sandwich_ Islands, the _Marquesas_, the _Society_ Islands, the
_Friendly_ Islands, _Bourbon_, and, finally, _Ararat_.
[Illustration: Fig. 10.--Existing crater of Vesuvius.]
The mouths of volcanic chimneys are, almost always, situated near the
summit of a more or less isolated conical mountain; they usually consist
of an opening in the form of a funnel, which is called the _crater_, and
which descends into the interior of the volcanic chimney. But in the
course of ages the crater becomes extended and enlarged, until, in some
of the older volcanoes, it has attained almost incredible dimensions. In
1822 the crater of Vesuvius was 2,000 feet deep, and of a very
considerable circumference. The crater of Kilauea, in the Sandwich
Islands group, is an immense chasm 1,000 feet deep, with an outer
circle no less than from two to three miles in diameter, in which lava
is usually seen, Mr. Dana tells us, to boil up at the bottom of a lake,
the level of which varies continually according to the active or
quiescent state of the volcano. The cone which supports these craters,
and which is designated the _cone of ejection_, is composed for the most
part of lava or _scoriæ_, the products of eruption. Many volcanoes
consist only of a _cone of scoriæ_. Such is that of Barren Isle, in the
Bay of Bengal. Others, on the contrary, present a very small cone,
notwithstanding the considerable height of the volcanic chain. As an
example we may mention the new crater of Vesuvius, which was produced in
1829 within the former crater (Fig. 10).
[Illustration: Fig. 11.--Fissures near Locarno.]
The frequency and intensity of the eruptions bear no relation to the
dimensions of the volcanic mountain. The eruption of a volcano is
usually announced by a subterranean noise, accompanied by shocks,
quivering of the ground, and sometimes by actual earthquakes. The noise,
which usually proceeds from a great depth, makes itself heard, sometimes
over a great extent of country, and resembles a well-sustained fire of
artillery, accompanied by the rattle of musketry. Sometimes it is like
the heavy rolling of subterranean thunder. Fissures are frequently
produced during the eruptions, extending over a considerable radius, as
represented in the woodcut on page 57 of the fissures of Locarno (Fig.
11), where they present a singular appearance; the clefts radiating from
a centre in all directions, not unlike the starred fracture in a cracked
pane of glass. The eruption begins with a strong shock, which shakes the
whole interior of the mountain; masses of heated vapour and fluids begin
to ascend, revealing themselves in some cases by the melting of the snow
upon the flanks of the cone of ejection; while simultaneously with the
final shock, which overcomes the last resistance opposed by the solid
crust of the ground, a considerable body of gas, and more especially of
steam, escapes from the mouth of the crater.
The steam, it is important to remark, is essentially the cause of the
terrible mechanical effects which accompany volcanic eruptions.
Granitic, porphyritic, trachytic, and sometimes even basaltic matters,
have reached the surface without producing any of those violent
explosions or ejections of rocks and stones which accompany modern
volcanic eruptions; the older granites, porphyries, trachytes, and
basalts were discharged without violence, because steam did not
accompany those melted rocks--a sufficient proof of the comparative calm
which attended the ancient as compared with modern eruptions. Well
established by scientific observations, this is a fact which enables us
to explain the cause of the tremendous mechanical effects attending
modern volcanic eruptions, contrasted with the more tranquil eruptions
of earlier times.
During the first moments of a volcanic eruption, the accumulated masses
of stones and ashes, which fill the crater, are shot up into the sky by
the suddenly and powerfully developed elasticity of the steam. This
steam, which has been disengaged by the heat of the fluid lava, assumes
the form of great rounded bubbles, which are evolved into the air to a
great height above the crater, where they expand as they rise, in clouds
of dazzling whiteness, assuming that appearance which Pliny the Younger
compared to a stone pine rising over Vesuvius. The masses of clouds
finally condense and follow the direction of the wind.
These volcanic clouds are grey or black, according to the quantity of
_ashes_, that is, of pulverulent matter or dust, mixed with watery
vapour, which they convey. In some eruptions it has been observed that
these clouds, on descending to the surface of the soil, spread around an
odour of hydrochloric or sulphuric acid, and traces of both these acids
are found in the rain which proceeds from the condensation of these
clouds.
The fleecy clouds of vapour which issue from the volcanoes are streaked
with lightning, followed by continuous peals of thunder; in condensing,
they discharge disastrous showers, which sweep the sides of the
mountain. Many eruptions, known as _mud volcanoes_, and _watery
volcanoes_, are nothing more than these heavy rains, carrying down with
them showers of ashes, stones, and scoriæ, more or less mixed with
water.
Passing on to the phenomena of which the crater is the scene at the time
of an eruption, it is stated that at first there is an incessant rise
and fall of the lava which fills the interior of the crater. This double
movement is often interrupted by violent explosions of gas. The crater
of Kilauea, in the Island of Hawaii, contains a lake of molten matter
1,600 feet broad, which is subject to such a double movement of
elevation and depression. Each of the vaporous bubbles as it issues from
the crater presses the molten lava upwards, till it rises and bursts
with great force at the surface. A portion of the lava, half-cooled and
reduced to scoriæ, is thus projected upwards, and the several fragments
are hurled violently in all directions, like those of a shell at the
moment when it bursts.
The greater number of the fragments being thrown vertically into the
air, fall back into the crater again. Many accumulating on the edge of
the opening add more and more to the height of the cone of eruption. The
lighter and smaller fragments, as well as the fine ashes, are drawn
upwards by the spiral vapours, and sometimes transported by the winds
over almost incredible distances.
In 1794 the ashes from Vesuvius were carried as far as the extremity of
Calabria. In 1812 the volcanic ashes of Saint Vincent, in the Antilles,
were carried eastward as far as Barbadoes, spreading such obscurity over
the island, that, in open day, passengers could not see their way.
Finally, some of the masses of molten lava are shot singly into the air
during an eruption with a rapid rotatory motion, which causes them to
assume the rounded shape in which they are known by the name of
_volcanic bombs_.
We have already remarked that the lava, which in a fluid state fills the
crater and the internal vent or chimney of the volcano, is forced
upwards by gaseous fluids, and by the steam which has been generated
from the water, entangled with the lava. In some cases the mechanical
force of this vapour is so great as to drive the lava over the edge of
the crater, when it forms a fiery torrent, spreading over the sides of
the mountain. This only happens in the case of volcanoes of
inconsiderable height; in lofty volcanoes it is not unusual for the lava
thus to force an outlet for itself near the base of the mountain,
through which the fiery stream discharges itself over the surrounding
country. In such circumstances the lava cools somewhat rapidly; it
becomes hard and presents a scoriaceous crust on the surface, while the
vapour escapes in jets of steam through the interstices. But under this
superficial crust the lava retains its fluid state, cooling slowly in
the interior of the mass, while the thickening stream moves sluggishly
along, impeded in its progress by the fragments of rock which this
burning river drives before it.
The rate at which a current of lava moves along depends upon its mass,
upon its degree of fluidity, and upon the inclination of the ground. It
has been stated that certain streams of lava have traversed more than
3,000 yards in an hour; but the rate at which they travel is usually
much less, a man on foot being often able to outstrip them. These
streams, also, vary greatly in dimensions. The most considerable stream
of lava from Etna had, in some parts, a thickness of nearly 120 feet,
with a breadth of a geographical mile and a half. The largest
lava-stream which has been recorded issued from the Skaptár Jokul, in
Iceland, in 1783. It formed two currents, whose extremities were twenty
leagues apart, and which from time to time presented a breadth of from
seven to fifteen miles and a thickness of 650 feet.
A peculiar effect, and which only simulates volcanic activity, is
observable in localities where _mud volcanoes_ exist. Volcanoes of this
class are for the most part conical hills of low elevation, with a
hollow or depression at the centre, from which they discharge the mud
which is forced upwards by gas and steam. The temperature of the ejected
matter is only slightly elevated. The mud, generally of a greyish
colour, with the odour of petroleum, is subject to the same alternating
movements which have been already ascribed to the fluid lava of
volcanoes, properly so called. The gases which force out this liquid
mud, mixed with salts, gypsum, naphtha, sulphur, sometimes even of
ammonia, are usually carburetted hydrogen and carbonic acid. Everything
leads to the conclusion that these compounds proceed, at least in great
part, from the reaction produced between the various elements of the
subsoil under the influence of infiltrating water between bituminous
marls, complex carbonates, and probably carbonic acid, derived from
acidulated springs. M. Fournet saw in Languedoc, near Roujan, traces of
some of these formations; and not far from that neighbourhood is the
bituminous spring of Gabian.
[Illustration: IV.--Mud volcano at Turbaco, South America.]
Mud volcanoes, or _salses_, exist in rather numerous localities. Several
are found in the neighbourhood of Modena. There are some in Sicily,
between Aragona and Girgenti. Pallas observed them in the Crimea--in the
peninsula of Kertch, and in the Isle of Tamàn. Von Humboldt has
described and figured a group of them in the province of Cartagena, in
South America. Finally, they have been observed in the Island of
Trinidad and in Hindostan. In 1797 an eruption of mud ejected from
Tunguragua, in Quito, filled a valley 1,000 feet wide to a depth of 600
feet. On the opposite page is represented the mud volcano of Turbaco, in
the province of Cartagena (PLATE IV.), which is described and figured by
Von Humboldt in his “Voyage to the Equatorial Regions of America.”
In certain countries we find small hillocks of argillaceous formation,
resulting from ancient discharges of mud volcanoes, from which all
disengagement of gas, water, and mud has long ceased. Sometimes,
however, the phenomenon returns and resumes its interrupted course with
great violence. Slight shocks of earthquakes are then felt; blocks of
dried earth are projected from the ancient crater, and new waves of mud
flow over its edge, and spread over the neighbouring ground.
To return to ordinary volcanoes, that is to say, those which eject lava.
At the end of a lava-flow, when the violence of the volcanic action
begins to subside, the discharge from the crater is confined to the
disengagement of vaporous gases, mixed with steam, which make their
escape in more or less abundance through a multitude of fissures in the
ground.
The great number of volcanoes which have thus become extinct form what
are called _solfataras_. The sulphuretted hydrogen, which is given out
through the fissures in the ground, is decomposed by contact with the
air, water being formed by the action of the oxygen of the atmosphere,
and sulphur deposited in considerable quantities on the walls of the
crater, and in the cracks of the ground. Such is the geological source
of the sulphur which is collected at Pozzuoli, near Naples, and in many
other similar regions--a substance which plays a most important part in
the industrial occupations of the world. It is, in fact, from sulphur
extracted from the ground about the mouths of extinct volcanoes, that is
to say from the products of _solfataras_, that sulphuric acid is
frequently made--sulphuric acid being the fundamental agent, one of the
most powerful elements, of the manufacturing productions of both worlds.
The last phase of volcanic activity is the disengagement of carbonic
acid gas without any increase of temperature. In places where these
continued emanations of carbonic acid gas manifest themselves, the
existence of ancient volcanoes may be recognised, of which these
discharges are the closing phenomenon. This is seen in a most remarkable
manner in Auvergne, where there are a multitude of acidulated springs,
that is to say, springs charged with carbonic acid. During the time when
he was opening the mines of Pontgibaud, M. Fournet had to contend with
emanations which sometimes exhibited themselves with explosive power.
Jets of water were thrown to great heights in the galleries, roaring
with the noise of steam when escaping from the boiler of a locomotive
engine. The water which filled an abandoned mine-shaft was, on two
separate occasions, upheaved with great violence--half emptying the
pit--while vast volumes of the gas overspread the whole valley,
suffocating a horse and a flock of geese. The miners were compelled to
fly in all haste at the moment when the gas burst forth, holding
themselves as upright as possible, to avoid plunging their heads into
the carbonic acid gas, which, from its low specific gravity, was now
filling the lower parts of the galleries. It represented on a small
scale the effect of the _Grotto del Cane_, which excites such surprise
among the ignorant near Naples; passing, also, for one of the marvels of
Nature all over the world. M. Fournet states that all the minute
fissures of the metalliferous gneiss near Clermont are quite saturated
with free carbonic acid gas, which rises plentifully from the soil
there, as well as in many parts of the surrounding country. The
components of the gneiss, with the exception of the quartz, are softened
by it; and fresh combinations of the acid with lime, iron, and manganese
are continually taking place. In short, long after volcanoes have become
extinct, hot springs, charged with mineral ingredients, continue to flow
in the same area.
The same facts as those of the _Grotto del Cane_ manifest themselves
with even greater intensity in Java, in the so-called Valley of Poison,
which is an object of terror to the natives. In this celebrated valley
the ground is said to be covered with skeletons and carcases of tigers,
goats, stags, birds, and even of human beings; for asphyxia or
suffocation, it seems, strikes all living things which venture into this
desolate place. In the same island a stream of sulphurous water, as
white as milk, issues from the crater of Mount Idienne, on the east
coast; and on one occasion, as cited by Nozet in the _Journal de
Géologie_, a great body of hot water, charged with sulphuric acid, was
discharged from the same volcano, inundating and destroying all the
vegetation of a large tract of country by its noxious fumes and
poisonous properties.
[Illustration: V.--Great Geyser of Iceland.]
It is known that the alkaline waters of Plombières, in the Vosges, have
a temperature of 160° Fahr. For 2,000 years, according to Daubrée,
through beds of concrete, of lime, brick, and sandstone, these hot
waters have percolated until they have originated calcareous spar,
aragonite, and fluor spar, together with siliceous minerals, such as
opal, which are found filling the interstices of the bricks and mortar.
From these and other similar statements, “we are led,” says Sir Charles
Lyell,[26] “to infer that when in the bowels of the earth there are
large volumes of molten matter, containing heated water and various
acids, under enormous pressure, these subterraneous fluid masses will
gradually part with their heat by the escape of steam and various gases
through fissures producing hot springs, or by the passage of the same
through the pores of the overlying and injected rocks.” “Although,” he
adds,[27] “we can only study the phenomena as exhibited at the surface,
it is clear that the gaseous fluids must have made their way through the
whole thickness of the porous or fissured rocks, which intervene between
the subterraneous reservoirs of gas and the external air. The extent,
therefore, of the earth’s crust which the vapours have permeated, and
are now permeating, may be thousands of fathoms in thickness, and their
heating and modifying influence may be spread throughout the whole of
this solid mass.”
[26] “Elements of Geology,” p. 732.
[27] Ibid, p. 733.
The fountains of boiling water, known under the name of _Geysers_, are
another emanation connected with ancient craters. They are either
continuous or intermittent. In Iceland we find great numbers of these
gushing springs--in fact, the island is one entire mass of eruptive
rock. Nearly all the volcanoes are situated upon a broad band of
trachyte, which traverses the island from south-west to north-east. It
is traversed by immense fissures, and covered with masses of lava, such
as no other country presents. The volcanic action, in short, goes on
with such energy that certain paroxysms of Mount Hecla have lasted for
six years without interruption. But the Great Geyser, represented on the
opposite page (PLATE V.), is, perhaps, even more an object of curiosity.
This water-volcano projects a column of boiling water, eight yards in
diameter, charged with silica, to the height, it has been said, of about
150 feet, depositing vast quantities of silica as it cools after
reaching the earth.
* * * * *
The volcanoes in actual activity are, as we have said, very numerous,
being more than 200 in number, scattered over the whole surface of the
globe, but mostly occurring in tropical regions. The island of Java
alone contains about fifty, which have been mapped and described by Dr.
Junghahn. Those best known are Vesuvius, near Naples; Etna, in Sicily;
and Stromboli, in the Lipari Islands. A rapid sketch of a few of these
may interest the reader.
Vesuvius is of all volcanoes that which has been most closely studied;
it is, so to speak, the classical volcano. Few persons are ignorant of
the fact that it opened--after a period of quiescence extending beyond
the memory of living man--in the year 79 of our era. This eruption cost
the elder Pliny his life, who fell a sacrifice to his desire to witness
one of the most imposing of natural phenomena. After many mutations the
present crater of Vesuvius consists of a cone, surrounded on the side
opposite the sea by a semicircular crest, composed of pumiceous matter,
foreign to Vesuvius properly speaking, for we believe that Mount
Vesuvius was originally the mountain to which the name of _Somma_ is now
given. The cone which now bears the name of Vesuvius was probably formed
during the celebrated eruption of 79, which buried under its showers of
pumiceous ashes the cities of Pompeii and Herculaneum. This cone
terminates in a crater, the shape of which has undergone many changes,
and which has, since its origin, thrown out eruptions of a varied
character, together with streams of lava. In our days the eruptions of
Vesuvius have only been separated by intervals of a few years.
The Lipari Isles contain the volcano of Stromboli, which is continually
in a state of ignition, and forms the natural lighthouse of the
Tyrrhenian Sea; such it was when Homer mentioned it, such it was before
old Homer’s time, and such it still appears in our days. Its eruptions
are incessant. The crater whence they issue is not situated on the
summit of the cone, but upon one of its sides, at nearly two-thirds of
its height. It is in part filled with fluid lava, which is continually
subjected to alternate elevation and depression--a movement provoked by
the ebullition and ascension of bubbles of steam which rise to the
surface, projecting upwards a tall column of ashes. During the night
these clouds of vapour shine with a magnificent red reflection, which
lights up the whole isle and the surrounding sea with a lurid glow.
Situated on the eastern coast of Sicily, Etna appears, at the first
glance, to have a much more simple structure than Vesuvius. Its slopes
are less steep, more uniform on all sides; its vast base nearly
represents the form of a buckler. The lower portion of Etna, or the
cultivated region of the mountain, has an inclination of about three
degrees. The middle, or forest region, is steeper, and has an
inclination of about eight degrees. The mountain terminates in a cone
of an elliptical form of thirty-two degrees of inclination, which bears
in the middle, above a nearly horizontal terrace, the cone of eruption
with its circular crater. The crater is 10,874 feet high. It gives out
no lava, but only vomits forth gas and vapour, the streams of lava
issuing from sixteen smaller cones which have been formed on the slopes
of the mountain. The observer may, by looking at the summit, convince
himself that these cones are disposed in rays, and are based upon clefts
or fissures which converge towards the crater as towards a centre.
But the most extraordinary display of volcanic phenomena occurs in the
Pacific Ocean, in the Sandwich Islands, and in Java. Mauna Loa and Mauna
Kea, in Hawaii, are huge flattened cones, 14,000 feet high. According to
Mr. Dana, these lofty, featureless hills sometimes throw out successive
streams of lava, not very far below their summits, often two miles in
breadth and six-and-twenty in length; and that not from one vent, but in
every direction, from the apex of the cone down slopes varying from four
to eight degrees of inclination. The lateral crater of Kilauea, on the
flank of Mauna Loa, is from 3,000 to 4,000 feet above the level of the
sea--an immense chasm 1,000 feet deep, with an outer circuit two to
three miles in diameter. At the bottom lava is seen to boil up in a
molten lake, the level of which rises or falls according to the active
or quiescent state of the volcano; but in place of overflowing, the
column of melted rock, when the pressure becomes excessive, forces a
passage through subterranean communications leading to the sea. One of
these outbursts, which took place at an ancient wooded crater six miles
east of Kilauea, was observed by Mr. Coan, a missionary, in June, 1840.
Another indication of the subterranean progress of the lava took place a
mile or two beyond this, in which the fiery flood spread itself over
fifty acres of land, and then found its way underground for several
miles further, to reappear at the bottom of a second ancient wooded
crater which it partly filled up.[28]
[28] Lyell’s “Elements of Geology,” p. 617.
The volcanic mountains of Java constitute the highest peaks of a
mountain-range running through the island from east to west, on which
Dr. Junghahn described and mapped forty-six conical eminences, ranging
from 4,000 to 11,000 feet high. At the top of many of the loftiest of
these Dr. Junghahn found the active cones and craters of small size, and
surrounded by a plain of ashes and sand, which he calls the “old crater
wall,” sometimes exceeding 1,000 feet in vertical height, and many of
the semicircular walls enclosing large cavities or _calderas_, four
geographical miles in diameter. From the highest parts of many of these
hollows rivers flow, which, in the course of ages, have cut out deep
valleys in the mountain’s side.[29]
[29] Lyell’s “Elements of Geology,” p. 620.
To this rapid sketch of actually existing volcanic phenomena we may add
a brief notice of submarine volcanoes. If these are known to us only in
small numbers, the circumstance is explained by the fact that their
appearance above the bosom of the sea is almost invariably followed by a
more or less complete disappearance; at the same time such very striking
and visible phenomena afford a sufficient proof of the continued
persistence of volcanic action beneath the bed of the sea-basin. At
various times islands have suddenly appeared, amid the ocean, at points
where the navigator had not before noticed them. In this manner we have
witnessed the island called Graham’s, Ferdinanda, or Julia, which
suddenly appeared off the south-west coast Sicily in 1831, and was swept
away by the waves two months afterwards.[30] At several periods also,
and notably in 1811, new islands were formed in the Azores, which raised
themselves above the waves by repeated efforts all round the islands,
and at many other points.
[30] Ibid, p. 620.
The island which appeared in 1796 ten leagues from the northern point of
Unalaska, one of the Aleutian group of islands, is specially remarkable.
We first see a column of smoke issuing from the bosom of the ocean,
afterwards a black point appears, from which bundles of fiery sparks
seem to rise over the surface of the sea. During the many months that
these phenomena continue, the island increases in breadth and in height.
Finally smoke only is seen; at the end of four years, even this last
trace of volcanic convulsion altogether ceases. The island continued,
nevertheless, to enlarge and to increase in height, and in 1806 it
formed a cone, surmounted by four other smaller ones.
In the space comprised between the isles of Santorin, Tharasia, and
Aspronisi, in the Mediterranean, there arose, 160 years before our era,
the island of _Hyera_, which was enlarged by the upheaval of islets on
its margin during the years 19, 726, and 1427. Again, in 1773,
Micra-Kameni, and in 1707, Nea-Kameni, made their appearance. These
islands increased in size successively in 1709, in 1711, in 1712.
According to ancient writers, Santorin, Tharasia, and Aspronisi, made
their appearance many ages before the Christian era, at the termination
of earthquakes of great violence.
METAMORPHIC ROCKS.
The rocks composing the terrestrial crust have not always remained in
their original state. They have frequently undergone changes which have
altogether modified their properties, physical and chemical.
When they present these characteristics, we term them _Metamorphic
Rocks_. The phenomena which belong to this subject are at once important
and new, and have lately much attracted the attention of geologists. We
shall best enlighten our readers on the metamorphism of rocks, if we
treat of it under the heads of _special_ and _general_ metamorphism.
When a mass of eruptive rock penetrates the terrestrial crust it
subjects the rocks through which it passes to a special metamorphism--to
the effects of _heat_ produced by _contact_. Such effects may almost
always be observed near the margin of masses of eruptive rock, and they
are attributable either to the communicated heat of the eruptive rock
itself, or to the disengagement of gases, of steam, or of mineral and
thermal waters, which have accompanied its eruption. The effects vary
not only with the rock ejected, but even with the nature of the rock
surrounding it.
In the case of volcanic lava ejected in a molten state, for instance,
the modifications it effects on the surrounding rock are very
characteristic. Its structure becomes prismatic, full of cracks, often
cellular and scoriaceous. Wood and other combustibles touched by the
lava are consumed or partially carbonised. Limestone assumes a granular
and crystalline texture. Siliceous rocks are transformed, not only into
quartz like glass, but they also combine with various bases, and yield
vitreous and cellular silicates. It is nearly the same with argillaceous
rocks, which adhere together, and frequently take the colour of red
bricks.
The surrounding rock is frequently impregnated with specular iron-ore,
and penetrated with hydrochloric or sulphuric acid, and by divers salts
formed from these acids.
At a certain distance from the place of contact with the lava, the
action of water aided by heat produces silica, carbonate of lime,
aragonite, zeolite, and various other minerals.
From immediate contact with the lava, then, the metamorphic rocks denote
the action of a very strong heat. They bear evident traces of
calcination, of softening, and even of fusion. When they present
themselves as hydrosilicates and carbonates, the silica and associated
minerals are most frequently at some distance from the points of
contact; and the formation of these minerals is probably due to the
combination of water and heat, although this last ceases to be the
principal agent.
The hydrated volcanic rocks, such as the basalts and trappean rocks in
general, continue to produce effects of metamorphism, in which heat
operates, although its influence is inconsiderable, water being much the
more powerful agent. The metamorphosis which is observable in the
structure and mineralogical composition of neighbouring rocks is as
follows:--The structure of separation becomes fragmentary, columnar, or
many-sided, and even prismatic. It becomes especially prismatic in
combustibles, in sandstones, in argillaceous formations, in felspathic
rocks, and even in limestones. Prisms are formed perpendicular to the
surface of contact, their length sometimes exceeding six feet. Most
commonly they still contain water or volatile matter. These characters
may be observed at the junction of the basalts which has been ejected
upon the argillaceous strata near Clermont in Auvergne, at Polignac, and
in the neighbourhood of Le Puy-en-Velay.
If the vein of Basalt or Trap has traversed a bed of coal or of lignite,
we find the combustible strongly _metamorphosed_ at the point of
contact. Sometimes it becomes cellular and is changed into _coke_. This
is especially the case in the coal-basin of Brassac. But more frequently
the coal has lost all, or part of, its bituminous and volatile
matter--it has been metamorphosed into anthracite--as an example we may
quote the lignite of Mont Meisner.
Again, in some exceptional cases, the combustible may even be changed
into graphite near to its junction with Trap. This is observed at the
coal-mine of New Cumnock in Ayrshire.
When near its junction with a _trappean_ rock, a combustible has been
metamorphosed into _coke_ or anthracite, it is also frequently
impregnated by hydrated oxide of iron, by clay, foliated carbonate of
lime, iron pyrites, and by various mineral veins. It may happen that the
combustible has been reduced to a pulverulent state, in which case it is
unfit for use. Such is the case in a coal-mine at Newcastle, where the
coal lies within thirty yards of a dyke of Trap.
When Basalt and Trap have been ejected through limestone rock, the
latter becomes more or less altered. Near the points of contact, the
metamorphism which they have undergone is revealed by the change of
colour and aspect, which is exhibited all around the vein, often also by
the development of a crystalline structure. Limestone becomes granular
and saccharoid--it is changed into marble. The most remarkable instance
of this metamorphism is the Carrara marble, a non-fossiliferous
limestone of the Oolite series, which has been altered and the fossils
destroyed; so that the marble of these celebrated quarries, once
supposed to have been formed before the creation of organic beings, is
now shown to be an altered limestone of the Oolitic period, and the
underlying crystalline schists are sandstones and shales of secondary
age modified by plutonic action.
The action of basalt upon limestone is observable at Villeneuve de Berg,
in Auvergne; but still more in the neighbourhood of Belfast, where we
may see the Chalk changed into saccharoid limestone near to its contact
with the Trap. Sometimes the metamorphism extends many feet from the
point of contact; nay, more than that, some zeolites and other minerals
seem to be developed in the crystallised limestone.
When sandstone is found in contact with trappean rock, it presents
unequivocal traces of metamorphism; it loses its reddish colour and
becomes white, grey, green, or black; parallel veins may be detected
which give it a jaspideous structure; it separates into prisms
perpendicular to the walls of the injected veins, when it assumes a
brilliant and vitreous lustre. Sometimes it is even also found
penetrated by zeolites, a family of minerals which melt before the
blowpipe with considerable ebullition. The mottled sandstones of
Germany, which are traversed by veins of basalt, often exhibit
metamorphism, particularly at Wildenstern, in Würtemberg.
Argillaceous rocks, like all others, are subject to metamorphism when
they come in contact with eruptive trappean rocks. In these
circumstances they change colour and assume a varied or prismatic
structure; at the same time their hardness increases, and they become
lithoidal or stony in structure. They may also become cellular--form
zeolites in their cavities with foliated carbonate of lime, as well as
minerals which commonly occur in amygdaloid. Sometimes even the fissures
are coated by the metallic minerals, and the other minerals which
accompany them in their metalliferous beds. Generally they lose a part
of their water and of their carbonic acid. In other circumstances they
combine with oxide of iron and the alkalies. This has been asserted, for
example, at Essey, in the department of the Meurthe, where a very
argillaceous sandstone is found, charged with jasper porcellanite, near
to the junction of the rock with a vein of basalt.
Hitherto we have spoken only of the metamorphosis the result of volcanic
action. A few words will suffice to acquaint the reader with the
metamorphism exercised by the porphyries and granites. By contact with
granite, we find coal changed into anthracite or graphite. It is
important to note, however, that coal has seldom been metamorphosed into
coke. As to the limestone, it is sometimes, as we have seen, transformed
into marble; we even find in its interior divers minerals, notably
silicates with a calcareous base, such as garnets, pyroxene, hornblende,
&c. The sandstones and clay-slates have alike been altered.
The surrounding deposit and the eruptive rock are both frequently
impregnated with quartz, carbonate of lime, sulphate of baryta,
fluorides, and, in a word, with the whole tribe of metalliferous
minerals, which present themselves, besides, with the characteristics
which are common to them in the veins.
GENERAL METAMORPHISM.
Sedimentary rocks sometimes exhibit all the symptoms of metamorphism
where there is no evidence of direct eruptive action, and that upon a
scale much grander than in the case of special metamorphism. It is
observable over whole regions, in which it has modified and altered
simultaneously all the surrounding rocks. This state of things is called
general, or normal, metamorphism. The fundamental gneiss, which covers
such a vast extent of country, is the most striking instance known of
general metamorphism. It was first described by Sir W. E. Logan,
Director of the Canadian Geological Survey, who estimates its thickness
at 30,000 feet. The Laurentian Gneiss is a term which is used by
geologists to designate those metamorphic rocks which are known to be
older than the Cambrian system. They are parts of the old pre-Cambrian
continents which lie at the base of the great American continent,
Scandinavia, the Hebrides, &c.; and which are largely developed on the
west coast of Scotland. In order to give the reader some idea of this
metamorphism, we shall endeavour to trace its effects in rocks of the
same nature, indicating the characters successively presented by the
rocks according to the intensity of the metamorphism to which they have
been subjected.
Combustibles, which have a special composition, totally different from
all other rocks, are obviously the first objects of examination. When we
descend in the series of sedimentary deposits, the combustibles are
observed completely to change their characters. From the _peat_ which is
the product of our own epoch, we pass to _lignite_, to _coal_, to
_anthracite_, and even to _graphite_; and find that their density
increases, varying up to at least double. Hydrogen, nitrogen, and,
above all, oxygen, diminish rapidly. Volatile and bituminous matters
decrease, while carbon undergoes a proportionate increase.
This metamorphism of the combustible minerals, which takes place in
deposits of different ages, may also be observed even in the same bed.
For instance, in the coal formations of America, which extend to the
west of the Alleghany mountains, the Coal-measures contain a certain
proportion of volatile matter, which goes on diminishing in proportion
as we approach the granite rocks; this proportion rises to fifty per
cent. upon the Ohio, but it falls to forty upon the Manon-Gahela, and
even to sixteen in the Alleghanies. Finally, in the regions where the
strata have been most disturbed, in Pennsylvania and Massachusetts, the
coal has been metamorphosed into anthracite and even into graphite or
plumbago.
Limestone is one of the rocks upon which we can most easily follow the
effects of general metamorphism. When it has not been modified, it is
usually found in sedimentary rocks in the state of compact limestone, of
coarse limestone, or of earthy limestone such as chalk. But let us
consider it in the mountains, especially in mountains which are at the
same time granitic, such as the Pyrenees, the Vosges, and the Alps. We
shall then see its characters completely modified. In the long and deep
valleys of the Alps, for example, we can follow the alterations of the
limestone for many leagues, the beds losing more and more their
regularity in proportion as we approach the central chain, until they
lose themselves in solitary pinnacles and projections enclosed in
crystalline schists or granitic rocks. Towards the upper regions of the
Alps the limestone divides itself into pseudo-regular fragments, it is
more strongly cemented, more compact, more sonorous; its colour becomes
paler, and it passes from black to grey by the gradual disappearance of
organic and bituminous matter with which it has been impregnated, at the
same time its crystalline structure increases in a manner scarcely
perceptible. It may even be observed to be metamorphosed into an
aggregate of microscopic crystals, and finally to pass into a white
saccharoid limestone.
This metamorphism is produced without any decomposition of the
limestone; it has rather been softened and half melted by the heat, that
is, rendered plastic, so to speak, for we find in it fossils still
recognisable, and among these, notably, some Ammonites and Belemnites,
the presence of which enables us to state that it is the greyish-black
Jurassic limestone, which has been transformed into white saccharoid or
granular limestone. If the limestone subjected to this transformation
were perfectly pure, it would simply take a crystalline structure; but
it is generally mixed with sand and various argillaceous matters, which
have been deposited along with it, matters which go to form new
minerals. These new minerals, however, are not disseminated by chance;
they develop themselves in the direction of the lamination, so to speak,
of the limestone, and in its fissures, in such a manner that they
present themselves in nodules, seams, and sometimes in veins.
Among the principal minerals of the saccharoid limestone we may mention
graphite, quartz, some very varied silicates, such as andalusite,
disthene, serpentine, talc, garnet, augite, hornblende, epidote,
chlorite, the micas, the felspars; finally, spinel, corundum, phosphate
of lime, oxide of iron and oligiste, iron pyrites, &c. Besides these,
various minerals in veins figure among those which exist more commonly
in the saccharoid limestone.
When metamorphic limestone is sufficiently pure, it is employed as
statuary marble. Such is the geological origin of Carrara marble, which
is quarried in the Apuan Alps on a great scale; such, also, was the
marble of Paros and Antiparos, still so celebrated for its purity. On
examination, however, with the lens the Carrara marble exhibits blackish
veins and spangles of graphite; the finest blocks, also, frequently
contain nodules of ironstone, which are lined with perfectly limpid
crystals of quartz. These accidental defects are very annoying to the
sculptor, for they are very minute, and nothing on the exterior of the
block betrays their existence. In the marble of Paros, even when it is
strongly translucent, specks of mica are often found. In the ancient
quarries the nodules are so numerous as to have hindered their being
worked, up even to the present time.
When the mica which occurs in granular limestone takes a green colour
and forms veins, it constitutes the Cipoline marble, which is found in
Corsica, and in the Val Godemar in the Alps. Some white marbles are
quarried in France, chiefly at Loubie, at Sost, at Saint-Béat in the
Pyrenees, and at Chippal in the Vosges. In our country, and especially
in Ireland, there are numerous quarries of marble, veined and coloured
of every hue, but none of a purity suitable for the finest statuary
purposes. All these marbles are only metamorphosed limestones.
The white marbles employed almost all over the world are those of
Carrara. They result from the metamorphism of limestone of the Lias.
They have not been penetrated by the eruptive rocks, but they have been
subjected upon a great scale to a general metamorphism, to which their
crystalline structure may be attributed.
It is easily understood that the calcareous strata have not undergone
such an energetic metamorphism without the beds of sandstone and clay,
associated with them, having also undergone some modification of the
same kind. The siliceous beds accompanying the saccharoid limestone
have, in short, a character of their own. They are formed of small
grains of transparent quartz more or less cemented one to the other in a
manner strongly resembling those of the saccharoid limestone. Between
these grains are usually developed some lamellæ of mica of brilliant and
silky lustre, of which the colour is white, red, or green; in a word, it
has produced a _quartzite_. Some veins of quartz frequently traverse
this quartzite in all directions. Independent of the mica, it may
contain, besides, the different minerals already mentioned as occurring
in the limestone, and particularly silicates--such as disthene,
andalusite, staurotide, garnet, and hornblende.
The argillaceous beds present a series of metamorphisms analogous to the
preceding. We can follow them readily through all their gradations when
we direct our attention towards such granitic masses as those which
constitute the Alps, Pyrenees, the Bretagne Mountains, or our own
Grampians. The schists may perhaps be considered the first step towards
the metamorphism of certain argillaceous rocks; in fact, the schists are
not susceptible of mixing with water like clay; they become stony, and
acquire a much greater density, but their chief characteristic is a
foliated structure.
Experiment proves that when we subject a substance to a great pressure a
foliated structure is produced in a direction perpendicular to that in
which the pressure is exercised. Everything leads us, therefore, to
believe that pressure is the principal cause of the schistous texture,
and of the foliation of clay-slates, the most characteristic variety of
which is the roofing-slate which is quarried so extensively in North
Wales, in Cumberland, and various parts of Scotland in the British
Islands; in the Ardennes; and in the neighbourhood of Angers, in France.
In some localities the slate becomes siliceous and is charged with
crystals of felspar. Nevertheless, it still presents itself in parallel
beds, and contains the same fossil remains still in a recognisable
state. For example, in the neighbourhood of Thann, in the Vosges,
certain vegetable imprints are perfectly preserved in the metamorphic
schist, and in their midst are developed some crystals of felspar.
Mica-schist, which is formed of layers of quartz and mica, is found
habitually associated with rocks which have taken a crystalline
structure, proceeding evidently from an energetic metamorphism of beds
originally argillaceous. Chiastolite, disthene, staurotide, hornblende,
and other minerals are found in it. Mica-schists occur extensively in
Brittany, in the Vosges, in the Pyrenees. In all cases, as we approach
the masses of granite, in these regions, the crystalline structure
becomes more and more marked.
In describing the various facts relating to the metamorphism of rocks,
we have said little of the causes which have produced it. The causes
are, indeed, in the region of hypothesis, and somewhat mysterious.
In what concerns special metamorphism, the cause is supposed to admit of
easy explanation--it is heat. When a rock is ejected from the interior
of the earth in a state of igneous fusion, we comprehend readily enough
that the strata, which it traverses, should sustain alterations due to
the influence of heat, and varying with its intensity. This is clear
enough in the case of _lava_. On the other hand, as water always exists
in the interior of the earth’s crust, and as this water must be at a
very high temperature in the neighbourhood of volcanic fires, it
contributes, no doubt, largely to the metamorphism. If the rocks have
not been ejected in a state of fusion, it is evidently water, with the
different mineral substances it holds in solution, which is the chief
actor in the special metamorphism which is produced.
In general metamorphism, water appears still to be the principal agent.
As it is infiltered through the various beds it will modify their
composition, either by dissolving certain substances, or by introducing
into the metalliferous deposits certain new substances, such as may be
seen forming, even under our eyes, in mineral springs. This has tended
to render the sedimentary deposits plastic, and has permitted the
development of that crystalline structure, which is one of the principal
characteristics of metamorphic rocks. This action has been seconded by
other causes, notably by heat and pressure, which would have an immense
increase of power and energy when metamorphism takes place at a great
depth beneath the surface. Dr. Holl, in an able paper descriptive of the
geology of the Malvern Hills, read before the Geological Society in
February, 1865, adopts this hypothesis as explanatory of the vast
phenomena which are there displayed. After describing the position of
this interesting and strangely-mingled range of rocks, he adds: “These
metamorphic rocks are for the most part highly inclined, and often in a
position nearly vertical. Their disturbance and metamorphism, their
being traversed by granitic veins, and still later their invasion by
trap-dykes and their subsequent elevation above the sea-level, were all
events which must have occupied no inconsiderable period, even of
geological time. I presume,” he adds, “that it will not be maintained
in the present day that the metamorphism of rocks over areas of any but
very moderate extent is due to the intrusion of veins and erupted
masses. The insufficiency of such agency becomes the more obvious when
we consider the slight effects produced by even tolerably extensive
outbursts, such as the Dartmoor granite; while in the case of the
Malverns there is an absence of any local cause whatever. The more
probable explanation in the case of these larger areas is, that they
were faulted down, or otherwise depressed, so as to be brought within
the influence of the earth’s internal heat, and this is the more likely
as they belong to an epoch when the crust is believed to have been
thinner.” When it is considered that, according to the doctrine of
modern geology, the Laurentian rocks, or their equivalents, lie at the
base of all the sedimentary deposits; that this, like other systems of
stratified rocks, was deposited in the form of sand, mud, and clay, to
the thickness of 30,000 feet; and that over an area embracing
Scandinavia, the Hebrides, great part of Scotland, and England as far
south as the Malverns, besides a large proportion of the American
continent, with certain forms of animal life, as recent investigations
demonstrate--can the mind of man realise any other cause by which this
vast extent of metamorphism could have been produced?
Electric and galvanic currents, circulating in the stratified crust, are
not to be overlooked. The experiments of Mr. R. W. Fox and Mr. Robert
Hunt suggest that, in passing long-continued galvanic currents through
masses of moistened clay, there is a tendency to produce cleavage and a
semi-crystalline arrangement of the particles of matter.[31]
[31] Report of the Royal Cornwall Polytechnic Society for 1837. Robert
Hunt, in “Memoirs of the Geological Survey of Great Britain,”
vol. i., p. 433.
THE BEGINNING.
The theory which has been developed, and which considers the earth as an
extinct sun, as a star cooled down from its original heated condition,
as a nebula, or luminous cloud, which has passed from the gaseous to the
solid state--this fine conception, which unites so brilliantly the
kindred sciences of astronomy and geology, belongs to the French
mathematician, Laplace, the immortal author of the “Mécanique Céleste.”
The hypothesis of Laplace assigns to the sun, and to all bodies which
gravitate in what Descartes calls his _tourbillon_, a common origin. “In
the primitive state in which we must suppose the sun to be,” he says,
“it resembles one of those nebulæ which the telescope reveals to us,
consisting of a more or less brilliant central _nucleus_, surrounded by
luminous clouds, which clouds, condensing at the surface, become
transformed into a star.”
It has been calculated that the centre of the earth has a temperature of
about 195,000° Cent., a degree of heat which surpasses all that the
imagination can conceive. We can have no difficulty in admitting that,
at a heat so excessive, all the substances which now enter into the
composition of the globe would be reduced to the state of gas or vapour.
Our planet, then, must have been originally an aggregation of aëriform
fluids--a mass of matter entirely gaseous; and if we reflect that
substances in their gaseous state occupy a volume eighteen hundred times
larger than when solid, we shall have some conception of the enormous
volume of this gaseous mass. It would be as large as the sun, which is
fourteen hundred thousand times larger than the terrestrial sphere. In
Fig. 12 we have attempted to give an idea of the vast difference of
volume between the earth in its present solid state and in its primitive
gaseous condition. One of the globes, A, represents the former, B the
latter. It is simply a comparison of size, which is made the more
strikingly apparent by means of these geometrical figures--one the
twentieth part of an inch in diameter, the other two inches and three
quarters.
[Illustration: VI.--The Earth circulating in space in the state of a
gaseous star.]
[Illustration: Fig. 12.--Comparative volume of the earth in the gaseous
and solid state.]
At this excessive temperature the gaseous mass, which we have described,
would shine in space as the sun does at the present day; and with the
same brilliancy as that with which, to our eyes, the fixed stars and
planets shine in the serenity of night, as represented on the opposite
page (PLATE VI.). Circulating round the sun in obedience to the laws of
universal gravitation, this incandescent gaseous mass was necessarily
regulated by the laws which govern other material substances. As it got
cooler it gradually transferred part of its warmth to the glacial
regions of the inter-planetary spaces, in the midst of which it traced
the line of its flaming orbit. Consequent on its continual cooling (but
at the end of a period of time of which it would be impossible, even
approximately, to fix the duration), the star, originally gaseous, would
attain a liquid state. It would then be considerably diminished in
volume.
The laws of mechanics teach us that liquid bodies, when in a state of
rotation, assume a spherical form; it is one of the laws of their being,
emanating from the Creator, and is due to the force of attraction. Thus
the Earth takes the spheroidal form, belonging to it, in common with the
greater number of the celestial bodies.
The Earth is subject to two distinct movements; namely, a movement of
translation round the sun, and a movement of rotation on its own
axis--the latter a uniform movement, which produces the regular
alternations of days and nights. Mechanics have also established the
fact, which is confirmed by experiment, that a fluid mass in motion
produces (as the result of the variation of the centrifugal force on its
different diameters), a swelling towards the equatorial diameter of the
sphere, and a flattening at the poles or extremities of its axis. It is
in consequence of this law, that the Earth, when it was in a liquid
state, became swollen at the equator, and depressed at its two poles;
and that it has passed from its primitive spherical form to the
spheroidal--that is, has become flattened at each of its polar
extremities, and has assumed its present shape of an oblate spheroid.
This bulging at the equator and flattening towards the poles afford the
most direct proofs, that can be adduced, of the original liquid state of
our planet. A solid and non-elastic sphere--a stone ball, for
example--might turn for ages upon its axis, and its form would sustain
no change; but a fluid ball, or one of a pasty consistence, would swell
out towards the middle, and, in the same proportion, become flattened at
the extremities of its axis. It was upon this principle, namely, by
admitting the primitive fluidity of the globe, that Newton announced _à
priori_ the bulging of the globe at the equator and its flattening at
the poles; and he even calculated the amount of this depression. The
actual measurement, both of this expansion and flattening, by
Maupertuis, Clairaut, Camus, and Lemonnier, in 1736, proved how exact
the calculations of the great geometrican were. Those gentlemen,
together with the Abbé Outhier, were sent into Lapland by the Academy of
Sciences; the Swedish astronomer, Celsius, accompanied them, and
furnished them with the best instruments for measuring and surveying. At
the same time the Academy sent Bouguer and Condamine to the equatorial
regions of South America. The measurements taken in both these regions
established the existence of the equatorial expansion and the polar
depression, as Newton had estimated it to be in his calculations.
It does not follow, as a consequence of the partial cooling down of the
terrestrial mass, that all the gaseous substances composing it should
pass into a liquid state; some of these might remain in the state of gas
or vapour, and form round the terrestrial spheroid an outer envelope or
_atmosphere_ (from the Greek words ατμος, _vapour_, and σφαιρα,
_sphere_). But we should form a very inexact idea of the atmosphere
which surrounded the globe, at this remote period, if we compared it
with that which surrounds it now. The extent of the gaseous matter which
enveloped the primitive earth must have been immense; it doubtless
extended to the moon. It included, in short, in the state of vapour, the
enormous body of water which, as such, now constitutes our existing
seas, added to all the other substances which preserve their gaseous
state at the temperature then exhibited by the incandescent earth; and
it is certainly no exaggeration to place this temperature at 2,000°
Centigrade. The atmosphere would participate in this temperature; and
acted on by such excessive heat, the pressure that it would exert on the
Earth would be infinitely greater than that which it exercises at the
present time. To the gases which form the component parts of the present
atmospheric air--namely, nitrogen, oxygen, and carbonic acid--to
enormous masses of watery vapour, must be added vast quantities of
mineral substances, metallic or earthy, reduced to a gaseous state, and
maintained in that state by the temperature of this gigantic furnace.
The metals, the chlorides--metallic, alkaline, and earthy--sulphur, the
sulphides, and even the silicates of alumina and lime; all, at this
temperature, would exist in a vaporous form in the atmosphere
surrounding the primitive globe.
It is to be inferred that, under these circumstances, the different
substances composing this atmosphere would be ranged round the globe in
the order of their respective densities; the first layer--that nearest
to the surface of the globe--being formed of the heavier vapours, such
as those of the metals, of iron, platinum, and copper, mixed doubtless
with clouds of fine metallic dust produced by the partial condensation
of their vapours. This first and heaviest zone, and the thickest also,
would be quite opaque, although the surface of the earth was still at a
red heat. Above it would come the more vaporisable substances, such as
the metallic and alkaline chlorides, particularly the chloride of sodium
or common salt, sulphur and phosphorus, with all the volatile
combinations of these substances. The upper zone would contain matter
still more easily converted into vapour, such as water (steam), together
with others naturally gaseous, as oxygen, nitrogen, and carbonic acid.
This order of superposition, however, would not always be preserved. In
spite of their differences of density, these three atmospheric layers
would often become mixed, producing formidable storms and violent
ebullitions; frequently throwing down, rending, upheaving, and
confounding these incandescent zones.
As to the globe itself, without being so much agitated as its hot and
shifting atmosphere, it would be no less subject to perpetual tempests,
occasioned by the thousand chemical actions which took place in its
molten mass. On the other hand, the electricity resulting from these
powerful chemical actions, operating on such a vast scale, would induce
frightful electric detonations, thunder adding to the horror of this
primitive scene, which no imagination, no human pencil could trace, and
which constitutes that gloomy and disastrous chaos of which the
legendary history of every ancient race has transmitted the tradition.
In this manner would our globe circulate in space, carrying in its train
the flaming streaks of its multiple atmosphere, unfitted, as yet, for
living beings, and impenetrable to the rays of the sun, around which it
described its vast orbit.
The temperature of the planetary regions is infinitely low; according to
Laplace it cannot be estimated at less than 100° below zero. The glacial
regions traversed in its course by the incandescent globe would
necessarily cool it, at first superficially, when it would assume a
pasty consistence. It must not be forgotten that the earth, on account
of its liquid state, would be obedient in all its mass to the action of
flux and reflux, which proceeds from the attraction of the sun and moon,
but to which the sea alone is now subject. This action, to which all its
liquid and movable particles were subject, would singularly accelerate
the commencement of the solidification of the terrestrial mass. It would
thus gradually assume that sort of consistence which iron attains, when
it is first withdrawn from the furnace, in the process of puddling.
As the earth cooled, beds of concrete substances would necessarily be
formed, which, floating at first in isolated masses on the surface of
the semi-fluid matter, would in course of time come together,
consolidate, and form continuous banks; just as we see with the ice of
the present Polar Seas, which, when brought in contact by the agitation
of the waves, coalesces and forms icebergs, more or less movable. By
extending this phenomenon to the whole surface of the globe, the
solidification of its entire surface would be produced. A solid, but
still thin and fragile crust, would thus envelop the whole earth,
enclosing entirely its still fluid interior. The entire consolidation
would necessarily be a much slower process--one which, according to the
received hypothesis, is very far from being completed at the present
time; for it is estimated that the actual thickness of the earth’s crust
does not exceed thirty miles, while the mean radius or distance from the
centre of the terrestrial sphere, approaches 4,000 miles, the mean
diameter being 7,912·409 miles; so that the portion of our planet,
supposed to be solidified, represents only a very small fraction of its
total mass.
[Illustration: Fig. 13.--Relative volumes of the solid crust and liquid
mass of the globe.]
We say thirty miles, for such is the ordinary estimated thickness of the
earth’s crust, usually admitted by savants; and the following is the
process by which this result has been obtained.
We know that the temperature of the earth increases one degree
Centigrade for every hundred feet of descent. This result has been
borne out by a great number of measurements, made in many of the mines
of France, in the tin mines of Cornwall, in the mines of the Erzgeberge,
of the Ural, of Scotland, and, above all, in the soundings effected in
the Artesian wells of Grenelle and Passy, near Paris, of St. André de
Iregny, and at a great number of other points.
The greatest depth to which miners have hitherto penetrated is about 973
yards, which has been reached in a boring executed in Monderf, in the
Grand Duchy of Luxembourg. At Neusalzwerk, near Minden, in Prussia,
another boring has been carried to the depth of 760 yards. In the
coal-mines of Monkwearmouth the pits have been sunk 525 yards, and at
Dukinfield 717 yards. The mean of the thermometic observations made, at
all these points, leads to the conclusion that the temperature increases
about one degree Fahrenheit for every sixty feet (English) of descent
after the first hundred.
In admitting that this law of temperature exists for all depths of the
earth’s crust, we arrive at the conclusion that, at a depth of from
twenty-five to thirty-five miles--which is only about five times the
height of the highest mountains--the most refractory matter would be in
a state of fusion. According to M. Mitscherlich, the flame of hydrogen,
burning in free air, acquires a temperature of 1,560° Centigrade. In
this flame platinum would be in a state of fusion. Granite melts at a
lower temperature than soft iron, that is at about 1,300°; while silver
melts at 1,023°. In imagining an increase of temperature equal to one
degree for every hundred feet of descent, the temperature at twenty-five
miles will be 1,420° C. or 2,925° F.; thirty miles below the surface
there will be a probable temperature of 1,584° C. or 3,630° F.; it
follows, if these arguments be admitted, and the calculation correct,
that the thickness of the solid crust of the globe does not much exceed
thirty miles.
This result, which gives to the terrestrial crust a thickness equal to
1/190 of the earth’s diameter, has nothing, it is true, of absolute
certainty.
Prof. W. Hopkins, F.R.S., an eminent mathematician, has much insisted
upon the fact, that the conductibility of granite rocks, for heat, is
much greater than that of sedimentary rocks; and he argues that in the
lower stratum of the earth the temperature increases much more slowly
than it does nearer the surface. This consideration has led Mr. Hopkins
to estimate the probable thickness of the earth’s solid crust at a
minimum of 200 miles.
In support of this estimate Mr. Hopkins puts forward another argument,
based upon the precession of the equinoxes. We know that the terrestrial
axis, instead of always preserving the same direction in space,
revolves in a cone round the pole of the ecliptic. Our globe, it is
calculated, will accomplish its revolution in about 25,000 years. In
about this period it will return to its original position. This
balancing, which has been compared to that of a top when about to cease
spinning, produces the movement known as the _precession of the
equinoxes_. It is due to the attraction which the sun and moon exercise
upon the swelling equatorial of the globe. This attraction would act
very differently upon a globe entirely solid, and upon one with a liquid
interior, covered by a comparatively thin crust. Mr. Hopkins subjected
this curious problem to mathematical analysis, and he calculated that
the precession of the equinoxes, observed by astronomers, could only be
explained by admitting that the solid shell of the earth could not be
less than from about 800 to 1,000 miles in thickness.
In his researches on the _rigidity of the earth_, Sir William Thomson
finds that the phenomena of precession and nutation require that the
earth, if not solid to the core, must be nearly so; and that no
continuous liquid vesicle at all approaching 6,000 miles in diameter can
possibly exist in the earth’s interior, without rendering the phenomena
in question very sensibly different from what they are.
The calculations of Mr. Hennessey are in direct opposition to those of
Sir William Thomson, and show that the earth’s crust cannot be less than
eighteen miles, or more than 600 miles in thickness.
Admitting, for the present, that the terrestrial crust is only thirty
miles in thickness, we can express in a familiar, but very intelligible
fashion, the actual relation between the dimensions of the liquid
nucleus and the solid crust of the earth. If we imagine the earth to be
an orange, a tolerably thick sheet of paper applied to its surface will
then represent, approximately, the thickness of the solid crust which
now envelopes the globe. Fig. 13 will enable us to appreciate this fact
still more correctly. The terrestrial sphere having a mean diameter of
7,912 miles, or a mean radius of 3,956 miles, and a solid crust about
thirty miles thick, which is 1/260 of the diameter, or 1/130 of the
radius, the engraving may be presumed to represent these proportions
with sufficient accuracy.
To determine, even approximately, the time such a vast body would take
in cooling, so as to permit of the formation of a solid crust, or to fix
the duration of the transformations which we are describing, would be an
impossible task.
[Illustration: Fig. 14.--Formation of primitive granitic mountains.]
The first terrestrial crust formed, as indicated, would be incapable of
resisting the waves of the ocean of internal fire, which would be
depressed and raised up at its daily flux and reflux in obedience to
the attraction of the sun and moon. Who can trace, even in imagination,
the fearful rendings, the gigantic inundations, which would result from
these movements! Who would dare to paint the sublime horrors of these
first mysterious convulsions of the globe! Amid torrents of molten
matter, mixed with gases, upheaving and piercing the scarcely
consolidated crust, large crevices would be opened, and through these
gaping cracks waves of liquid granite would be ejected, and then left to
cool and consolidate on the surface. Fig. 14 represents the formation of
a primitive granitic mountain, by the eruption of the internal granitic
matter which forces its way to the surface through a fracture in the
crust. In some of these mountains, Ben Nevis for example, three
different stages of the eruption can be traced. “Ben Nevis, now the
undoubted monarch of the Scottish mountains,” says Nicol, “shows well
the diverse age and relations of igneous rocks. The Great Moor from
Inverlochy and Fort William to the foot of the hill is gneiss. Breaking
through, and partly resting on the gneiss is granite, forming the lower
two-thirds of the mountain up to the small tarn on the shoulder of the
hill. Higher still is the huge prism of porphyry, rising steep and
rugged all around.” In this manner would the first mountains be formed.
In this way, also, might some metallic veins be ejected through the
smaller openings, true injections of eruptive matter produced from the
interior of the globe, traversing the primitive rocks and constituting
the precious depository of metals, such as copper, zinc, antimony, and
lead. Fig. 15 represents the internal structure of some of these
metallic veins. In this case the fracture is only a fissure in the rock,
which soon became filled with injected matter, often of different kinds,
which in crystallising would completely fill the hollow of this cleft,
or crack; but sometimes forming cavities or geodes as a result of the
contraction of the mass.
[Illustration: Fig. 15.--Metallic veins.]
But some eruptions of granitic and other substances, ejected from the
interior, never reach the surface at all. In such cases the clefts and
crevices--longitudinal or oblique--are filled, but the fissures in the
crust do not themselves extend to the surface. Fig. 16 represents an
eruption of granite through a mass of sedimentary rock--the granite
ejected from the centre fills all the clefts and fractures, but it has
not been sufficiently powerful to force its way to the surface.
[Illustration: Fig. 16.--Eruption of granite.]
On the surface of the earth, then, which would be at first smooth and
unbroken, there were formed, from the very beginning, swelling
eminences, hollows, foldings, corrugations, and crevices, which would
materially alter its original aspect; its arid and burning surface
bristled with rugged protuberances, or was traversed by enormous
fissures and cracks. Nevertheless, as the globe continued to cool, a
time arrived when its temperature became insufficient to maintain, in a
state of vapour, the vast masses of water which floated in the
atmosphere. These vapours would pass into the liquid state, and then the
first rain fell upon the earth. Let us here remark that these were
veritable rains of boiling water; for in consequence of the very
considerable pressure of the atmosphere, water would be condensed and
become liquid at a temperature much above 100° Centigrade (212° Fahr.)
[Illustration: VII.--Condensation and rainfall on the primitive globe.]
The first drop of water, which fell upon the still heated terrestrial
sphere, marked a new period in its evolution--a period the mechanical
and chemical effects of which it is important to analyse. The contact of
the condensed water with the consolidated surface of the globe opens up
a series of modifications of which science may undertake the examination
with a degree of confidence, or at least with more positive elements of
appreciation than any we possess for the period of chaos; some of the
features of which we have attempted to represent, leaving of necessity
much to the imagination, and for the reader to interpret after his own
fashion.
The first water which fell, in the liquid state, upon the slightly
cooled surface of the earth would be rapidly converted into steam by the
elevation of its temperature. Thus, rendered much lighter than the
surrounding atmosphere, these vapours would rise to the utmost limits of
the atmosphere, where they would become condensed afresh, in consequence
of their
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