Young Folks' Library, Volume XI (of 20) by Various

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Even now our knowledge carries us far enough to warrant the assertion
that there was a time when our earth was in a state of igneous fusion,
when no ocean bathed it and no atmosphere surrounded it, when no wind
blew over it and no rain fell upon it, but an intense heat held all
its materials in solution. In those days the rocks which are now the
very bones and sinews of our mother Earth--her granites, her
porphyries, her basalts, her syenites--were melted into a liquid mass.
As I am writing for the unscientific reader, who may not be familiar
with the facts through which these inferences have been reached, I
will answer here a question which, were we talking together, he might
naturally ask in a somewhat sceptical tone. How do you know that this
state of things ever existed, and, supposing that the solid materials
of which our earth consists were ever in a liquid condition, what
right have you to infer that this condition was caused by the action
of heat upon them? I answer, Because it is acting upon them still;
because the earth we tread is but a thin crust floating on a liquid
sea of molten materials; because the agencies that were at work then
are at work now, and the present is the logical sequence of the past.
From artesian wells, from mines, from geysers, from hot springs, a
mass of facts has been collected, proving incontestably the heated
condition of all substances at a certain depth below the earth's
surface; and if we need more positive evidence, we have it in the
fiery eruptions that even now bear fearful testimony to the molten
ocean seething within the globe and forcing its way but from time to
time. The modern progress of Geology has led us by successive and
perfectly connected steps back to a time when what is now only an
occasional and rare phenomenon was the normal condition of our earth;
when the internal fires were enclosed by an envelope so thin that it
opposed but little resistance to their frequent outbreak, and they
constantly forced themselves through this crust, pouring out melted
materials that subsequently cooled and consolidated on its surface. So
constant were these eruptions, and so slight was the resistance they
encountered, that some portions of the earlier rock-deposits are
perforated with numerous chimneys, narrow tunnels as it were, bored by
the liquid masses that poured out through them and greatly modified
their first condition.

[Illustration: IDEAL SECTION OF A VOLCANO IN ACTION.]

The question at once suggests itself, How was even this thin crust
formed? what should cause any solid envelope, however slight and filmy
when compared to the whole bulk of the globe, to form upon the surface
of such a liquid mass? At this point of the investigation the
geologist must appeal to the astronomer; for in this vague and
nebulous border-land, where the very rocks lose their outlines and
flow into each other, not yet specialized into definite forms and
substances,--there the two sciences meet. Astronomy shows us our
planet thrown off from the central mass of which it once formed a
part, to move henceforth in an independent orbit of its own. That
orbit, it tells us, passed through celestial spaces cold enough to
chill this heated globe, and of course to consolidate it externally.
We know, from the action of similar causes on a smaller scale and on
comparatively insignificant objects immediately about us, what must
have been the effect of this cooling process upon the heated mass of
the globe. All substances when heated occupy more space than they do
when cold. Water, which expands when freezing, is the only exception
to this rule. The first effect of cooling the surface of our planet
must have been to solidify it, and thus to form a film or crust over
it. That crust would shrink as the cooling process went on; in
consequence of the shrinking, wrinkles and folds would arise upon it,
and here and there, where the tension was too great, cracks and
fissures would be produced. In proportion as the surface cooled, the
masses within would be affected by the change of temperature
outside of them, and would consolidate internally also, the crust
gradually thickening by this process.

[Illustration: A VOLCANO.]

But there was another element without the globe, equally powerful in
building it up. Fire and water wrought together in this work, if not
always harmoniously, at least with equal force and persistency. I have
said that there was a time when no atmosphere surrounded the earth;
but one of the first results of the cooling of its crust must have
been the formation of an atmosphere, with all the phenomena connected
with it,--the rising of vapors, their condensation into clouds, the
falling of rains, the gathering of waters upon its surface. Water is a
very active agent of destruction, but it works over again the
materials it pulls down or wears away, and builds them up anew in
other forms. As soon as an ocean washed over the consolidated crust of
the globe, it would begin to abrade the surfaces upon which it moved,
gradually loosening and detaching materials, to deposit them again as
sand or mud or pebbles at its bottom in successive layers, one above
another. Thus, in analyzing the crust of the globe, we find at once
two kinds of rocks, the respective work of fire and water: the first
poured out from the furnaces within, and cooling, as one may see any
mass of metal cool that is poured out from a smelting-furnace to-day,
in solid crystalline masses, without any division into separate layers
or leaves; and the latter in successive beds, one over another, the
heavier materials below, the lighter above, or sometimes in alternate
layers, as special causes may have determined successive deposits of
lighter or heavier materials at some given spot.

There were many well-fought battles between geologists before it was
understood that these two elements had been equally active in building
up the crust of the earth. The ground was hotly contested by the
disciples of the two geological schools, one of which held that the
solid envelope of the earth was exclusively due to the influence of
fire, while the other insisted that it had been accumulated wholly
under the agency of water. This difference of opinion grew up very
naturally; for the great leaders of the two schools lived in different
localities, and pursued their investigations over regions where the
geological phenomena were of an entirely opposite character,--the one
exhibiting the effect of volcanic eruptions, the other that of
stratified deposits. It was the old story of the two knights on
opposite sides of the shield, one swearing that it was made of gold,
the other that it was made of silver; and almost killing each other
before they discovered that it was made of both. So prone are men to
hug their theories and shut their eyes to any antagonistic facts, that
it is related of Werner, the great leader of the Aqueous school, that
he was actually on his way to see a geological locality of especial
interest, but, being told that it confirmed the views of his
opponents, he turned round and went home again, refusing to see what
might force him to change his opinions. If the rocks did not confirm
his theory, so much the worse for the rocks,--he would none of them.
At last it was found that the two great chemists, fire and water, had
worked together in the vast laboratory of the globe, and since then
scientific men have decided to work together also; and if they still
have a passage at arms occasionally over some doubtful point, yet the
results of their investigations are ever drawing them nearer to each
other,--since men who study truth, when they reach their goal, must
always meet at last on common ground.

The rocks formed under the influence of heat are called, in geological
language, the Igneous, or, as some naturalists have named them, the
Plutonic rocks, alluding to their fiery origin, while the others have
been called Aqueous or Neptunic rocks, in reference to their origin
under the agency of water. A simpler term, however, quite as
distinctive, and more descriptive of their structure, is that of the
stratified and massive or unstratified rocks. We shall see hereafter
how the relative position of these two classes of rocks and their
action upon each other enable us to determine the chronology of the
earth, to compare the age of her mountains, and, if we have no
standard by which to estimate the positive duration of her continents,
to say at least which was the first-born among them, and how their
characteristic features have been successfully worked out. I am aware
that many of these inferences, drawn from what is called "the
geological record," must seem to be the work of the imagination. In a
certain sense this is true,--for imagination, chastened by correct
observation, is our best guide in the study of Nature. We are too apt
to associate the exercise of this faculty with works of fiction, while
it is in fact the keenest detective of truth.

[Illustration: DIKES.]

Besides the stratified and massive rocks, there is still a third set,
produced by the contact of these two, and called, in consequence of
the changes thus brought about, the Metamorphic rocks. The effect of
heat upon clay is to bake it into slate; limestone under the influence
of heat becomes quick-lime, or, if subjected afterwards to the action
of water, it is changed to mortar; sand under the same agency is
changed to a coarse kind of glass. Suppose, then, that a volcanic
eruption takes place in a region of the earth's surface where
successive layers of limestone, of clay, and of sandstone, have been
previously deposited by the action of water. If such an eruption has
force enough to break through these beds, the hot, melted masses will
pour out through the rent, flow over its edges, and fill all the
lesser cracks and fissures produced by such a disturbance. What will
be the effect upon the stratified rocks? Wherever these liquid masses,
melted by a heat more intense than can be produced by any artificial
means, have flowed over them or cooled in immediate contact with them,
the clays will be changed to slate, the limestone will have assumed a
character more like marble, while the sandstone will be vitrified.
This is exactly what has been found to be the case, wherever the
stratified rocks have been penetrated by the melted masses from
beneath. They have been themselves partially melted by the contact,
and when they have cooled again, their stratification, though still
perceptible, has been partly obliterated, and their substance changed.
Such effects may often be traced in dikes, which are only the cracks
in rocks filled by materials poured into them at some period of
eruption when the melted masses within the earth were thrown out and
flowed like water into any inequality or depression of the surface
around. The walls enclosing such a dike are often found to be
completely altered by contact with its burning contents, and to have
assumed a character quite different from the rocks of which they make
a part; while the mass itself which fills the fissure shows by the
character of its crystallization that it has cooled more quickly on
the outside, where it meets the walls, than at the centre.

The first two great classes of rocks, the unstratified and stratified
rocks, represent different epochs in the world's physical history: the
former mark its revolutions, while the latter chronicle its periods of
rest. All mountains and mountain-chains have been upheaved by great
convulsions of the globe, which rent asunder the surface of the earth,
destroyed the animals and plants living upon it at the time, and were
then succeeded by long intervals of repose, when all things returned
to their accustomed order, ocean and river deposited fresh beds in
uninterrupted succession, the accumulation of materials went on as
before, a new set of animals and plants were introduced, and a time of
building up and renewing followed the time of destruction. These
periods of revolution are naturally more difficult to decipher than
the periods of rest; for they have so torn and shattered the beds they
uplifted, disturbing them from their natural relations to each other,
that it is not easy to reconstruct the parts and give them coherence
and completeness again. But within the last half-century this work has
been accomplished in many parts of the world with an amazing degree of
accuracy, considering the disconnected character of the phenomena to
be studied; and I think I shall be able to convince my readers that
the modern results of geological investigation are perfectly sound
logical inferences from well-established facts. In this, as in so many
other things, we are but "children of a larger growth." The world is
the geologist's great puzzle-box; he stands before it like the child
to whom the separate pieces of his puzzle remain a mystery till he
detects their relation and sees where they fit, and then his fragments
grow at once into a connected picture beneath his hand....

When geologists first turned their attention to the physical history
of the earth, they saw at once certain great features which they took
to be the skeleton and basis of the whole structure. They saw the
great masses of granite forming the mountains and mountain-chains,
with the stratified rocks resting against their slopes; and they
assumed that granite was the first primary agent, and that all
stratified rocks must be of a later formation. Although this involved
a partial error, as we shall see hereafter when we trace the upheavals
of granite even into comparatively modern periods, yet it held an
important geological truth also; for, though granite formations are by
no means limited to those early periods, they are nevertheless very
characteristic of them, and are indeed the foundation-stones on which
the physical history of the globe is built.

Starting from this landmark, the earlier geologists divided the
world's history into three periods. As the historian recognizes
Ancient History, the Middle Ages, and Modern History as distinct
phases in the growth of the human race, so they distinguished between
what they called the Primary period, when, as they believed, no life
stirred on the surface of the earth; the Secondary or middle period,
when animals and plants were introduced, and the land began to assume
continental proportions; and the Tertiary period, or comparatively
modern geological times, when the physical features of the earth as
well as its inhabitants were approaching more nearly to the present
condition of things. But as their investigations proceeded, they found
that every one of these great ages of the world's history was divided
into numerous lesser epochs, each of which had been characterized by a
peculiar set of animals and plants, and had been closed by some great
physical convulsion, disturbing and displacing the materials
accumulated during such a period of rest.

The further study of these subordinate periods showed that what had
been called Primary formations, namely, the volcanic or Plutonic rocks
formerly believed to be confined to the first geological ages,
belonged to all the periods, successive eruptions having taken place
at all times, pouring up through the accumulated deposits, penetrating
and injecting their cracks, fissures, and inequalities, as well as
throwing out large masses on the surface. Up to our own day there has
never been a period when such eruptions have not taken place, though
they have been constantly diminishing in frequency and extent. In
consequence of this discovery, that rocks of igneous character were by
no means exclusively characteristic of the earliest times, they are
now classified together upon very different grounds from those on
which geologists first united them; though, as the name _Primary_ was
long retained, we still find it applied to them, even in geological
works of quite recent date. This defect of nomenclature is to be
regretted, as likely to mislead the student, because it seems to refer
to time; whereas it no longer signifies the age of the rocks, but
simply their character. The name Plutonic or Massive rocks is,
however, now almost universally substituted for that of Primary.

A wide field of investigation still remains to be explored by the
chemist and the geologist together, in the mineralogical character of
the Plutonic rocks, which differs greatly in the different periods.
The earlier eruptions seem to have been chiefly granitic, though this
must not be understood in too wide a sense, since there are granite
formations even as late as the Tertiary period; those of the middle
periods were mostly porphyries and basalts; while in the more recent
ones, lavas predominate. We have as yet no clew to the laws by which
this distribution of volcanic elements in the formation of the earth
is regulated; but there is found to be a difference in the crystals of
the Plutonic rocks belonging to different ages, which, when fully
understood may enable us to determine the age of any Plutonic rock by
its mode of crystallization; so that the mineralogist will as readily
tell you by its crystals whether a bit of stone of igneous origin
belongs to this or that period of the world's history, as the
palaeontologist will tell you by its fossils whether a piece of rock
of aqueous origin belongs to the Silurian or Devonian or Carboniferous
deposits.

Although subsequent investigations have multiplied so extensively not
only the number of geological periods, but also the successive
creations that have characterized them, yet the first general division
into three great eras was nevertheless founded upon a broad and true
generalization. In the first stratified rocks in which any organic
remains are found, the highest animals are fishes, and the highest
plants are cryptogams; in the middle periods reptiles come in,
accompanied by fern and moss forests; in later times quadrupeds are
introduced, with a dicotyledonous vegetation. So closely does the
march of animal and vegetable life keep pace with the material
progress of the world, that we may well consider these three
divisions, included under the first general classification of its
physical history, as the three Ages of Nature; the more important
epochs which subdivide them may be compared to so many great
dynasties, while the lesser periods are the separate reigns contained
therein. Of such epochs there are ten, well known to geologists; of
the lesser periods about sixty are already distinguished, while many
more loom up from the dim regions of the past, just discerned by the
eye of science, though their history is not yet unravelled.

Before proceeding further, I will enumerate the geological epochs in
their succession, confining myself, however, to such as are perfectly
well established, without alluding to those of which the limits are
less definitely determined, and which are still subject to doubts and
discussions among geologists. As I do not propose to make here any
treatise of Geology, but simply to place before my readers some
pictures of the old world, with the animals and plants that have
inhabited it at various times, I shall avoid, as far as possible, all
debatable ground, and confine myself to those parts of my subject
which are best known, and can therefore be more clearly presented.

[Illustration: FOSSIL SCORPION.--SILURIAN PERIOD.]

First, we have the Azoic period, _devoid of life_, as its name
signifies,--namely, the earliest stratified deposits upon the heated
film forming the first solid surface of the earth, in which no trace
of living thing has ever been found. Next comes the Silurian period,
when the crust of the earth had thickened and cooled sufficiently to
render the existence of animals and plants upon it possible, and when
the atmospheric conditions necessary to their maintenance were already
established. Many of the names given to these periods are by no means
significant of their character, but are merely the result of accident:
as, for instance, that of Silurian, given by Sir Roderick Murchison to
this set of beds, because he first studied them in that part of Wales
occupied by the ancient tribe of the Silures. The next period, the
Devonian, was for a similar reason named after the country of
Devonshire in England, where it was first investigated. Upon this
follows the Carboniferous period, with the immense deposits of coal
from which it derives its name. Then comes the Permian period, named,
again, from local circumstances, the first investigation of its
deposits having taken place in the province of Permia in Russia. Next
in succession we have the Triassic period, so called from the trio of
rocks, the red sandstone, Muschel Kalk (shell-limestone), and Keuper
(clay), most frequently combined in its formations; the Jurassic, so
amply illustrated in the chain of the Jura, where geologists first
found the clew to its history; and the Cretaceous period, to which the
chalk cliffs of England and all the extensive chalk deposits belong.
Upon these follow the so-called Tertiary formations, divided into
three periods, all of which have received most characteristic names in
this epoch of the world's history we see the first approach to a
condition of things resembling that now prevailing, and Sir Charles
Lyell has most fitly named its three divisions, the Eocene, Miocene,
and Pliocene. The termination of the three words is made from the
Greek word _Kainos_, recent; while _Eos_ signifies dawn, _Meion_ less,
and _Pleion_ more. Thus Eocene indicates the dawn of recent species,
Pliocene their increase, while Miocene, the intermediate term, means
less recent. Above these deposits comes what has been called in
science the present period,--_the modern times_ of the geologist,--that
period to which man himself belongs, and since the beginning of which,
though its duration be counted by hundreds of thousands of years,
there has been no alteration in the general configuration of the
earth, consequently no important modification of its climatic
conditions, and no change in the animals and plants inhabiting it.

[Illustration: CRUSTACEA.--DEVONIAN PERIOD.]

[Illustration: FISH OF THE DEVONIAN PERIOD.]

[Illustration: FISH OF THE CARBONIFEROUS PERIOD.]

[Illustration: FOSSIL VEGETATION OF CARBONIFEROUS PERIOD.]

[Illustration: FISH OF THE PERMIAN PERIOD.]

I have spoken of the first of these periods, the Azoic, as having
been absolutely devoid of life, and I believe this statement to be
strictly true; but I ought to add that there is a difference of
opinion among geologists upon this point, many believing that the
first surface of our globe may have been inhabited by living beings,
but that all traces of their existence have been obliterated by the
eruptions of melted materials, which not only altered the character of
those earliest stratified rocks, but destroyed all the organic remains
contained in them. It will be my object to show, not only that the
absence of the climatic and atmospheric conditions essential to
organic life, as we understand it, must have rendered the previous
existence of any living beings impossible, but also that the
completeness of the Animal Kingdom in those deposits where we first
find organic remains, its intelligible and coherent connections with
the successive creations of all geological times and with the animals
now living, afford the strongest internal evidence that we have indeed
found in the lower Silurian formations, immediately following the
Azoic, the beginning of life upon earth. When a story seems to us
complete and consistent from the beginning to the end, we shall not
seek for a first chapter, even though the copy in which we have read
it be so torn and defaced as to suggest the idea that some portion of
it may have been lost. The unity of the work, as a whole, is an
incontestable proof that we possess it in its original integrity. The
validity of this argument will be recognized, perhaps, only by those
naturalists to whom the Animal Kingdom has begun to appear as a
connected whole. For those who do not see order in Nature it can have
no value.

[Illustration: FOSSILS OF TRIASSIC VEGETATION.]

[Illustration: BIRD OF THE JURASSIC PERIOD.(The Oldest Bird.)]

[Illustration: SKELETON OF BIRD OF THE CRETACEOUS PERIOD.]

[Illustration: SKELETON OF ANIMAL OF THE EOCENE PERIOD.]

For a table containing the geological periods in their succession, I
would refer to any modern text-book of Geology, or to an article in
the _Atlantic Monthly_ for March, 1862, upon "Methods of Study in
Natural History," where they are given in connection with the order of
introduction of animals upon earth.

Were these sets of rocks found always in the regular sequence in which
I have enumerated them, their relative age would be easily
determined, for their superposition would tell the whole story: the
lowest would, of course, be the oldest, and we might follow without
difficulty the ascending series, till we reached the youngest and
uppermost deposits. But their succession has been broken up by
frequent and violent alterations in the configuration of the globe.
Land and water have changed their level,--islands have been
transformed to continents,--sea-bottoms have become dry land, and dry
land has sunk to form sea-bottoms,--Alps and Himalayas, Pyrenees and
Apennines, Alleghanies and Rocky Mountains, have had their stormy
birthdays since many of these beds have been piled one above another,
and there are but few spots on the earth's surface where any number of
them may be found in their original order and natural position. When
we remember that Europe, which lies before us on the map as a
continent, was once an archipelago of islands,--that, where the
Pyrenees raise their rocky barrier between France and Spain, the
waters of the Mediterranean and Atlantic met,--that, where the British
Channel flows, dry land united England and France, and Nature in those
days made one country of the lands parted since by enmities deeper
than the waters that run between,--when we remember, in short, all the
fearful convulsions that have torn asunder the surface of the earth,
as if her rocky record had indeed been written on paper, we shall find
a new evidence of the intellectual unity which holds together the
whole physical history of the globe in the fact that through all the
storms of time the investigator is able to trace one unbroken thread
of thought from the beginning to the present hour.

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