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The Atlantic Monthly, Volume 11, No. 65, March, 1863 online

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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 unstratified or massive rocks. We shall see hereafter how
the relative position of these two kinds of rocks and their action upon
each other enables 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 successively 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.

Beside the stratified and unstratified 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 sandstones 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 that inclose 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.

It is a curious fact in the history of progress, that, by a kind of
intuitive insight, the earlier observers seem to have had a wider, more
comprehensive recognition of natural phenomena as a whole than their
successors, who far excel them in their knowledge of special points,
but often lose their grasp of broader relations in the more minute
investigation of details. 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
a great 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 great 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 as distinct
phases in the growth of the human race Ancient History, the Middle Ages,
and Modern History, 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 aspect of the earth as well as its inhabitants was 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, that disturbed and displaced
the materials accumulated during such a period of rest. The further
study of these subordinate periods showed that what had been called
Primary formations, 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.

There is still a wide field of investigation 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 clue 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, enables 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 farther, 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 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.

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 county 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
clue 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," or the dawn, the
"Miocene," meaning the continuance and increase of that light, and
lastly, the "Pliocene," signifying its fulness and completion. 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.

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 in this series of papers, 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 connection with the
successive creations of all geological times and with the animals now
living, affords 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.

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 ago 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-bottom, - 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.

The tree is known by its fruits, - and the fruits of chance are
incoherence, incompleteness, unsteadiness, the stammering utterance of
blind, unreasoning force. A coherence that binds all the geological ages
in one chain, a stability of purpose that completes in the beings born
to-day an intention expressed in the first creatures that swam in the
Silurian ocean or crept upon its shores, a steadfastness of thought,
practically recognized by man, if not acknowledged by him, whenever
he traces the intelligent connection between the facts of Nature and
combines them into what he is pleased to call his system of Geology, or
Zoology, or Botany, - these things are not the fruits of chance or of an
unreasoning force, but the legitimate results of intellectual power.
There is a singular lack of logic, as it seems to me, in the views of
the materialistic naturalists. While they consider classification, or,
in other words, their expression of the relations between animals or
between physical facts of any kind, as the work of their intelligence,
they believe the relations themselves to be the work of physical causes.
The more direct inference surely is, that, if it requires an intelligent
mind to recognize them, it must have required an intelligent mind to
establish them. These relations existed before man was created; they
have existed ever since the beginning of time; hence, what we call
the classification of facts is not the work of his mind in any direct
original sense, but the recognition of an intelligent action prior to
his own existence.

There is, perhaps, no part of the world, certainly none familiar to
science, where the early geological periods can be studied with so
much ease and precision as in the United States. Along their northern
borders, between Canada and the United States, there runs the low line
of hills known as the Laurentian Hills. Insignificant in height, nowhere
rising more than fifteen hundred or two thousand feet above the level
of the sea, these are nevertheless the first mountains that broke the
uniform level of the earth's surface and lifted themselves above the
waters. Their low stature, as compared with that of other more lofty
mountain-ranges, is in accordance with an invariable rule, by which the
relative age of mountains may be estimated. The oldest mountains are the
lowest, while the younger and more recent ones tower above their
elders, and are usually more torn and dislocated also. This is easily
understood, when we remember that all mountains and mountain-chains are
the result of upheavals, and that the violence of the outbreak must have
been in proportion to the strength of the resistance. When the crust of
the earth was so thin that the heated masses within easily broke through
it, they were not thrown to so great a height, and formed comparatively
low elevations, such as the Canadian hills or the mountains of Bretagne
and Wales. But in later times, when young, vigorous giants, such as the
Alps, the Himalayas, or, later still, the Rocky Mountains, forced their
way out from their fiery prison-house, the crust of the earth was
much thicker, and fearful indeed must have been the convulsions which
attended their exit.

The Laurentian Hills form, then, a granite range, stretching from
Eastern Canada to the Upper Mississippi, and immediately along its base
are gathered the Azoic deposits, the first stratified beds, in which the
absence of life need not surprise us, since they were formed beneath a
heated ocean. As well might we expect to find the remains of fish or
shells or crabs at the bottom of geysers or of boiling springs, as on
those early shores bathed by an ocean of which the heat must have been
so intense. Although, from the condition in which we find it, this
first granite range has evidently never been disturbed by any violent
convulsion since its first upheaval, yet there has been a gradual
rising of that part of the continent, for the Azoic beds do not lie
horizontally along the base of the Laurentian Hills in the position in
which they must originally have been deposited, but are lifted and rest
against their slopes. They have been more or less dislocated in this
process, and are greatly metamorphized by the intense heat to which they
must have been exposed. Indeed, all the oldest stratified rocks have
been baked by the prolonged action of heat.

It may be asked how the materials for those first stratified deposits
were provided. In later times, when an abundant and various soil covered
the earth, when every river brought down to the ocean, not only its
yearly tribute of mud or clay or lime, but the _débris_ of animals and
plants that lived and died in its waters or along its banks, when every
lake and pond deposited at its bottom in successive layers the lighter
or heavier materials floating in its waters and settling gradually
beneath them, the process by which stratified materials are collected
and gradually harden into rock is more easily understood. But when the
solid surface of the earth was only just beginning to form, it would
seem that the floating matter in the sea can hardly have been in
sufficient quantity to form any extensive deposits. No doubt there was
some abrasion even of that first crust; but the more abundant source of
the earliest stratification is to be found in the submarine volcanoes
that poured their liquid streams into the first ocean. At what rate
these materials would be distributed and precipitated in regular strata
it is impossible to determine; but that volcanic materials were so
deposited in layers is evident from the relative position of the
earliest rocks. I have already spoken of the innumerable chimneys
perforating the Azoic beds, narrow outlets of Plutonic rock, protruding
through the earliest strata. Not only are such funnels filled with the
crystalline mass of granite that flowed through them in a liquid state,
but it has often poured over their sides, mingling with the stratified
beds around. In the present state of our knowledge, we can explain such
appearances only by supposing that the heated materials within the
earth's crust poured out frequently, meeting little resistance, - that
they then scattered and were precipitated in the ocean around, settling


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Online LibraryVariousThe Atlantic Monthly, Volume 11, No. 65, March, 1863 → online text (page 16 of 20)