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.
[Illustration: SKELETON OF ANIMAL OF THE MIOCENE PERIOD.]
[Illustration: SKELETON OF ANIMAL OF THE PLIOCENE PERIOD.]
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 Zoölogy, 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.
[Illustration: A PHYSICAL MAP OF THE UNITED STATES.]
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 in successive strata
at its bottom,--that through such strata the heated masses within
continued to pour again and again, forming for themselves the
chimney-like outlets above mentioned.
Such, then, was the earliest American land,--a long, narrow island,
almost continental in its proportions, since it stretched from the
eastern borders of Canada nearly to the point where now the base of
the Rocky Mountains meets the plain of the Mississippi Valley. We may
still walk along its ridge and know that we tread upon the ancient
granite that first divided the waters into a northern and southern
ocean; and if our imaginations will carry us so far, we may look down
toward its base and fancy how the sea washed against this earliest
shore of a lifeless world. This is no romance, but the bald, simple
truth; for the fact that this granite band was lifted out of the
waters so early in the history of the world, and has not since been
submerged, has, of course, prevented any subsequent deposits from
forming above it. And this is true of all the northern part of the
United States. It has been lifted gradually, the beds deposited in one
period being subsequently raised, and forming a shore along which
those of the succeeding one collected, so that we have their whole
sequence before us. In regions where all the geological deposits
(Silurian, Devonian, carboniferous, permian, triassic, etc.) are piled
one upon another, and we can get a glimpse of their internal relations
only where some rent has laid them open, or where their ragged edges,
worn away by the abrading action of external influences, expose to
view their successive layers, it must, of course, be more difficult to
follow their connection. For this reason the American continent offers
facilities to the geologist denied to him in the so-called Old World,
where the earlier deposits are comparatively hidden, and the broken
character of the land, intersected by mountains in every direction,
renders his investigation still more difficult. Of course, when I
speak of the geological deposits as so completely unveiled to us here,
I do not forget the sheet of drift which covers the continent from
north to south, and which we shall discuss hereafter, when I reach
that part of my subject. But the drift is only a superficial and
recent addition to the soil, resting loosely above the other
geological deposits, and arising, as we shall see, from very different
causes.
In this article I have intended to limit myself to a general sketch of
the formation of the Laurentian Hills with the Azoic stratified beds
resting against them. In the Silurian epoch following the Azoic we
have the first beach on which any life stirred; it extended along the
base of the Azoic beds, widening by its extensive deposits the narrow
strip of land already upheaved. I propose ... to invite my readers to
a stroll with me along that beach.
With what interest do we look upon any relic of early human history!
The monument that tells of a civilization whose hieroglyphic records
we cannot even decipher, the slightest trace of a nation that vanished
and left no sign of its life except the rough tools and utensils
buried in the old site of its towns or villages, arouses our
imagination and excites our curiosity. Men gaze with awe at the
inscription on an ancient Egyptian or Assyrian stone; they hold with
reverential touch the yellow parchment-roll whose dim, defaced
characters record the meagre learning of a buried nationality; and the
announcement, that for centuries the tropical forests of Central
America have hidden within their tangled growth the ruined homes and
temples of a past race, stirs the civilized world with a strange, deep
wonder.
To me it seems, that to look on the first land that was ever lifted
above the waste of waters, to follow the shore where the earliest
animals and plants were created when the thought of God first
expressed itself in organic forms, to hold in one's hand a bit of
stone from an old sea-beach, hardened into rock thousands of
centuries ago, and studded with the beings that once crept upon its
surface or were stranded there by some retreating wave, is even of
deeper interest to men than the relies of their own race, for these
things tell more directly of the thoughts and creative acts of God.
Standing in the neighborhood of Whitehall, near Lake George, one may
look along such a seashore, and see it stretching westward and sloping
gently southward as far as the eye can reach. It must have had a very
gradual slope, and the waters must have been very shallow; for at that
time no great mountains had been uplifted, and deep oceans are always
the concomitants of lofty heights. We do not, however, judge of this
by inference merely; we have an evidence of the shallowness of the sea
in those days in the character of the shells found in the Silurian
deposits, which shows that they belonged in shoal waters.
Indeed, the fossil remains of all times tell us almost as much of the
physical condition of the world at different epochs as they do of its
animal and vegetable population. When Robinson Crusoe first caught
sight of the footprint on the sand, he saw in it more than the mere
footprint, for it spoke to him of the presence of men on his desert
island. We walk on the old geological shores, like Crusoe along his
beach, and the footprints we find there tell us, too, more than we
actually see in them. The crust of our earth is a great cemetery,
where the rocks are tombstones on which the buried dead have written
their own epitaphs. They tell us not only who they were and when and
where they lived, but much also of the circumstances under which they
lived. We ascertain the prevalence of certain physical conditions at
special epochs by the presence of animals and plants whose existence
and maintenance required such a state of things, more than by any
positive knowledge respecting it. Where we find the remains of
quadrupeds corresponding to our ruminating animals, we infer not only
land, but grassy meadows also, and an extensive vegetation; where we
find none but marine animals, we know the ocean must have covered the
earth; the remains of large reptiles, representing, though in gigantic
size, the half aquatic, half terrestrial reptiles of our own period,
indicate to us the existence of spreading marshes still soaked by the
retreating waters; while the traces of such animals as live now in
sand and shoal waters, or in mud, speak to us of shelving sandy
beaches and of mud-flats. The eye of the Trilobite tells us that the
sun shone on the old beach where he lived; for there is nothing in
nature without a purpose, and when so complicated an organ was made to
receive the light, there must have been light to enter it. The immense
vegetable deposits in the Carboniferous period announce the
introduction of an extensive terrestrial vegetation; and the
impressions left by the wood and leaves of the trees show that these
first forests must have grown in a damp soil and a moist atmosphere.
In short, all the remains of animals and plants hidden in the rocks
have something to tell of the climatic conditions and the general
circumstances under which they lived, and the study of fossils is to
the naturalist a thermometer by which he reads the variations of
temperature in past times, a plummet by which he sounds the depths of
the ancient oceans,--a register, in fact, of all the important
physical changes the earth has undergone.
But although the animals of the early geological deposits indicate
shallow seas by their similarity to our shoal-water animals, it must
not be supposed that they are by any means the same. On the contrary,
the old shells, crustacea, corals, etc., represent types which have
existed in all times with the same essential structural elements, but
under different specific forms in the several geological periods. And
here it may not be amiss to say something of what are called by
naturalists _representative types_.
The statement that different sets of animals and plants have
characterized the successive epochs is often understood as indicating
a difference of another kind than that which distinguishes animals now
living in different parts of the world. This is a mistake. There are
so-called representative types all over the globe, united to each
other by structural relations and separated by specific differences of
the same kind as those that unite and separate animals of different
geological periods. Take, for instance, mud-flats or sandy shores in
the same latitudes of Europe and America; we find living on each,
animals of the same structural character and of the same general
appearance, but with certain specific differences, as of color, size,
external appendages, etc. They represent each other on the two
continents. The American wolves, foxes, bears, rabbits, are not the
same as the European, but those of one continent are as true to their
respective types as those of the other; under a somewhat different
aspect they represent the same groups of animals. In certain
latitudes, or under conditions of nearer proximity, these differences
may be less marked. It is well known that there is a great monotony
of type, not only among animals and plants, but in the human races
also, throughout the Arctic regions; and some animals characteristic
of the high North reappear under such identical forms in the
neighborhood of the snow-fields in lofty mountains, that to trace the
difference between the ptarmigans, rabbits, and other gnawing animals
of the Alps, for instance, and those of the Arctics, is among the most
difficult problems of modern science.
And so it is also with the animated world of past ages; in similar
deposits of sand, mud, or lime, in adjoining regions of the same
geological age, identical remains of animals and plants may be found;
while at greater distances, but under similar circumstances,
representative species may occur. In very remote regions, however,
whether the circumstances be similar or dissimilar, the general aspect
of the organic world differs greatly, remoteness in space being thus
in some measure an indication of the degree of affinity between
different faunæ. In deposits of different geological periods
immediately following each other, we sometimes find remains of animals
and plants so closely allied to those of earlier or later periods that
at first sight the specific differences are hardly discernible. The
difficulty of solving these questions, and of appreciating correctly
the differences and similarities between such closely allied
organisms, explains the antagonistic views of many naturalists
respecting the range of existence of animals, during longer or shorter
geological periods; and the superficial way in which discussions
concerning the transition of species are carried on, is mainly owing
to an ignorance of the conditions above alluded to. My own personal
observation and experience in these matters have led me to the
conviction that every geological period has had its own
representatives, and that no single species has been repeated in
successive ages.
The laws regulating the geographical distribution of animals, and
their combination into distinct zoölogical provinces called faunæ,
with definite limits, are very imperfectly understood as yet; but so
closely are all things linked together from the beginning that I am
convinced we shall never find the clew to their meaning till we carry
on our investigations in the past and the present simultaneously. The
same principle according to which animal and vegetable life is
distributed over the surface of the earth now, prevailed in the
earliest geological periods. The geological deposits of all times have
had their characteristic faunæ under various zones, their zoölogical
provinces presenting special combinations of animal and vegetable life
over certain regions, and their representative types reproducing in
different countries, but under similar latitudes, the same groups with
specific differences.
Of course, the nearer we approach the beginning of organic life, the
less marked do we find the differences to be, and for a very obvious
reason. The inequalities of the earth's surface, her mountain-barriers
protecting whole continents from the Arctic winds, her open plains
exposing others to the full force of the polar blasts, her snug
valleys and her lofty heights, her tablelands and rolling prairies,
her river-systems and her dry deserts, her cold ocean-currents pouring
down from the high North on some of her shores, while warm ones from
tropical seas carry their softer influence to others,--in short, all
the contrasts in the external configuration of the globe, with the
physical conditions attendant upon them, are naturally accompanied by
a corresponding variety in animal and vegetable life.
But in the Silurian age, when there were no elevations higher than the
Canadian hills, when water covered the face of the earth, with the
exception of a few isolated portions lifted above the almost universal
ocean, how monotonous must have been the conditions of life! And what
should we expect to find on those first shores? If we are walking on a
sea-beach to-day, we do not look for animals that haunt the forests or
roam over the open plains, or for those that live in sheltered valleys
or in inland regions or on mountain-heights. We look for Shells, for
Mussels and Barnacles, for Crabs, for Shrimps, for Marine Worms, for
Star-Fishes and Sea-Urchins, and we may find here and there a fish
stranded on the sand or tangled in the seaweed.
[Illustration]
SOME RECORDS OF THE ROCKS
(FROM A FIRST BOOK IN GEOLOGY.)
BY N.S. SHALER, S.D.[1]
[Footnote 1: Copyright, 1884, by N.S. Shaler.]
[Illustration]
The geologist cannot find his way back in the record of the great
stone book, to the far-off day when life began. The various changes
that come over rocks from the action of heat, of water, and of
pressure, have slowly modified these ancient beds, so that they no
longer preserve the frames of the animals that were buried in them.