Atlantic Monthly, Vol. XII. July, 1863, No. LXIX. by Various

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[Footnote 2: I say nothing of the traces of Birds in the Secondary
deposits, because the so-called bird-tracks seem to me of very
doubtful character; and it is also my opinion that the remains of
a feathered animal recently found in the Solenhofen lithographic
limestone, and believed to be a bird by some naturalists, do not
belong to a genuine bird, but to one of those synthetic types
before alluded to, in which reptilian structure is combined with
certain birdlike features.]

There was an age in the physical history of the world when great ranges of
mountains bound together in everlasting chains the islands which had
already grown to continental dimensions,--when wide tracts of land,
hitherto insular in character, became soldered into one by the upheaval of
Plutonic masses which stretched across them all and riveted them forever
with bolts of granite, of porphyry, and of basalt. Thus did the Rocky
Mountains and the Andes bind together North and South America; the
Pyrenees united Spain to France; the Alps, the Caucasus, and the Himalayas
bound Europe to Asia. The class of Mammalia were now at the head of the
animal kingdom; huge quadrupeds possessed the earth, and dwelt in forests
characterized by plants of a higher order than any preceding ones,--the
Beeches, Birches, Maples, Oaks, and Poplars of the Tertiaries. But though
the continents had assumed their permanent outlines, extensive tracts of
land still remained covered with ocean. Inland seas, sheets of water like
the Mediterranean, so unique in our world, were then numerous. Physically
speaking, this was the age of continents broken by large inland seas;
while in the organic world it was the age of Mammalia among animals, and
of extensive Dicotyledonous forests among plants. In a certain sense it
was the age of completion,--the one which ushered in the crowning work of
creation.

There was an age in the physical history of the world (it is in its
infancy still) when Man, with the animals and plants that were to
accompany him, was introduced upon the globe, which had acquired all its
modern characters. At last the continents were redeemed from the water,
and all the earth was given to this new being for his home. Among all the
types born into the animal kingdom before, there had never been one to
which positive limits had not been set by a law of geographical
distribution absolutely impassable to all. For Man alone those boundaries
were removed. He, with the domestic animals and plants which were to be
the companions of all his pilgrimages, could wander over the whole earth
and choose his home. Placed at the head of creation, gifted with intellect
to make both animals and plants subservient to his destinies, his
introduction upon the earth marks the last great division in the history
of our planet. To designate these great divisions in time, I would urge,
for the reasons above stated, that the term which is indeed often, though
not invariably, applied to them, be exclusively adopted,--that of the Ages
of Nature.

But these Ages are themselves susceptible of subdivisions, which should
also be accurately defined. What is the nature of these subdivisions? They
are all connected with sudden physical changes in the earth's surface,
more or less limited in their action, these changes being themselves
related to important alterations in the organic world. Although I have
stated that one general character prevailed during each of the Ages, yet
there was nevertheless a constant progressive action running through them
all, and at various intervals both the organic and the physical world
received a sudden impulse in consequence of marked and violent changes in
the earth-crust, bringing up new elevations, while at the same time the
existing animal creation was brought to a close, and a new set of beings
was introduced. These changes are not yet accurately defined in America,
because the age of her mountains is not known with sufficient accuracy;
but their limits have been very extensively traced in Europe, and this
coincidence of the various upheavals with the introduction of a new
population differing entirely from, the preceding one has been
demonstrated so clearly that it may be considered as an ascertained law.
What name, then, is most appropriate for the divisions thus marked by
sudden and violent changes? It seems to me, from their generally accepted
meaning, that the word Epoch or Era, both of which have been widely,
though indiscriminately, used in geology, is especially applicable here.
In their common use, they imply a condition of things determined by some
decisive event. In speaking of human affairs, we say, "It was an epoch or
an era in history,"--or in a more limited sense, "It was an epoch in the
life of such or such a man." It at once conveys the idea of an important
change connected with or brought about by some striking occurrence. Such
were those divisions in the history of the earth when a violent convulsion
in the surface of the globe and a change in its inhabitants ushered in a
new aspect of things.

I have said that we owe to Elie de Beaumont the discovery of this
connection between the successive upheavals and the different sets of
animals and plants which have followed each other on the globe. We have
seen in the preceding article upon the formation of mountains, that the
dislocations thus produced show the interruptions between successive
deposits: as, for instance, where certain strata are raised upon the sides
of a mountain, while other strata rest _unconformably_, as it is called,
above them at its base,--this term, unconformable, signifying merely that
the two sets of strata are placed at an entirely different angle, and must
therefore belong to two distinct sets of deposits. But there are two
series of geological facts connected with this result which are often
confounded, though they arise from very different causes. One is that
described above, in which a certain series of beds having been raised out
of their natural horizontal position, another series has been deposited
upon them, thus resting unconformably above. The other is where, one set
of beds having been deposited over any given region, at a later time, in
consequence of a recession of the sea-shore, for instance, or of some
other gradual disturbance of the surface, the next set of beds accumulated
above them cover a somewhat different area, and are therefore not
conformable with the first, though parallel with them. This difference,
however slight, is sufficient to show that some shifting of the ground on
which they were accumulated must have taken place between the two series
of deposits.

This distinction must not be confounded with that made by Elie de
Beaumont: we owe it to D'Orbigny, who first pointed out the importance of
distinguishing the dislocations produced by gradual movements of the earth
from those caused by mountain-upheavals. The former are much more numerous
than the latter, and in every epoch geologists have distinguished a number
of such changes in the surface of the earth, accompanied by the
introduction of a new set of animals, though the changes in the organic
world are not so striking as those which coincide with the
mountain-upheavals. Still, to the eye of the geologist they are quite as
distinct, though less evident to the ordinary observer. To these divisions
it seems to me that the name of Period is rightly applied, because they
seem to have been brought about by the steady action of time, and by
gradual changes, rather than by any sudden or violent convulsion.

It was my good fortune to be in some degree connected with the
investigations respecting the limitation of Periods, for which the geology
of Switzerland afforded peculiar facilities. My early home was near the
foot of the Jura, where I constantly faced its rounded domes, and the
slope by which they gently descend to the plain of Switzerland. I have
heard it said that there is something monotonous in the continuous
undulations of this range, so different from the opposite one of the Alps.
But I think it is only by contrast that it seems wanting in vigor and
picturesqueness; and those who live in its neighborhood become very much
attached to the more peaceful character of its scenery. Perhaps my readers
will pardon the digression, if I interrupt our geological discussion for a
moment, to offer them a word of advice, though it be uncalled for. I have
often been asked by friends who were intending to go to Europe what is the
most favorable time in the day and the best road to enter Switzerland in
order to have at once the finest impression of the mountains. My answer is
always,--Enter it in the afternoon over the Jura. If you are fortunate,
and have one of the bright, soft afternoons that sometimes show the Alps
in their full beauty, as you descend the slope of the Jura, from which you
command the whole panorama of the opposite range, you may see, as the day
dies, the last shadow pass with strange rapidity from peak to peak of the
Alpine summits. The passage is so rapid, so sudden, as the shadow vanishes
from one height and appears on the next, that it seems like the step of
some living spirit of the mountains. Then, as the sun sinks, it sheds a
brilliant glow across them, and upon that follows--strangest effect of
all--a sudden pallor, an ashy paleness on the mountains, that has a
ghastly, chilly look. But this is not their last aspect: after the sun has
vanished out of sight, in place of the glory of his departure, and of the
corpse-like pallor which succeeded it, there spreads over the mountains a
faint blush that dies gradually into the night. These changes--the glory,
the death, the soft succeeding life--really seem like something that has a
spiritual existence. While, however, I counsel my friends to see the Alps
for the first time in the afternoon, if possible, I do not promise them
that the hour will bring with it such a scene as I have tried to describe.
Perfect sunsets are rare in any land; but, nevertheless, I would advise
travellers to choose the latter half of the day and a road over the Jura
for their entrance into Switzerland.[3]

[Footnote 3: The two most imposing views of the Alps from the Jura
are those of Latourne, on the road from Pontarlier to Neufchatel,
and of St. Cergues, on the road from Lons le Saulnier to Nyon; the
next best is to be had above Boujean, on the road from Basle to
Bienne. Very extensive views may be obtained from any of the
summits in the southern range of the Jura; among which the
Weissenstein above Soleure, the Chasseral above Bienne, the
Chanmont above Neufchatel, the Chasseron above Grancon, the Suchet
above Orbe, the Mont Tendre or the Noirmont above Morges, and the
Dole above Nyon, are the most frequented. Of all these pointe
Chaumont is unquestionably to be preferred, as it commands at the
same time an equally extensive view of the Bernese Alps and the
Mont Blanc range.]

It was from the Jura itself that one of the great epochs in the history of
the globe received its name. It was in a deep gorge of the Jura, that,
more than half a century ago, Leopold von Buch first perceived the mode of
formation of mountains; and it was at the foot of the Jura, in the
neighborhood of Neufchatel, that the investigations were made which first
led to the recognition of the changes connected with the Periods. As I
shall have occasion hereafter to enter into this subject more at length, I
will only allude briefly here to the circumstances. In so doing I am
anticipating the true geological order, because I must treat of the
Jurassic and Cretaceous deposits, which are still far in advance of us;
but as it was by the study of these deposits that the circumscription of
the Periods, as I have defined them above, was first ascertained, I must
allude to them in this connection.

Facing the range of the Jura from the Lake of Neufchatel, there seems to
be but one uninterrupted slope by which it descends to the shore of the
lake. It will, however, be noticed by the most careless observer that this
slope is divided by the difference in vegetation into two strongly marked
bands of color: the lower and more gradual descent being of a lighter
green, while the upper portion is covered by the deeper hue of the
forest-trees, the Beeches, Birches, Maples, etc., above which come the
Pines. When the vegetation is fully expanded, this marked division along
the whole side of the range into two broad bands of green, the lighter
below and the darker above, becomes very striking. The lighter band
represents the cultivated portion of the slope, the vineyards, the farms,
the orchards, covering the gentler, more gradual part of the descent; and
the whole of this cultivated tract, stretching a hundred miles east and
west, belongs to the Cretaceous epoch. The upper slope of the range, where
the forest-growth comes in, is Jurassic. Facing the range, you do not, as
I have said, perceive any difference in the angle of inclination; but the
border-line between the two bands of green does in fact mark the point at
which the Cretaceous beds abut with a gentler slope against the Jurassic
strata, which continue their sharper descent, and are lost to view beneath
them.

This is one of the instances in which the contact of two epochs is most
directly traced. There is no question, from the relation of the deposits,
that the Jura in its upheaval carried with it the strata previously
accumulated. At its base there was then no lake, but an extensive stretch
of ocean; for the whole plain of Switzerland was under water, and many
thousand years elapsed before the Alps arose to set a new boundary to the
sea and inclose that inland sheet of water, gradually to be filled up by
more modern accumulations, and transformed into the fertile plain which
now lies between the Jura and the Alps. If the reader will for a moment
transport himself in imagination to the time when the southern side of the
Jurassic range sloped directly down to the ocean, he will easily
understand how this second series of deposits was collected at its base,
as materials are collected now along any sea-shore. They must, of course,
have been accumulated horizontally, since no loose materials could keep
their place even at so moderate an angle as that of the present lower
slope of the range; but we shall see hereafter that there were many
subsequent perturbations of this region, and that these Cretaceous
deposits, after they had become consolidated, were raised by later
upheavals from their original position to that which they now occupy on
the lower slope of the Jura, resting immediately, but in geological
language _unconformably_, against it. The two adjoining wood-cuts are
merely theoretical, showing by lines the past and the present relation of
these deposits; but they may assist the reader to understand my meaning.

[Illustration]

Figure 1 represents the Jura before the Alps were raised, with the
Cretaceous deposits accumulating beneath the sea at its base. The line
marked S indicates the ocean-level; the letter c, the Cretaceous deposits;
the letter j, the Jurassic strata, lifted on the side of the mountain.

[Illustration]

Figure 2 represents the Jura at the present time, when the later upheavals
have lifted the Jurassic strata to a sharper inclination with the
Cretaceous deposits, now raised and forming the lower slope of the
mountain, at the base of which is the Lake of Neufchatel.

Although this change of inclination is hardly perceptible, as one looks up
against the face of the Jura range, there is a transverse cut across it
which seems intended to give us a diagram of its internal structure.
Behind the city of Neufchatel rises the mountain of Chaumont, so called
from its bald head, for neither tree nor shrub grows on its summit.
Straight through this mountain, from its northern to its southern side,
there is a natural road, formed by a split in the mountain from top to
bottom. In this transverse cut, which forms one of the most romantic and
picturesque gorges leading into the heart of the Jura range, you get a
profile view of the change in the inclination of the strata, and can
easily distinguish the point of juncture between the two sets of deposits.
But even after this dislocation of strata had been perceived, it was not
known that it indicated the commencement of a new epoch, and it is here
that my own share in the work, such as it is, belongs. Accustomed as a boy
to ramble about in the beautiful gorges and valleys of the Jura, and in
riper years, as my interest in science increased, to study its formation
with closer attention, this difference in the inclination of the slope had
not escaped my observation. I was, however, still more attracted by the
fossils it contained than by its geological character: and, indeed, there
is no better locality for the study of extinct forms of life than the
Jura. In all its breaks and ravines, wherever the inner surface of the
rock is exposed, it is full of organic remains; and to take a handful of
soil from the road-side is often to gather a handful of shells. It is
actually built of the remains of animals, and there are no coral reefs in
existing seas presenting a better opportunity for study to the naturalist
than the coral reefs of the Jura. Being already tolerably familiar with
the fossils of the Jura, it occurred to me to compare those of the upper
and lower slope; and to my surprise I found that they were everywhere
different, and that those of the lower slope were invariably Cretaceous in
character, while those of the upper slope were Jurassic. In the course of
this investigation I discovered three periods in the Cretaceous and four
in the Jurassic epoch, all characterized by different fossils. This led to
a more thorough investigation of the different sets of strata, resulting
in the establishment by D'Orbigny of a still greater number of periods,
marked by the successive deposits of the Jurassic and Cretaceous seas, all
of which contained different organic remains. The attention of geologists
being once turned in this direction, the other epochs were studied with
the same view, and all were found to be susceptible of division into a
greater or less number of such periods.

I have dwelt at greater length on the Jurassic and Cretaceous divisions,
because I believe that we have in the relation of these two epochs, as
well as in that of the Cretaceous epoch with the Tertiary immediately
following it, facts which are very important in their bearing on certain
questions, now loudly discussed, not only by scientific men, but by all
who are interested in the mode of origin of animals. Certainly, in the
inland seas of the Cretaceous and subsequent Tertiary times, where we can
trace in the same sheet of water not only the different series of deposits
belonging to two successive epochs in immediate juxtaposition, but those
belonging to all the periods included within these epochs, with the
organic remains contained in each,--there, if anywhere, we should be able
to trace the transition-types by which one set of animals is said to have
been developed out of the preceding. We hear a great deal of the
interruption in geological deposits, of long intervals, the record of
which has vanished, and which may contain those intermediate links for
which we vainly seek. But here there is no such gap in the evidence. In
the very same sheets of water, covering limited areas, we have the
successive series of deposits containing the remains of animals which
continue perfectly unchanged during long intervals, and then, with a more
or less violent shifting of the surface,[4] traceable by the consequent
discordance of the strata, is introduced an entirely new set of animals,
differing as much from those immediately preceding them as do those of the
present period from the old Creation, (our predecessors, but _not_ our
ancestors,) traced by Cuvier in the Tertiary deposits underlying those of
our own geological age. I subjoin here a tabular view giving the Epochs in
their relation to the Ages, and indicating, at least approximately, the
number of Periods contained in each Epoch.

[Footnote 4: I use surface often in its geological significance,
meaning earth-crust, and applied to sea-bottom as well as to dry
land.]

Age of Man Present.

Tertiary Age: { Pliocene }
Age of Mammalia { Miocene } with at least twelve Periods.
{ Eocene }

Secondary Age: { Cretaceous }
Age of Reptiles { Jurassic } with at least twenty Periods.
{
{ Triassic }
{ Permian } with eight or nine Periods.
{ Carboniferous }

Palaeozoic or Primary Age: { Devonian }
Age of Fishes { Silurian } with ten or twelve Periods.

It will be noticed by those who have any knowledge of geological
divisions, that in this diagram I consider the Carboniferous epoch as
forming a part of the Secondary age. Some geologists have been inclined,
from the marked and peculiar character of its vegetation, to set it apart
as forming in itself a distinct geological age, while others have united
it with the Palaeozoic age. For many years I myself adopted the latter of
these two views, and associated the Carboniferous epoch with the Palaeozoic
age. But it is the misfortune of progress that one is forced not only to
unlearn a great deal, but, if one has been in the habit of communicating
his ideas to others, to destroy much of his own work. I now find myself in
this predicament; and after teaching my students for years that the
Carboniferous epoch belongs to the Palaeozoic or Primary age, I am
convinced--and this conviction grows upon me constantly as I free myself
from old prepossessions and bias on the subject--that with the
Carboniferous epoch we have the opening of the Secondary age in the
history of the world. A more intimate acquaintance with organic remains
has shown me that there is a closer relation between the character of the
animal and vegetable world of the Carboniferous epoch, as compared with
that of the Permian and Triassic epochs, than between that of the
Carboniferous epoch and any preceding one. Neither do I see any reason for
separating it from the others as a distinct age. The plants as well as the
animals of the two subsequent epochs seem to me to show, on the contrary,
the same pervading character, indicating that the Carboniferous epoch
makes an integral part of that great division which I have characterized
as the Secondary age.

Within the Periods there is a still more limited kind of geological
division, founded upon the special character of local deposits. These I
would call geological Formations, indicating concrete local deposits,
having no cosmic character, but circumscribed within comparatively narrow
areas, as distinguished from the other terms, Ages, Epochs, Periods, which
have a more universal meaning, and are, as it were, cosmopolitan in their
application. Let me illustrate my meaning by some formations of the
present time. The accumulations along the coast of Florida are composed
chiefly of coral sand, mixed of course with the remains of the animals
belonging to that locality; those along the coast of the Southern States
consist principally of loam, which the rivers bring down from their swamps
and low, muddy grounds; those upon the shores of the Middle States are
made up of clay from the disintegration of the eastern slopes of the
Alleghanies; while those farther north, along our own coast, are mostly
formed of sand from the New-England granites. Such deposits are the local
work of one period, containing the organic remains belonging to the time
and place. From the geological point of view, I would call them
Formations; from the naturalist's point of view, I would call them
Zooelogical Provinces.

Of course, in urging the application of these names, I do not intend to
assume any dictatorship in the matter of geological nomenclature. But I do
feel very strongly the confusion arising from an indiscriminate use of
terms, and that, whatever names be selected as most appropriate or
descriptive for these divisions, geologists should agree to use them in
the same sense.

There is one other geological term, bequeathed to us by a great authority,
and which cannot be changed for the better: I mean that of Geological
Horizon, applied by Humboldt to the whole extent of any one geological
division,--as, for instance, the Silurian horizon, including the whole
extent of the Silurian epoch. It indicates one level in time, as the
horizon which limits our view indicates the farthest extension of the
plain on which we stand in space.

* * * * *

We left America at the close of the Carboniferous epoch, when the central
part of the United States was already raised above the water. Let us now
give a glance at Europe in those early days, and see how far her physical
history has advanced. What European countries loom up for us out of the
Azoic sea, corresponding in time and character to the low range of hills
which first defined the northern boundary of the United States? what did
the Silurian and Devonian epochs add to these earliest tracts of dry land
in the Old World? and where do we find the coal basins which show us the
sites of her Carboniferous forests? Since the relation between the epochs
of comparative tranquillity and the successive upheavals has been so
carefully traced in Europe, I will endeavor, while giving a sketch of that
early European world, to point out, at the same time, the connection of
the different systems of upheaval with the successive stratified deposits,
without, however, entering into such details as must necessarily become
technical and tedious.

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