The observer who glances over a rich and fertile plain, watered by
rivers and streams which have, during a long series of ages, pursued the
same uniform and tranquil course; the traveller who contemplates the
walls and monuments of a great city, the first founding of which is lost
in the night of ages, testifying, apparently, to the unchangeableness of
things and places; the naturalist who examines a mountain or other
locality, and finds the hills and valleys and other accidents of the
soil in the very spot and condition in which they are described by
history and tradition--none of these observers would at first suspect
that any serious change had ever occurred to disturb the surface of the
globe. Nevertheless, the earth has not always presented the calm aspect
of stability which it now exhibits; it has had its convulsions, and its
physical revolutions, whose story we are about to trace. The earth, like
the body of an animal, is wasted, as the philosophical Hutton tells us,
at the same time that it is repaired. It has a state of growth and
augmentation; it has another state, which is that of diminution and
decay: it is destroyed in one part to be renewed in another; and the
operations by which the renewal is accomplished are as evident to the
scientific eye as those by which it is destroyed. A thousand causes,
aqueous, igneous, and atmospheric, are continually at work modifying the
external form of the earth, wearing down the older portions of its
surface, and reconstructing newer out of the older; so that in many
parts of the world denudation has taken place to the extent of many
thousand feet. Buried in the depths of the soil, for example, in one of
those vast excavations which the intrepidity of the miner has dug in
search of coal or other minerals, there are numerous phenomena which
strike the mind of the inquirer, and carry their own conclusions with
them. A striking increase of temperature in these subterranean places
is one of the most remarkable of these. It is found that the temperature
of the earth rises one degree for every sixty or seventy feet of descent
from its surface. Again: if the mine be examined vertically, it is found
to consist of a series of layers or beds, sometimes horizontal, but more
frequently inclined, upright, or contorted and undulating--even folded
back upon themselves. Then, instances are numerous where horizontal and
parallel beds have been penetrated, and traversed vertically or
obliquely by veins of ores or minerals totally different in their
appearance and nature from the surrounding rocks. All these undulations
and varying inclinations of strata are indications that some powerful
cause, some violent mechanical action, has intervened to produce them.
Finally, if the interior of the beds be examined more minutely--if,
armed with the miner’s pick and hammer, the rock is carefully broken
up--it is not impossible that the very first efforts at mining may be
rewarded by the discovery of some fossilised organic form no longer
found in the living state. The remains of plants and animals belonging
to the earlier ages of the world, are, in fact, very common; entire
strata are sometimes formed of them; and in some localities the rocks
can scarcely be disturbed without yielding fragments of bones and
shells, or the impressions of fossilised animals and vegetables--the
buried remains of extinct creations.
These bones--these remains of animals or vegetables which the hammer of
the geologist has torn from the rock--belong possibly to some organism
which no longer any where exists: it may not be identical with any
animal or plant living in our times: but it is evident that these
beings, whose remains are now so deeply buried, have not always been so
covered; they once lived on the surface of the earth as plants and
animals do in our days, for their organisation is essentially the same.
The beds in which they now repose must, then, in older times have formed
the surface of the earth; and the presence of these fossils proves that
the earth has suffered great mutations at some former period of its
history.
Geology explains to us the various transformations which the earth has
passed through before it arrived at its present condition. We can
determine, with its help, the comparative epoch to which any beds
belong, as well as the order in which others have been superimposed upon
them. Considering that the stratigraphical crust of the earth with which
the geologist has to deal may be some ten miles thick, and that it has
been deposited in distinct layers in a definite order of succession, the
dates or epochs of each formation may well be approached with hesitation
and caution.
Dr. Hutton, the earliest of our philosophical geologists, eloquently
observes, in his “Theory of the Earth,” that the solid earth is
everywhere wasted at the surface. The summits of the mountains are
necessarily degraded. The solid and weighty materials of these mountains
have everywhere been carried through the valleys by the force of running
water. The soil which is produced in the destruction of the solid earth
is gradually transported by the moving waters, and is as constantly
supplying vegetation with its necessary aid. This drifted soil is at
last deposited upon some coast, where it forms a fertile country. But
the billows of the ocean again agitate the loose material upon the
shore, wearing away the coast with endless repetitions of this act of
power and imparted force; the solid portion of our earth, thus sapped to
its foundations, is carried away into the deep and sunk again at the
bottom of the sea whence it had originated, and from which sooner or
later it will again make its appearance. We are thus led to see a
circulation of destruction and renewal in the matter of which the globe
is formed, and a system of beautiful economy in the works of Nature.
Again, discriminating between the ordinary and scientific observer, the
same writer remarks, that it is not given to common observation to see
the operation of physical causes. The shepherd thinks the mountain on
which he feeds his flock has always been there. The inhabitant of the
valley cultivates the soil as his fathers did before him, and thinks the
soil coeval with the valley or the mountain. But the scientific observer
looks into the chain of physical events, sees the great changes that
have been made, and foresees others that must follow from the continued
operation of like natural causes. For, as Pythagoras taught 2,350 years
ago, “the minerals and the rocks, the islands and the continents, the
rivers and the seas, and all organic Nature, are perpetually changing;
there is nothing stationary on earth.” To note these changes--to
decipher the records of this system of waste and reconstruction, to
trace the physical history of the earth--is the province of GEOLOGY,
which, the latest of all modern sciences, is that which has been
modified most profoundly and most rapidly. In short, resting as it does
on observation, it has been modified and transformed according to every
series of facts recorded; but while many of the facts of geology admit
of easy and obvious demonstration, it is far otherwise with the
inferences which have been based upon them, which are mostly
hypothetical, and in many instances from their very nature incapable of
proof. Its applications are numerous and varied, projecting new and
useful lights upon many other sciences. Here we ask of it the teachings
which serve to explain the origin of the globe--the evidence it
furnishes of the progressive formation of the different rocks and
mineral masses of which the earth is composed--the description and
restoration of the several species of animals and vegetables which have
existed, have died and become extinct, and which form, in the language
of naturalists, the _Fauna_ and _Flora_ of the ancient world.
* * * * *
In order to explain the origin of the earth, and the cause of its
various revolutions, modern geologists invoke three orders of facts, or
fundamental considerations: