All that is known of the extinct plants and animals which inhabited the
earth before man began the practice of recording his observations has
been obtained from a study of the rocks. The few possible exceptions to
this rule, in which animal and plant remains have been preserved by
freezing or drying, are so unusual as to be hardly worth mentioning.
Explanation of this is that plant and animal tissues quickly decay under
ordinary conditions when life ceases. Unless protected from destructive
agencies which are especially active at the surface of the ground, even
the heavier bones of animals and the large trunks of fallen trees will
soon crumble into shapeless masses. The usual method employed by nature
to prepare a fossil specimen is so closely related to the natural
process of rock making that a little knowledge of that subject will be
necessary in order to know what fossils are and how they are preserved
for so long a time.
It should be understood first that a fossil is some record, commonly
preserved in rock, of a kind of plant or animal which no longer exists
as a living type. This, at least, is the ordinary sense of the word and
more elaborate definitions are of small service to anyone. It may be
necessary to add, however, that all things which have lived at any time
are regarded as either plants or animals.
Nature’s way of producing rocks and fossils remains a mystery to many of
us because we are so wrapped up with the importance of finding names for
things and materials that we frequently neglect the consideration of
sources and histories. Everyone knows a rock when he sees it in a large
mass, but when he looks at sand, mud, dust, or soil, he seldom thinks of
it as related in any way to rocks. Although the difference is almost
entirely a matter of size, our use of words makes it seem unreasonable
to speak of the finer particles as rock.
There can be no reality or meaning in the natural record for an
individual who has failed to observe a few simple facts which involve
changes going on in all parts of the world at the present time. With
regard to rocks, it is supposed that what happens in our day also
occurred under like circumstances ages ago. Anyone wishing to do so may
see for himself that rock masses break down wherever they are exposed to
the elements, that the larger pieces are reduced to smaller fragments,
and that the final product is sand or dust.
He may also note that this finely ground material is being moved and
sorted, by rain, wind, and streams, transported to lower levels and
accumulated in great quantities wherever it finds a resting place. Along
with it go sticks and leaves, bugs, shells, bones and carcasses of
animals, some of which in time may become fossils. In large lakes and
seas there is a steady distribution of such materials over broad areas,
yesterday’s accumulation of sediments being buried by the contributions
of today, the most recent of the settlings always resting upon older
ones until something happens to disturb that arrangement.
Not so readily observed are other parts of the process, such as the
consolidation of sands and muds into the firm sandstones and shales
which we again recognize as rocks. Much of this requires more than the
few score years of a human lifetime for its accomplishment, but many of
us have seen muds become so solidified, by merely drying, that they
could hardly be distinguished from prehistoric shales. It is to be noted
also that some ancient fossils come from sandstones which are scarcely
more rock-like than the loose sands of an ocean beach. Thus we learn
that firm consolidation of rock-making materials is not always a sign of
great antiquity, and that hardness of rock is not always essential to
the preservation of imbedded plants and animals.
[Illustration: Rhinocerous Bones as Found in the Rock
This exhibit was taken from the famous fossil quarry at Agate,
Nebraska. The fossilized remains are still partially imbedded in the
sandstone which preserved them for millions of years.]
The rocks themselves must explain the many things which have happened
during the course of millions of years, and this they do remarkably well
when carefully studied, for many of the factors involved in their
histories leave characteristic marks. Changing climates, the draining of
seas, the uplifting of mountain ranges, all have ways of registering
their occurrence which are as convincing and reliable as anything ever
written by man. Piece by piece the story has been patched together
through the efforts of thousands of investigators. Parts of the
narrative remain buried at inaccessible depths, and whole chapters, no
doubt, have been destroyed by the same forces that composed this
tremendous record of prehistoric times.