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.
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.
VARIETIES OF FOSSILS
It would be a serious mistake to regard nature as divided into a number of distinct and independent schools of fossil making, each refusing to use the methods and devices of another. There are, however, certain features which stand out so prominently that a little classification becomes helpful. While this procedure brings out differences it should be understood that processes actually work together, several of them usually being involved in the production of any individual specimen.