Just as the regular succession of rock-strata has been gradually
learned, so the regular succession of different fossils is becoming
more and more understood. It is now known that some kinds of fossils
are always found in the oldest rocks, and in them only; that some
kinds are always found in the newest rocks, and in them only; that
some fossils are rarely or never found lower than certain layers; that
some fossils are rarely or never found higher than certain other
layers.

So this fossil arrangement is growing into quite a history of the
past. And a geologist, looking at certain rocks, pushed up from
underground, in England and in America, can say: "These are very
different kinds of rocks, it is true, and it would be impossible to
say how long the building up of the one might have taken place before
or after the other. But I see that in both these rocks there are
exactly the same kinds of fossil-remains, differing from those in the
rocks above and below. I conclude therefore that the two rocks belong
to about the same great age in the world's past history, when the
same animals were living upon the earth."

Observing and reasoning thus, geologists have drawn up a general plan
or order of strata; and the whole of the vast masses of water-built
rocks throughout the world have been arranged in a regular succession
of classes, rising step by step from earliest ages up to the present
time.

[Illustration]