strata were all left lying just as they were first formed, in smooth
level layers, one above another. But if it were so, we could know very
little about the lower layers.

We might indeed feel sure, as we do now, that the lowest layers were
the oldest and the top layers the newest, and that any fossils found
in the lower layers must belong to an age farther back than any
fossils found in the upper layers.

So much would be clear. And we might dig also and burrow a little way
down, through a few different kinds of rock, where they were not too
thick. But that would be all. There our powers would cease.

Now how different. Through the heavings and tiltings of the earth's
crust, the lower layers are often pushed quite up to the surface, so
that we are able to examine them and their fossils without the least
difficulty, and very often without digging underground at all.

You must not suppose that the real order of the rocks is changed by
these movements, for generally speaking it is not. The lower kinds are
rarely if ever found placed _over_ the upper kinds; only the ends of
them are seen peeping out above ground.

It is as if you had a pile of copy-books lying flat one upon another,
and were to put your finger under the lowest and push it up. All those
above would be pushed up also, and perhaps they would slip a little
way down, so that you would have a row of _edges_ showing side by
side, at very much the same height. The arrangement of the copy-books
would not be changed, for the lowest would still be the lowest in
actual position; but a general tilting or upheaval would have taken