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
place.

Just such a tilting or upheaval has taken place again and again with
the rocks forming our earth-crust. The edges of the lower rocks often
show side by side with those of higher layers.

But geologists know them apart. They are able to tell confidently
whether such and such a rock, peeping out at the earth's surface,
belongs really to a lower or a higher kind. For there is a certain
sort of order followed in the arrangement of rock-layers all over the
earth, and it is well known that some rocks are never found below some
other rocks, that certain particular kinds are never placed above
certain other kinds. Thus it follows that the fossils found in one
description of rock, must be the fossils of animals which lived and
died before the animals whose fossil remains are found in another
neighboring rock, just because this last rock-layer was built upon the
ocean-floor above and therefore later than the other.

All this is part of the foreign language of geology--part of the
piecing and arranging of the torn volume. Many mistakes are made; many
blunders are possible; but the mistakes and blunders are being
gradually corrected, and certain rules by which to read and understand
are becoming more and more clear.

It has been already said that unstratified rocks are those which have
been at some period, whether lately or very long ago, in a liquid
state from intense heat, and which have since cooled, either quickly
or slowly, crystallizing as they cooled.

Unstratified Rocks may be divided into two distinct classes.

[Illustration: SECTION OF A LAVA BOMB.]

First.--Volcanic Rocks, such as lava. These have been quickly cooled
at the surface of the earth, or not far below it.

Secondly.--Plutonic Rocks, such as granite. These have been slowly
cooled deep down in the earth under heavy pressure.

There is also a class of rocks, called metamorphic rocks, including
some kinds of marble. These are, strictly speaking, crystalline rocks,
and yet they are arranged in something like layers. The word
"metamorphic" simply means "transformed." They are believed to have
been once stratified rocks, perhaps containing often the remains of
animals; but intense heat has later transformed them into crystalline
rocks, and the animal remains have almost or quite vanished