[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.

[Illustration: LAVA-STREAM ON VESUVIUS.]

Just as the different kinds of Stratified Rocks are often called
Aqueous Rocks, or rocks formed by the action of water--so these
different kinds of Unstratified Rocks are often called Igneous Rocks,
or rocks formed by the action of fire--the name being taken from the
Latin word for fire. The Metamorphic Rocks are sometimes described as
"Aqueo-igneous," since both water and fire helped in the forming of
them.

It was at one time believed, as a matter of certainty, that granite
and such rocks belonged to a period much farther back than the periods
of the stratified rocks. That is to say, it was supposed that