.

[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
fire-action had come first and water-action second; that the fire-made
rocks were all formed in very early ages, and that only water-made
rocks still continued to be formed. So the name of Primary Rocks, or
First Rocks, was given to the granites and other such rocks, and the
name of Secondary Rocks to all water-built rocks; while those of the
third class were called Transition Rocks, because they seemed to be a
kind of link or stepping-stone in the change from the First to the
Second Rocks.

The chief reason for the general belief that fire-built rocks were
older than water-built ones was, that the former are as a rule found
to lie _lower_ than the latter. They form, as it were, the basement of
the building, while the top-stories are made of water-built rocks.

Many still believe that there is much truth in the thought. It is most
probable, so far as we are able to judge, that the _first-formed_
crust of rocks all over the earth was of cooled and crystallized
material. As these rocks were crumbled and wasted by the ocean,
materials would have been supplied for the building-up of rocks, layer
upon layer.

But this is conjecture. We cannot know with any certainty the course
of events so far back in the past. And geologists are now able to
state with tolerable confidence that, however old many of the granites
may be, yet a large amount of the fire-built rocks are no older than
the water-built rocks which lie over them.

So by many geologists the names of Primary, Transition, and Secondary
Formations are pretty well given up. It has been proposed to give
instead to the crystallized rocks of all kinds the name of Underlying
Rocks (Hypogene Rocks).

But if they really do lie under, how can they possibly be of the same
age? One would scarcely venture to suppose, in look