pposed that such a tremendous
furnace of heat--a furnace nearly eight thousand miles in
diameter--could not fail to break up and melt so slight a covering
shell.

Many believe, therefore, not that the whole interior of the earth is
liquid with heat, but that enormous fire-seas or lakes of melted rock
exist here and there, under or in the earth-crust. From these lakes
the volcanoes would be fed, and they would be the cause of earthquakes
and land-upheavals or land-sinkings. There are strong reasons for
supposing that the earth was once a fiery liquid body, and that she
has slowly cooled through long ages. Some hold that her centre
probably grew solid first from tremendous pressure; that her crust
afterwards became gradually cold; and that between the solid crust and
the solid inside or "nucleus," a sea of melted rock long existed, the
remains of which are still to be found in these tremendous fiery
reservoirs.

The idea accords well with the fact that large numbers of extinct or
dead volcanoes are scattered through many parts of the earth. If the
above explanation be the right one, doubtless the fire-seas in the
crust extended once upon a time beneath such volcanoes, but have since
died out or smouldered low in those parts.

A somewhat curious calculation has been made, to illustrate the
different modes of working of these two mighty powers--Fire and Water.

The amount of land swept away each year in mud, and borne to the ocean
by the River Ganges, was roughly reckoned, and also the amount of land
believed to have been upheaved several feet in the great Chilian
earthquake.

It was found that the river, steadily working month by month, would
require some four hundred years to carry to the sea the same weight of
material, which in one tremendous effort was upheaved by the fiery
underground forces.

Yet we must not carry this distinction too far. Fire does not always
work suddenly, or water slowly; witness the slow rising and sinking of
land in parts of the earth, continuing through centuries; and witness
also the effects of great floods and storms.

The crust of the earth is made of rock. But what is rock made of?

Certain leading divisions of rocks have been already considered:

The Water-made Rocks;

The Fire-made Rocks, both Plutonic and Volcanic;

The Water-and-Fire-made Rocks.

The first of these--Water-made Rocks--may be subdivided into three
classes. These are,--

I. _Flint Rocks_; II. _Clay Rocks_