vast lake or furnace of melted fiery-hot material, with only a thin
cooled crust covering it. Some in the present day are inclined to
question this, and hold rather that the earth is solid and cold
throughout, though with large lakes of liquid fire here and there,
under or in her crust, from which our volcanoes are fed....

The materials of which the crust is made are many and various; yet,
generally speaking, they may all be classed under one simple word, and
that word is--_Rock_.

It must be understood that, when we talk of rock in this geological
sense, we do not only mean hard and solid stone, as in common
conversation. Rock may be changed by heat into a liquid or "molten"
state, as ice is changed by heat to water. Liquid rock may be changed
by yet greater heat to vapor, as water is changed to steam, only we
have in a common way no such heat at command as would be needed to
effect this. Rock may be hard or soft. Rock maybe chalky, clayey, or
sandy. Rock may be so close-grained that strong force is needed to
break it; or it may be so porous--so full of tiny holes--that water
will drain through it; or it may be crushed and crumbled into loose
grains, among which you can pass your fingers.

The cliffs above our beaches are rock; the sand upon our seashore is
rock; the clay used in brick-making is rock; the limestone of the
quarry is rock; the marble of which our mantel-pieces are made is
rock. The soft sandstone of South Devon, and the hard granite of the
north of Scotland, are alike rock. The pebbles in the road are rock;
the very mould in our gardens is largely composed of crumbled rock. So
the word in its geological sense is a word of wide meaning.