But as ages go on and the process of cooling progresses, the crust
reaches a density when it supports itself, like a couple of great
arches; it no longer wrinkles; it no longer follows downward the
receding molten mass within; mountains cease to be formed; and at
length we have a red-hot ball revolving in a shell or crust, with a
space between the two, like the space between the dried and shrunken
kernel of the nut and the nut itself.

Volcanoes are always found on sea-shores or on islands. Why? Through
breaks in the earth the sea-water finds its way occasionally down
upon the breast of the molten mass; it is at once converted into gas,
steam; and as it expands it blows itself out through the escape-pipe
of the volcano; precisely as the gas formed by the gunpowder coming
in contact with the fire of the percussion-cap, drives the ball out
before it through the same passage by which it had entered. Hence,
some one has said, "No water, no volcano."

While the amount of water which so enters is small because of the
smallness of the cavity between the shell of the earth and the molten
globe within, this process is carried on upon a comparatively small
scale, and is a safe one for the earth. But suppose the process of
cooling to go on uninterruptedly until a vast space exists between the

{p. 72}

crust and the core of the earth, and that some day a convulsion of
the surface creates a great chasm in the crust, and the ocean rushes
in and fills up part of the cavity; a tremendous quantity of steam is
formed, too great to escape by the aperture through which it entered,