can pick up is heavy, how much heavier is a great boulder
that it takes a four-horse team to haul. What tremendous weight there is
in all the boulders scattered on a hillside! The hill itself could not
be made level without digging away thousands of tons of earth. The
earth's outer crust, with its miles in depth of mountains and level
ground, is a crushing weight lying on the heated under-substance. Every
foot of depth adds greatly to the pressure exerted upon the mass, for
the attraction of gravitation increases amazingly as the centre of the
earth is approached.

It is now believed that the earth is solid to its centre, though heated
to a high degree. Terrific pressure, which causes this heat, is exerted
by the weight of the crust. A crack in the crust may relieve this
pressure at some point, and a mass of substance may be forced out and
burst into a flaming stream of lava. Such an eruption is familiar in
volcanic regions. The fact that red-hot lava streams from the crater of
Vesuvius is no proof that it was seething and bubbling while far below
the surface.

Volcanoes, geysers, and hot springs prove that the earth's interior is
hot. The crust is frozen the year around in the polar regions, and never
between the Tropics of Cancer and Capricorn. The sun's rays produce our
different climates, but they affect only the surface. Underground, there
is a rise of a degree of temperature for every fifty feet one goes
down. The lowest mine shaft is about a mile deep. That is only one
four-thousandth of the distance to the earth's centre.

By an easy computation we could locate the known melting-point for
metals and other rock materials. But one degree for each fifty feet of
depth below the surface may not be correct for the second mile, as it is
for the first. Again, the melting-point is probably a great deal higher
for substances under great pressure. The weight of the crust is a burden
the under-rocks bear. Probably the pressure on every square inch reaches
thousands of tons. Could any substance become liquid with such a weight
upon it, whatever heat it attained? Nobody can answer this question.

The theory that volcanoes are chimneys connecting lakes of burning lava
with the surface of the earth is discredited by geologists. The weight
of the overlying crust would, they think, close such chambers, and
reduce liquids to a solid condition.

Since the first land rose above the sea, the crust of the earth has
gradually beco