a density five and a half times greater than water."

"If that is the case, why is it that the molten metal flows out of the
volcanoes?"

"There may be fissures in the earth, or portions less dense than others
which, by the general disarrangements of the adjacent parts, and by the
enormous pressure exerted by the force of gravity, are contracted, and
the movement causes such friction and intense heat as to liquefy the
rock. In doing so a large amount of gas is evolved, the movement of
which causes the disturbance of the earth's crust, which manifests
itself to us in the form of earthquakes. At the same time the confined
gases seek an outlet, which they find at the weakest part, and the
volcanoes spout forth the lava, flame, and gases. There is an undoubted
connection between earthquakes and volcanic eruptions. Earthquakes
usually precede volcanic action. This internal combustion is going on at
all times, and is only more violent at some period than at others. The
lava in the Crater of Stromboli has been in a liquid state for more than
two thousand years."

"Before we left home I saw in a paper that some scientist described the
kind of rock and other matter which was seven miles down in the earth."

"Was anyone ever down as far as that?"

"No; a little over a mile is as far as man has actually penetrated the
earth."

"Then, I should like to know how geologists can tell with any certainty
what the rock is like several miles down?"

"That is known just as positively as though a hole had been dug down
that distance."

"I don't see how that is possible."

"I am going to make you a sketch which you can examine at leisure, that
will show how he knows. Assuming that the earth has a crust--that is,
the outside or cooled part, let the first sketch (Figure 10) represent
this crust, before the mountains and valleys were formed. The slightly
curved horizontal lines merely represent the different layers of the
crust, such as rock, clay, coal, slate, and the like. When the cooling
process took place the earth grew smaller within, so that the crust was
forced together.

[Illustration: _Fig. 10. NORMAL CRUST OF THE EARTH_]

"The second sketch (Figure 11) shows this crust forced together, so that
when the upheaval took place, two mountain ranges, A and B, were formed,
with a valley (C) between them, and the broken lines (D), where the
crust separated, were exposed, and by that means examinations can be
readily made way down