will form a mountain range.

The great mountain systems border the oceans, for the lines of
weakness occur where the land dips steeply down beneath the water.
It sometimes happens that the fractures in the rocks where mountains
are being made are situated underneath the water, or in some position
where water passes down through them in large quantities.

What do you think would happen if such an underground stream of
water came in contact with hot or molten rocks far below the surface?
Note the effect produced by drops of water falling upon a hot stove.
Each one, as it strikes, is partly changed to steam with a slight
explosive sound. The result is similar when water is turned into
the hot and nearly empty boiler of a steam-engine--an explosion
is sure to follow.

When the pressure of steam suddenly formed within the earth is
too great, a volcanic explosion takes place at some point where
the overlying rocks are weakest, probably on or near one of the
lines of fracture about which we have been speaking. The explosion
is accompanied by thundering noises, tremblings of the earth, and
the hurling of rock and molten lava into the air. That the rocks
of the earth's crust are elastic is shown by the rebounding of a
pebble thrown against a large boulder. If a file be drawn across
the edge of a sheet of tin upon which sand has been sprinkled, the
tin vibrates over its whole extent, as is shown by the jumping
of the sand grains. Because of like elasticity in the materials
which make up the surface of the earth, the vibrations produced
by an explosion are carried through the solid earth for hundreds
of miles.

[Illustration: FIG. 20.--EARTHQUAKE FISSURES NEAR MONO LAKE, CALIFORNIA]

The records of earthquakes show that they are much more violent and
occur oftener where the crust of the earth is being disturbed by
folding. We have seen that there are two main causes of earthquakes:
the slipping of portions of the earth past each other along a fissure,
and the contact of water with very hot rocks far below the surface.
It is probable that the earthquakes which occur so often in the
western portion of the United States are due to the first of these
causes. The numerous extinct volcanoes show that at one period
this region was frequently shaken by explosive eruptions.

[Illustration: FIG. 21.--THE WASATCH RANGE

From Salt Lake City]

Mono Lake (see Fig. 42, page 99), at the eastern base of the Sierra
Nevada Range, has been