and deep. The gradual
cooling must have exerted continued pressure, and the wrinkles have
become larger. It is not likely that new wrinkles would be formed as
long as the old ones would crumple and draw up into narrower, steeper
slopes, in response to the lateral crushing.

We can imagine those first mountains rising as folds under the sea.
Gradually their bases were narrowed, and their crests lifted out of the
water. They rose as long, narrow islands, and grew in size as time went
on.

Why is the trend of the great mountain systems almost always north and
south? Study the map of the continents and see how few cross ranges are
shown, and how short they are, compared with the others. The molten
globe bulged at its equator, as it rotated on its axis. The moon added
its strong pulling force to make it bulge still more. As the crust
thickened, it became less responsive to the two forces that caused it to
bulge. The shrinkage was greatest where the globe had been most pulled
out of shape. The rate of the earth's rotation is believed to have
diminished. Every change tended to let the earth draw in its (imaginary)
belt, a notch at a time. The forces of contraction acted along the line
of the equator, and formed folds running toward the poles. In this early
time the great mountain systems were born, and they grew in size
gradually, from small beginnings.

These mountains of upheaval, made by the bending of the earth's crust,
and the formation of alternating ridges and depressed valleys, are many.
The earth is old and much wrinkled. Other mountains have been formed by
forces quite different. Volcanic mountains have been far more numerous
in ages gone than they are now.

Mt. Hood and Mt. Rainier are peaks built up by the materials thrown out
of the craters of volcanoes dead these thousands of years. Vesuvius is
at present showing us how volcanic mountains are made. Each eruption
builds larger the cone--that is, the chimney through which the molten
rocks, the ashes, and the steam are ejected. Side craters may open, the
main cone be broken and its form changed, but the mass of lava and
stones and ashes grows with each eruption. The mountain grows by the
additions it receives. AEtna is a mountain built of lava.

A third mountain system grew, not by addition, but by subtraction. The
Catskills illustrate this type. This group of mountains is the remnant
of a table-land made of level layers of red sandstone. The rest of the
high