ees. As
the river bed rises by the deposit of mud, the levees are built higher
to contain the rising waters. No longer does the rich soil of the
Mississippi flood plain receive layers of sediment from the river's
overflow. The river very rarely breaks through a levee. The United
States Government has spent great sums in walling in the river, and each
state along its banks does its share toward paying for this
self-protection.

By means of _jetties_ the river's current is directed into a
straightened course, and its power is expended upon the work of
deepening its own channel and carrying its sediment to the Gulf. Much as
the river has been forced to do in cleaning its own main channel,
dredging is needed at various harbours to keep the river deep enough for
navigation. The forests of the mountain slopes in Colorado are being
slaughtered, and the headwaters of the Missouri are carrying more and
more rocky debris to choke the current of the Mississippi. Colorado soil
is stolen to build land in the vast delta, which is pushing out into the
Gulf at the rate of six miles in a century--a mile in every sixteen
years. The Mississippi delta measures 14,000 square miles. With the
continued denuding of mountain slopes, we shall expect the rate of delta
growth to be greatly increased, until reforesting checks the destructive
work of wind and water.

THE MAKING OF MOUNTAINS

The gradual thickening and shrinking of the earth's crust as it cools
have made the wrinkles we call mountain systems. Through millions of
years the globe has been giving off heat to the cold sky spaces through
which it swings in its orbit around the sun. The cooling caused the
contraction of the outer layer to fit the shrinking of the mass. When a
plump peach dries on its pit, the skin wrinkles down to fit the dried
flesh. The fruit shrinks by loss of water, just as the face of an old
person shrinks by loss of fat. The skin becomes wrinkled in both cases.

The weakest places in the earth's crust were the places to crumple,
because they could not resist the lateral pressure that was exerted by
the shrinking process. Along the shores of the ancient seas the rivers
piled great burdens of sediment. This caused the thin crust to sink and
to become a basin alongside of a ridge. The wearing away of the land in
certain places lightened and weakened the crust at these places, so that
it bent upward in a ridge.

Perhaps the first wrinkles were not very high