ant lava cover
the softer rocks.

There are other canons in the plateau region which are fully as
remarkable as those which have been mentioned. That of the Des
Chutes River in central Oregon is in places a thousand feet deep,
with almost vertical walls of lava.

We have already seen how mountains have been formed upon the Columbia
plateau, by a bending of the earth upward. Other mountains of the
plateau are due to fractures in the solid rocks, often many miles
long. Upon one side of these fractures the surface has been depressed,
while upon the other it has been raised. The amount of the uplift
varies from a few hundred to thousands of feet. The mountains thus
formed have a long, gentle slope upon one side and a very steep
incline upon the other. They are known as "block mountains," and
those upon the Columbia plateau are the most interesting of their
kind in the world.

With the exception of a few large rivers, the greater portion of the
Columbia plateau is remarkable for its lack of surface streams. The
water which reaches the borders of the plateau from the surrounding
mountains often sinks into the gravel between the layers of lava and
forms underground rivers. The deep canons which have been mentioned
intercept some of these underground rivers, so that their waters
pour out and down over the sides of the canons in foaming cascades.
The greatest of these cascades is that known as the Thousand Springs
in the Snake River canon. The waters of the Blue Lakes in the canon
of the same river below Shoshone Falls also come from underneath
the lava. They are utilized in irrigating the most picturesque
fruit ranch in southern Idaho.

[Illustration: FIG. 11.--CANON OF CROOKED RIVER NEAR THE DES CHUTES
RIVER

Eroded in the Columbia plateau]

The climate of the plateau is dry, and its eastern portion is
practically a desert. Toward the west, however, the rainfall is
greater, and in central Washington and northern Oregon the plateau
becomes one vast grain-field. It is difficult to irrigate the plateau
because the streams flow in such deep canons, but above the point
where the canon of the Snake River begins there is an extensive
system of canals and cultivated fields. With a sufficient water
supply, the lava makes one of the richest and most productive of
soils. Along the Snake and Columbia rivers, wherever there is a
bit of bottom land, orchards have been planted. Little steamers
ply along these rivers between the rap