ing in some places and rising in others,
and that where the strain of the upheaval is too great the rocks are
broken. These convulsions sometimes cause earthquakes and sometimes
volcanic eruptions, when enormous quantities of molten rock are
poured out over the surface. In all the long history of our earth
probably no greater flood of lava than that which made the Columbia
plateau was ever spread over the surface of any region. Travel where
you will over the plains of southern Idaho, central Washington,
or Oregon, and examine the rocks which here and there rise above
the soil or are exposed in the canons, and you will find that they
all appear to have been formed by fire.

[Illustration: FIG. 7.--SNAKE RIVER AT IDAHO FALLS

Just beginning to cut a canon in the volcanic plateau]

These rocks are dark in color and very hard. They are not arranged
in regular layers like sandstone and shale; many of them show numerous
little cavities which once contained steam. These cavities give
to the rock a slag-like appearance. In this kind of rock, which
we shall call lava, there are, of course, no remains of shells
or bones of animals such as are often found in rocks formed from
sand or clay.

Do not picture to yourself the Columbia plateau as one continuous
stretch of level land, for it is broken by many mountain ranges.
Some of these are old mountains which were too tall to be buried
by the lava, but most of them have been formed out of the plateau
itself. The eruptions which made the plateau extended through a
very long time, perhaps hundreds of thousands of years, and the
older lava is deeply decayed and covered with soil. Some of the
later flows show extremely rough and rugged surfaces and are probably
only a few hundred years old.

[Illustration: MAP OF THE COLUMBIA PLATEAU]

Long ago, before the eruptions began, the geography of the Northwest
was very different from what it is now. Instead of a vast plateau
there were mountains and valleys. Lowlands occupied most of the
region where the Cascade Range now rises with its lofty volcanic
peaks. Portions of the basin of the present Columbia River were
occupied by lakes which extended southwest into California.

Movements of the earth began to affect the region of the present
plateau, and at many points the solid rocks were fissured and broken.
Then from that mysterious region far beneath the surface came steam
and gases, escaping through the fissures with explosive force.