The Western United States | Page 7

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 cañons, 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 cañon 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. In some
places cinder cones were built about the openings by the fragments of
lava which were hurled out. In other places, during periods of less
explosive eruption, molten lava flowed out in vast quantities. The lava
was very hot and almost as liquid as water, so that it spread in thin
sheets over hundreds of square miles of lowland.
One important series of fissures through which eruptions took place
marked the line where the Cascade Range was to be built. Other
volcanoes appeared over the surface of southern Idaho, central
Washington, Oregon, and northeastern California.
The eruptions were not continuous over the whole field; now in this
place, now in that, there came long periods of quiet. During such
periods the earthquakes ceased, the lava became cold, and the clouds of
volcanic ashes cleared from the air. Frequently the lava intercepted
streams and blocked the valleys so that large lakes were formed.
Whenever the periods of quiet were very long, plants spread over the
surface and animals of many kinds made their homes about the lakes.
In eastern Oregon the John Day River and its branches have eroded
cañons through the later lava and have exposed the sands, clays, and
gravels which collected at the bottom of one of those ancient lakes. In
these beds the skeletons of many strange and interesting animals have

been found. Evidently they had once lived about the borders of the lake,
and the streams had washed their bones into the water and mingled
them with the sediment.
[Illustration: FIG. 8.--BLUE LAKES, IDAHO
Formed by springs issuing from underneath the lava of the plateau]
One of these animals appears to have been an ancestor of the present
horse. It was about the size of a sheep, and had three toes instead of one.
Another, probably a very dangerous animal, was related to our present
hog, but stood nearly seven feet high. Others resembled the rhinoceros,
camel, tapir, or peccary. All but the peccary are now extinct upon this
continent. Of the carnivorous animals there were wolves and cats of
large size.
The eruptions continued, filling the valleys little by little, until in places
the lava reached a thickness of nearly four thousand feet. The lower
mountains were hidden from sight. We know of the existence of these
buried mountains because the wearing away of the lava in some places
has exposed