transformation. The hundreds of basins, if fed by streams from
the surrounding mountains, would then become lakes. The highest,
overflowing, would empty into a lower, and this in turn into a
still lower basin, until the water had accumulated in vast inland
seas. These seas, overflowing the rim of the Great Basin at its
lowest points, would send rivers hastening away to the ocean.

[Illustration: Map of the Great Basin showing the location and extent
of the ancient lake beds.]

What a region of lakes this would be for a time! Then they would
begin to disappear, for lakes are short-lived as compared with
mountains. Some would be filled with clay and gravel brought by
the streams. Others would be drained by a cutting down of their
outlets.

Great Salt Lake, which is the only body of water in the Basin that
has ever sent a stream to the ocean, was lowered four hundred feet
by the washing away of the rock and earth at its outlet.

We know that the rainfall never has been heavy in this region since
the Great Basin was formed, although at one time it was sufficiently
great to form two inland seas, one in northwestern Nevada, the
other in Utah.

The chief reason for the dryness of the Great Basin is the presence
of that lofty barrier, the Sierra Nevada mountain range, between
the Basin and the Pacific Ocean. The storms, which usually come
from the ocean, are intercepted by this range, and the greater
portion of their moisture is taken away. The little moisture that
remains falls upon the highlands of the Great Basin, and so relieves
its surface from utter barrenness. The adjacent slopes of the Sierra
Nevada and Wasatch ranges furnish numerous perennial streams which
feed the lakes about the borders of the Basin, such as Great Salt
Lake, Pyramid, Walker, Mono, Honey, and Owens lakes. The wet weather
streams, flowing down the desert mountains for a short time each
year, frequently form broad, shallow lakes which disappear with
the coming of the summer sun.

The climate of the Great Basin has changed from time to time. During
one period it was much drier than it is now, and the lakes were
nearly or quite dried up. It must have been a desolate region then,
shunned by animals and forbidden to man.

During the Glacial period, a few thousand years ago, the climate
was moister and cooler than it is now. The mountains were covered
with deep snows, and glaciers crept down the slopes of the higher
peaks. Great Salt Lake covere