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The Mohave Desert, which lies to the north of the dividing 
ranges, is triangular in outline and covers an area of approximately 
51,200 square kilometers. Itis chiefly an arid plateau with an 
elevation of 300 to 900 meters, but north of our limits is the low 
depression of Death Valley which is nearly 90 meters below sea 
level. The broad level expanse is broken here and there by short 
isolated ranges or “lone mountains,” the rocky barren slopes of 
which, save for their talus bases, rise abruptly from the floor-like 
plain. Between these elevations are numerous low depressions 
which have become sinks or “‘dry lakes.” ‘The surfaces of these 
are frequently crusted over with the deposits of soluble salts, and 
the margins lined with characteristic saline vegetation. 
To the south of the Cottonwood and the Chuckawalla Moun- 
tains lies the Colorado Desert, which extends southward along 
the gulf slope of Lower California, and eastward into southern 
Arizona and northwestern Sonora. Within the boundaries of 
California this desert, the area of which occupies a little over 
16,000 square kilometers, is principally the dry bed of a large 
inland sea or lake, with pebble-covered beaches that are still 
clearly discernible along the base of the surrounding mountains. 
At one time this depression was a continuation of the Gulf of Cali- 
fornia, from which it was cut off in comparatively recent times. 
This separation was accomplished partly, perhaps, by a slight 
elevation of the land between the vicinity of Yuma and the Cocopa 
Mountains, but chiefly by the delta formed at the mouth of 
the Colorado River, which latter carries great quantities of 
silt. Inward from the gravelly rim of the depression the character 
of the soil gradually changes from a sandy loam into the heaviest 
of clays, while the center, now covered by an accidental over- 
flow from the Colorado River, is normally a salt bed several feet 
in thickness. 
On the western or coastal side of the mountains the foot-hills 
and mountains give way here and there to valleys often of con- 
siderable extent. These valleys are mainly very fertile, supporting 
a luxuriant vegetation wherever water is plentiful, but areas of 
low alkaline soil, or dry gravelly mesas or washes are frequently 
interspersed. Along the southern base of the San Gabriel and 
the San Bernardino Mountains the original floor of the valley 
has been buried by debris of gravel and coarse sands, washed down 
by the winter torrents from the steep mountain slopes. Here 
