THE FOSSIL FRESH- WATER SHELLS OF THE COLORADO 
DESERT, THEIR DISTRIBUTION, ENVIRONMENT, AND 
VARIATION. 
B}'^ Robert E. C Stearns, Ph. D., 
Honorary Associate in Zoology. 
On turning to an}' authentic map it will be seen that the o-reat 
range of the Sierra Nevada, as it approaches the south, makes a rather 
abrupt divergence in a westerly direction, between latitudes 35- 30' 
and 36° 30'. Here also it throws out a great spur which, after extend- 
ing for a considerable distance toward the east in a nearly easterly 
and westerly course, then deflects in its main mas.s toward the .south- 
east, with broad flanks, finally breaking down into hills of greater or 
less elevation. These lower and somewhat detached portions reach 
the Avesterh' margin of the Colorado River in the vicinity of Fort 
Yuma. This, the main spur, is known as the San Bernardino range, 
and the northerly (east and west section) is called the San Gabriel 
range, though topographically a part of the other. The rocky ram- 
parts of the San Gabriel Mountains separate the depressed levels on 
the south from the more elevated plains of the Mojave desert. On 
the west, facing the westerly slopes of the San Bernardino range, are 
the San Jacinto Mountains, a part of the Peninsular range, which 
extends southerly into the Mexican territory of Lower California. 
These mountain walls inclose a vast area of arid, desolate waste, the 
Colorado desert of California. Surrounded by mountains except in 
the south, and there opening out upon the head of the Gulf of Cali- 
fornia, it is seen that the desert was a portion of the old gulf which at 
some former time extended 200 miles above its present limits. The 
cause of the separation of the upper end of the former gulf, making 
what is now the Colorado desert, is so apparent that a moment's exam- 
ination reveals it. The same agency is still operating, widening the 
space between the present gulf and the desert. Here, nearl}' 150 miles 
from the head of the ancient gulf, came in from the east side the Colo- 
rado River, bearing in its thick floods quicksands and the red mud 
from the great plateaus of northern Arizona. The contour of the 
country shows the gulf to have been narrow here. The tilling in 
went on unceasingh' as at the mouth of every great river which enters 
Proceedings U. S. National Museum, Vol. XXIV— No. 1256. 
271 
