1879. ] Fossil Shells from the Colorado Desert. 145 
of the present Colorado desert, now at an elevation of probably 
1000 feet.” ? 
The sedimentary deposits super-imposed upon the more ancient 
sea or gulf bed, the top of which now constitutes the floor of the 
desert are geologically of very recent origin. 
The perpendicular section of nearly fifty feet cut by this well, 
as shown by the character of the stratum cut through in connec- 
tion with the shells contained therein, indicates a gradual deposit, 
a slow settling of sediment; the fragility of the shells and their 
present unfractured condition, prove that they were never subjected 
to abrasive action; that they were not swept violently from dis- 
tant places to this, and here buried pell-mell beneath and among 
the varied detritus of great freshets. 
Here probably was the home of some of these species, the me- 
tropolis of Tryonia, if not its specific center, which has only been 
pecs 
Anodonta californiensis Lea. Rio Colorado, Cal. 
found sparingly elsewhere, and was supposed to be extinct. 
Ninety per cent., if not more, of the shells found here belong to 
this genus. 
agency is still at work, widening the space between the Gulf and the desert. Here 
nearly 150 miles from the head of the ancient gulf came in from the east side the 
Colorado River, bearing in its thick floods, quicksands and the red mud from the 
great plateaus of northern Arizona which gives the river its colorand its name. The 
contour of the country shows the gulf to have been narrow here. The filling in of 
this alluvial deposit went on unceasingly, as at the mouth of every great river which 
enters the sea ata sheltered point. The water grew constantly shoaler, until at 
length the separation was complete. The alluvial deposit has steadily increased the 
distance between the gulf and the low bed of the desert, until now the division is 
marked by a narrow neck of thirty or forty miles of land but little raised above the 
sea level. (Dr. Widney, in Overland Monthly, Vol. x. See also various ppro en 
Arizona, &c., and the Colorado river, in Vols. iv, vi, and ix, zd.) 
1 Cooper in Proc. Cal. Acad. Sci., Vol. v, p. 403. 
