1879. ] Fossil Shells from the Colorado Desert. 143 
barometer. Shall we indulge in a guess as to the depth of the 
water when these shells were alive; shall we add the depth of the 
well to the above figures, and again add the elevation of bench- 
marks, the ancient levels? which form terrace lines in some places 
along the distant hills, once a part of the shores of a now extinct 
lake, the walls of the basin which once enclosed and held a fresh- 
water sea? 
It may have been, however, that the lake was never as deep as 
the figures thus added would indicate, and that instead of a lake 
or series of lakes, there existed only a lagoon or chain of 
lagoons, connected or disconnected, according to the volume of 
water, which probably varied one season compared with another; 
a system of shallow reservoirs, receiving the catchment or sur- 
plus surface water in periods or seasons of unusual rainfall; some- 
times after a prolonged and widespread storm of great severity, 
uniting and forming an extensive expanse a few feet only in depth, as 
was seen in the valleys of California during the notable winter 
of 1861-2. 
The rate of depression may have been such as to continue to 
keep the lagoons supplied during ordinary seasons with catch- 
ment or surface water from the immediate neighborhood, and 
have been so proportioned (involving the mean of supply) to the 
rate of evaporation, that only within a very recent period has this 
depressed portion of the Colorado basin become bare and dry. 
Are the phenomena which this vast and remarkable region 
exhibits—of which that portion usually called the desert, is but a 
part—the result of catastrophic action, sudden, violent and wide- 
spread, or the result of gradual changes moving slowly through 
countless centuries? 
he physiognomy of the wider and more general region in 
some of its aspects indicates action which, in geologic time is 
unmistakably recent, and the varied phenomena here exhibited, 
are more likely the result of periods of slow movement as well 
1 For miles and miles ï traced with the a a strange, well-defined line along the 
mountain sides, awer at the same level * * Riding out to it I found it to be 
the old beach of a sea 
The rocks were worn and rounded up to that level, * * * above that line 
the rocks were sharp and jagged. The surveying party of the Southern Pacific rail- _ 
road, in running the line to Fort Yuma, struck the present sea-level the moment their _ 
instruments reached this ancient beach.—Dr. J. P. Widney in Overland deii, oe 
Vol. x, 1873. 
