1879. | Fossil Shells from the Colorado Desert. 153 
or catastrophic action, the present condition of the desert is the 
result of gradual and slowly working causes, and that within 
comparatively recent times, there has been a decreasing mean 
average rain and snowfall to the north, in the higher regions 
which are drained in this direction, and also an increasing but 
gradual elevation combined with increased evaporation. 
While Tryonia as before stated is apparently local, the other 
species are widely distributed. The Azodonte (mussels) not only 
inhabited the area from the Wahsatch mountains on the east, to 
the Pacific ocean on the west, but from the Colorado on the south 
to and north of the Columbia river in the north, their migration 
in the latter direction having been from the Wahsatch region also, 
as appears probable when we consider the statement of Dr. James 
Blake,! “that the divide between the waters of the Great Basin 
pe of the Columbia is the nog s g " in latitude 
° 33’ N., longitude 113° 29’ b, zi r id 600 feet 
fis height) above the level of the Salt Lake basin.” And further 
he remarks “ the height of the divide above the level of Queen’s 
River valley is about six hundred feet, but I am of the opinion 
that other outlets must exist, which allowed the waters of the 
basin to attain a still lower level, before their disappearance solely 
by evaporation began. There must have been a large body of 
water ae to aigappear by evaporation, as the concretionary depos- 
its are found at an elevation of two hundred 
feet above the level of the valley and they could only have been 
Sered as the water became concentrated by evaporation. a 
Large numbers of Anodonta shells are found on the 
stirface of the ground, entirely encased in this concretionary sub- 
stance, to the thickness of two or three inches.” 
Of the two forms of Planorbis (P. gracilentus Gould, found 
dead in the desert, and P. ammon Gould, found living in the Colo- 
rado river) one is closely similar to other widely distributed 
1 Proc, Cal. Acad. Sci. Vol. iv. (p. 277, Oct.) 1872. 
Dr. Hayden believes that his parties have fixed the position of the ancient outlet 
of the great lake that once filled the Salt Lake basin. This appears to have formerly _ 
drained into the Columbia river; and the lowest pass between the great basin and 
the drainage of the river is continued directly south at the head of Marsh creek, ue 
and so low is it that one marsh connects two streams, one flowing to the Bear river — 
and the Great Lake, and the other to the Portneuf and Snake rivers of the Colum- 
bia. This generalization was actually made several years ago, but only fully men o 
lished during the past season. teres s Weekiy, J March 2d, stn . 
