COLORADO DESERT. 17 
formed by sudden storms bursting upon surfaces covered with loose material, but bare of vege- 
tation. 
Emerging from the bed of Carisso creek, where its walls are formed of green, pink, brown, 
and white marls—the series of Miocene strata already mentioned, which are filled with shells 
and plates of selenite—the road, crossing the desert, descends for many miles along the slope 
formed by the materials washed down from the mountain. These materials consist of repre- 
sentatives of the granitic rocks of which I have spoken, with the addition of fragments of meta- 
morphic silicious slate, and a highly crystalline blue, or white limestone. The blocks which 
strew the surface in the vicinity of the mountains are generally angular, and frequently of con- 
siderable size. Further out in the desert they are smaller, and are usually rounded, evincing. 
in their form the attrition to which they have been subjected during their transport. Most of 
the stones which cover the surface are polished and blackened in a peculiar manner, many of 
them reflecting the sun’s rays like glass. All have been exposed to the action of a common 
influence, by which they have, in some cases, been curiously weathered and eroded. The blocks 
of limestone have been most affected by this influence, and the surfaces of all the calcareous 
fragments are etched into singular and sometimes beautiful figures. This is probably in great 
degree owing to the action of drifting sand, as we find nowhere else, upon similar material, 
exposed to ordinary weathering, a like effect produced. 
At Carisso creek we were at an elevation of about four hundred and fifty feet above the level 
of the sea. Within the first twenty miles after leaving the base of the mountains we had 
descended nearly to the sea level, and had left behind us the accumulations of boulders and 
gravel, and had come on to the fine alluvial clays and sand which form the desert surface for 
the greater part of the distance between the peninsula Sierra and the Colorado. 
This depressed area doubtless exhibits nearly the same features that it did before it was 
elevated to its present level. It is everywhere underlaid by a series of Tertiary strata, probably 
continuous with that containing the Ostreas, Anomias, and Gnathodons, about the mouth of Carisso 
creek, and of Miocene age. The fossils I have enumerated prove the strata which include them 
to have been, in part at least, deposited in brackish water, showing that the area of the desert 
was then occupied by a narrow arm of the sea into which was poured the fresh water drainage 
of a large surface. In other words, that the present gulf of California formerly extended much 
further north than now. 
The Tertiary strata underlying the central portion of the desert are fine clays and sands, and 
are doubtless the sediments deposited from comparatively deep and still water, precisely such 
as are now being deposited at the head of the Gulf of California. Like them, too, they are with- 
out fossils. * 
A portion of the Colorado desert is now scarcely higher than the sea level, and we have 
evidence that since the retreat of the Gulf much of it has been covered by a continuous sheet 
of fresh water. The evaporation of moisture has, however, in modern times, exceeded the 
precipitation, and most of the surface is now constantly dry; though at the lagoons there still 
remains a miniature representative of the wide-spread fresh water lake which once occupied 
the area surrounding them. 
From Sackett’s Wells to the Alamo Mocho, a ‘Seutaies of nearly biey miles, we were almost 
constantly travelling over a surface strewed with fresh water shells—Anodonta, Planorbis, Physa, 
and Amnicola—showing plainly that we were traversing the bed of a former lake. These shells 
are very abundant, the Amnicole being sometimes drifted by the wind till they cover and whiten 
the ground, and look like miniature snow wreaths. They have generally lost their epidermis, 
and the Anodontas are considerably broken and decayed, but on the whole are so well preserved 
that it seems impossible that many years have elapsed since they were inhabited by — 
animals. 
Through the depressed portion of the desert New river, which has been already fully des- — 
3——L 
