232 GEOLOGY. 
The polish of the pebbles, and the lustre of the blackened rocks, are doubtless due to a similar 
action of sand, or rather, of a much finer-grained sand and dust, driven to and fro by the winds, 
and not progressing in any one direction only, as in the gorge of the Pass. 
A pebble of a stratified rock, in silicious layers, some harder than the others, found near the 
Alamo Well, was cut on all sides, so that the hardest layers stood outin relief. "This specimen 
had evidently been cut while lying loosely on the surface, for the cutting had taken place on 
all sides alike. That its form was not due to solution of the softer parts by water was proved, 
not only by the hardness of the surface, but by the presence of little grooves, along which the 
sand had travelled, precisely as upon the rocks of San Gorgoño. Many of the specimens of 
silicified wood, found upon the surface of the Desert, are cut so deeply in grooves that it seems 
probable that they are less than half of their original size. The surfaces of such specimens are 
perfectly smooth, but are without that fine polish seen on the pebbles of the upper plain. The 
specimens which are exposed upon the open plain, where the wind is variable, and is constantly 
shifting the coarsest sand, are acted upon with much greater rapidity than on the upper plain, 
where it is believed the coarse sand has long since been chiefly rernoved by the prevailing north 
wind, leaving the pebbles to be acted on by the fine dust alone. 
ELEVATION OF THE SURFACE—TERRACES. 
No part of the Desert is much elevated above the level of the sea, and there is great reason 
to believe that most of the surface north of the emigrant road, from the Gila to the coast, is 
below it. It is below the level of the Colorado River, the stream having banks more elevated 
than the surface of the country a few miles back of it, as in the Mississippi and other rivers 
which carry down large quantities of silt and overflow their banks. "That these conditions 
exist in that region is proved by the fact that the overflow from the Colorado extends inland 
for about sixty miles, and sometimes forms a deep and rapidly flowing stream. This establishes 
the fact of a great depression, but it was likewise shown by the barometrical observations on the 
portion of the Desert still further north, and beyond the known limits of the overflow from the 
river. Thus, at the Cohuilla Springs, November 18th, a depression of 81 to 99 feet below mid- 
tide was indicated ; again, on the 20th, at the camp on Salt Creek, the mean of five observa- 
tions shows a depression of 42 feet. At these stations it was evident that there was much lower 
ground beyond, nearer to the middle of the Desert. This became very evident when the plain 
was viewed from the base of the mountains on the west side, near the point of rocks, on which 
the water-line of the ancient lake was so distinctly seen. 
From this point of view the ground appeared to slope off very gradually to the centre of the 
valley, about fifteen miles distant. The depression appeared to be as much as 500 feet. The 
extent of this depression, and the geological structure of that part of the Desert, is shown on 
Section 12, Sheet VIII. It is certain that this part of the Desert is lower than the banks of the 
Colorado. This is proved by the overflow of the river and the cu rent to the interior along the 
extended channel of New River. The surface of the Desert, therefore, ascends, slope-like, 
towards the Colorado. This observation will probably apply to the alluvial deposits of e. 
Desert only ; the upper or pebbly plain of the Desert probably does not partake of the inclina- 
tion, but remains nearly horizontal. It is probable, however, that on the northwestern margın 
of this plain it does not form a bluff or terrace, as along the course of the river and at Pilot Knob. 
