230 
GEOLOGY. 
The surface of this plain is paved with small pebbles of various hard rocks, principally of 
volcanic origin, including variously colored porphyries, agates, and carnelians. Specimens of 
silicified wood are also numerous. All the pebbles are beautifully rounded and water-worn, 
showing that they have long been subjected to attrition, and indicating that they have been 
transported for great distances. In some places the pebbles are so thickly spread that the sur¬ 
face is entirely formed of them, and no sand or soil is visible. They are also packed together so 
closely, and lie so even, and are so uniform in size, that the surface is like a floor. Indeed the 
pebbles look as if they had been pressed into a yielding surface by a heavy roller. 
These pebbles have a peculiar polished and brilliant Surface, looking as if they had been oiled 
or varnished. The effect produced by the reflection of the sun’s rays, from a plain covered with 
these polished convex pebbles, can scarcely be imagined ; each one gives back a ray of light, 
and the ground seems paved with gems. It is somewhat like the glittering reflection from the 
ripples of a sheet of water. 
All this polishing is undoubtedly produced by the constant action of loose sand upon the 
surface, when driven by the wind. The fact that all the fine sand or dust is removed from 
between the pebbles near the surface, while it is abundant a few inches below, indicates that the 
winds have gradually blown it away, leaving the heavy pebbles behind. They thus protect the 
sand that lies below them. Indeed, the protective power of this surface of pebbles is worthy of 
remark ; for, if they were removed, it could not be long before the thick, underlying strata of 
sand would be blown away by the impetuous winds of that region. The equivalent of this 
plain of pebbles does not appear to exist on the western side of the Desert, or at least on that 
part of it north of Carrizo Creek, unless we so regard the flat-topped hills formed by the strata 
in the valley of Carrizo Creek. These are not horizontal, and do not form a continuous plain, 
but are surmounted by a pebbly drift very similar to that north of Pilot Knob, although coarser, 
and mingled with larger blocks. A great number of these pebbles are silicious, and full of 
fossils. These are apparently of the Palaeozoic and Secondary periods, and, also, of the Ter¬ 
tiary. Many of them are very beautiful; the sections of the silicified shells or corals being 
visible on the rounded and polished surfaces. Several of these pebbles were obtained on Pilot 
Knob, but specimens of much greater interest and beauty were afterwards collected by Major 
George H. Thomas, U. S. A., commanding at Camp Yuma, and presented to me. 1 Most of 
the fossils are corals, and are probably Carboniferous. 
The rocks around the Desert, especially those on the northern side, rising above the pebbly 
plain, partake of the general polishing. Pilot Knob is, perhaps, one of the best examples. 
The surfaces are not merely polished, but are discolored ; a large portion being as black as 
ebony, and yet having the ordinary gray color of granite or gneiss within. A dark, blackish- 
brown is a common tint, but it is so deep that it is impossible to decide upon the nature of the 
rock without first breaking off a fragment. The discoloration is confined to the surface, and 
does not extend to a perceptible depth. It is found alike on the rocks, pebbles, and specimens 
1 One of these pebbles of a dark color has a conical cavity in one side nearly half an inch deep, and marked on the sides by 
longitudinal septse. It is the silicified imprint or portion of a coral allied to the CyalhophylUz. The structure of the coral is very 
clearly shown. It is without transverse sept®, and this indicates its modern or Cretaceous age. In another specimen the septse 
are distinct, and it is probably Carboniferous. 
One of the specimens, of a dark chocolate-brown color, is filled with small, white fossil shells, discoid, and apparently thickest 
in the centre, like Nummulite s. Some of the sections exposed on the worn surface are like those of Fusilina, to which it is 
probable the fossils may be appropriately referred. The pebbles containing stems of encrinites are the most numerous. It is 
regretted that it has not been possible, since the specimens were received, to prepare sections and give them that close examina¬ 
tion which they deserve. Specimens are deposited in the collection, No. 401. 
