84 
GEOLOGICAL REPORT—THIRTY-FIFTH PARALLEL. 
point further west. Dr. Shumard, as has already been stated, found the summit of the Llano, 
or the stratum immediately underneath the soil, to he composed of “ drift.” This is mentioned 
several times wherever sections of the bluffs were made. 1 The hills around the base of the Llano 
appear to have been partly composed of or capped with drift, containing agates, chalcedony, 
&c. On the 22d of June the occurrence in the drift of fossil-wood, agate, jasper, and a few 
water-worn fossil-shells of the genus Ostrea, is recorded. Similar pebbles were found on the 
25 th. 2 
These deposites evidently are connected with those noted by Mr. Marcou, who, it will be 
remembered, noted the same minerals and fossil-shells near Camp 31, east of the Antelope 
hills. 
These descriptions of the drift and alluvium, as it is called, will exactly apply to the extensive 
layers of pebbles and fossil-wood which are found to surmount the plains of the Colorado and 
Sonora deserts. There almost endless varieties of agates, jaspers, carnelians, and porphyries 
may be picked up, and these beautiful pebbles are mingled with no less beautiful specimens of 
silicified wood, with its internal structure perfectly preserved. This pebbly layer is derived from 
a thick bed of conglomerate, which rests horizontally upon sandstones, believed to be of Tertiary 
age. The same drift is found at another point, spread out over a thick bed of Tertiary fossils. 
I am much inclined to regard this drift of the Llano and the adjoining region as similar in age 
and in origin to that along the Colorado and Gila rivers. If, however, it be identical in these 
respects, it does not show that the strata upon which it is spread are of the same or of Tertiary age. 
Accumulations of pebbles, similar to these, were also found on the surface of the plateau west 
of the Sierra Madre, and it was there mingled with a larger quantity of fossil-wood. 
Alluvial deposites of the valley of the Rio Grande. —The characters of the valley of the Eio 
Grande and the Puerco have already been presented at some length. All the descriptions unite 
in asserting the sandy and generally sterile character of the soil, although it is much cultivated 
and with considerable success. Sand-dunes appear to be common on the uncultivated parts of 
the valley, and are noted by Major Emory and others. They show the sandy, arid nature of the 
soil and the prevalence of high winds. 
Alluvial or bottom-land of the Colorado. —Mr. Marcou describes the alluvium of the Colorado 
as black and fine, being without pebbles. In color it therefore differs very greatly from the 
silt which is transported by the stream ; which is a brick red, and is so abundant as to render 
the water almost opaque. Its turbid and highly colored condition is retained even to its mouth; 
and beloAvFort Yuma it deposites thick layers of its light-colored red mud in all the shallow and 
quiet bays along the shore. The bottom-land at this place also differs in color from the stream. 
The earth has a bluish-grey or ash color when dry, and is quite dark when moistened, and is 
very fertile. With regard to the fertility and agricultural availability of the bottom-land along 
the route, the best evidence is found in the large crops of maize and wheat, and vegetables, which 
are raised by the Indians. It will be seen by reference to Capt. Whipple’s report that they were 
willing to dispose of large quantities of grain for a few trifles and blankets. If Indians, with 
their rude implements and indolent habits, can provide themselves with such a superabundance 
of food, what may not be expected to result from enlightened agriculture ? 
According to Capt. Whipple, these bottom-lands are very extensive in some places, as in the 
valley of the Mojaves, having a breadth of several miles. The lands are undoubtedly favorable 
to the growth of tobacco, cotton, and many other productions of the South. 
Detritus or ivash of the Great Basin , and other localities. —In the region of the Great Basin, 
where there is an endless number of isolated mountains and ridges, entirely bare of vegetation, 
and thus subject to rapid disintegration by the action of snow and water, there is a very 
great accumulation of loose gravelly materials around all the elevations. These materials being 
chiefly derived from the rocks they surround, partake of their character and composition. The 
1 See Marcy’s Red River Report, pp. 190, 191. 
2 Ibid. 
3 See the writer’s Report on the Geology of the route explored in California by Lieutenant Williamson, (Inedited.) 
