ee ee RA T. 
TI NUMEN * SORES aD ee ee en 
"PROBABLE SUCCESS OF ARTESIAN BORINGS. ` 225 
In addition to the supply from the mountains, a considerable quantity of water is occasionally 
received in the form of rain, which partly compensates for the loss by evaporation from the 
surface. Thus, during our sojourn at the Depót Camp, at Ocoya creek, in August, the party in 
the mountains, near Walker's Pass, experienced a heavy rain, by which they were thoroughly 
drenched. It appeared to rain quite as hard upon the surface of the Basin, and its effects were 
afterwards seen, although at a distance from the mountains the fall did not appear to have been 
very great. ۱ 
The evidences of the existence of strata of clay underlying the slopes are of great importance 
to the determination of the question as to the possibility of obtaining water from ordinary wells 
or by boring. Such strata may be regarded as comparatively impervious to water ; and as they 
occupy the hollows and basin-shaped depressions between the Lost Mountains, we have most of 
the conditions necessary for the successful construction of Artesian wells. These clayey strata 
doubtless alternate with beds of sand and gravel, and subterranean waters may either flow 
between them or at the bottom of the whole series next to the underlying granite. We have 
seen that the underground surface of the granite must be exceedingly irregular. It doubtless 
presents a series of ridges and valleys similar to those exhibited in the portions elevated above 
the general level of the sedimentary accumulations. These sedimentary strata do not lie in 
horizontal planes, but are more or less uplifted ; and even when they rest undisturbed they dip - 
gently away from the elevated ridges, as is seen along the base of the Sierra Nevada and Ber- 
nardino Sierra wherever sections are exposed. We may therefore conclude that the sediments 
conform in their stratification very nearly with the shape of the valleys, so that the lowest 
points in each are nearly in the same vertical line. 
These conditions should be carefully studied before commencing to bore for water, and it may 
be regarded as a general rule that the shape of the surface is, in the main, correspondent with 
that of the subjacent granite. In other words, the principal valleys or depressions in the Basin 
correspond with the lowest places in the granite. 
In these lowest places both the surface-and. subterranean waters collect; and in the rainy 
season, when the streams are swollen in the mountains, or during heavy rains, the temporary 
shallow ponds are formed, which leave the level expanse of clay or playa on drying up. As in 
these low places only the finer portions of the slope are brought down and deposited, it ig 
probab® that the clay extends to a great depth, or at least to the surface of older sediments or 
the bed-rock of granite, without being mingled with coarse or rudely stratified materials. All 
these conditions are favorable to the construction of Artesian wells, and it is probable that water 
can be obtained at distances convenient for railroad purposes in the portions of the Great Basin 
examined by the Expedition. 
. In selecting places for boring operations it will be necessary not only to regard the form of 
the surface of underlying rocks, and the dips of the impervious strata, but attention should be 
given to the presence and position of ridges or dykes of erupted rocks, which may greatly modify 
the general direction of the valleys in the granite, and also act as walls or barriers to the flow 
or percolation of the subterranean water. Much assistance in determining the best place to 
commence operations may be obtained by observations upon the scanty vegetation. I observed 
that in some of the low places, especially where water had been standing, that the tufts of 
ft bunch-grass’’ were not only more numerous, but more full and luxuriant, and retained some 
traces of green at their roots, while those on the higher parts of the slopes were completely 
dried. Other aids in discovering water will be found in currents of cool moist air, and in the 
29 F 
