Reports of explorations and surveys, to ascertain the most practicable and economical route for a railroad from the Mississippi River to the Pacific Ocean: Vol. 5, Pt. 2

PROBABLE SUCCESS OF ARTESIAN BORINGS.
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 Depot 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.
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 Bernardino 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 is
probable 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 mlodify
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 lowe places, especially where water had been standing, that the tufts of
" 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
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