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

GEOLOGY.
tions of much more enduring and resisting materials. In fact, we must refer the detailed
topography of all countries to the action of water upon the frame-work of rocks that has been
prepared and elevated by subterranean forces. It is only when the results are presented in a
form that can be readily comprehended that they become astounding.
The materials of which this formation is composed are exceedingly light and yielding, and
they are not protected by any consolidated layers of great extent. The hills, also, being devoid
of trees and vegetation, are not covered by a soil or sod bound together by roots; the surface
water has, therefore, free action upon the earth, and the course of its drainage is not obstructed.
Under these conditions, every shower that pours down there in the winter season acts rapidly
upon the soft materials, and every rivulet, stream, and torrent, in its course towards the rivers,
becomes turbid with immense quantities of clay held in suspension. It is by this continually
repeated action of the water that the deep valleys of the Ocoya and Posuncula rivers, and their
thousand tributary ravines, each with its multiplied diverging channels, have been excavated
from what was formerly an elevated and unbroken plateau. The quantity of earth that has
been removed in this manner is enormous, and may be regarded as nearly equal to the amount
that still remains in the hills. All this removed material has been transported down into the
Tulare valley, from which a portion may have again been carried into the San Joaquin. When
we consider the extent of the erosion of these valleys, we can readily conceive that the alluvial
deposits of the Tulares have a very great thickness. They are very broad, and cover a much
greater area than is now occupied by the lakes.
Some of the highest hills of this formation are found along the banks of Ocoya creek, where
the best natural sections that were found occur. They are near the spot marked as Depot camp
on the general map. The hills in this vicinity rise about eight hundred feet above the bed of
the stream; and the lines of horizontal stratification are traceable from one hill to another across
deep ravines.
The great inclination of the slopes of these hills is worthy of mention; many of those near
our camp were so steep that it was impossible to ascend them without winding around their bases
in some of the side ravines. The perfectly even surface that they present, entirely bare of rocks
or vegetation of any size, renders the ascent of the steep slopes impossible. I measured the
slopes of several hills with the clinometer, and found them to vary from 300 to 380; at other
points the inclination amounts to 45~. The general aspect of the hills along the side valleys is
shown in View X, accompanying this chapter.
Lithological Characters.-Although by far the greater portion of the materials composing tbe
formation are extremely light, fine, and unconsolidated, there are, in some places, layers of
sandstone and conglomerate, which offer more resistance to the action of the weather than the
other strata, and that slightly modify the rounded contour of the hill-sides. The principal constituent of the formation is a fine gray sand, mingled, in some of the beds, with a considerable
portion of clay, and alternating with layers in which clay predominates. volcanic materials, or
sands derived from their abrasion, constitute a large part of the strata. Thick beds are formed
almost wholly of white pumice-ston,irounded mse, ori a fine, powd, sirki finetpone sand,
regularly bedded. The color of these beds is white, but the lines of stratification are rendered
very distinct by the stains produced by the percolation of impure waters; also, by layers of the
same ingredients, differing in their fineness, and by occasional seams of cJzarcoal, in fragments.
Thin layers of pebbles are also numerous even among the strata of the finest materials. They
show that the waters, under which the beds were deposited, were liable to considerable disturb
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