166 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 subterganean 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 30° to 38^ و‎ 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 the 
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 con- 
stituent of the formation is a fine gray sand, mingled, in some of the beds, with a considerable 
portion of clay, and alternating with layersin 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-stone, in rounded masses, or in a fine powder, like fine 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 charcoal, 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- 
