SLOPES-DRIFT-TERTIARY STRATA-SANDSTONE. 
219 
by a thick deposit of detritus or drift—the debris of the rocks of the Basin—which forms nearly 
the whole surface, and constitutes the material of the slopes. 
The only points at which the strata are exposed by sections are near the mountains, along the 
courses of the principal streams which reach the slopes, or in the banks of the numerous dry 
channels, in which no water appears to flow, at least, for the greater part of the year. There 
are many of these sections, but they are of slight depth, and do not extend far from the moun¬ 
tains. Some of these sections exhibit strata of sandstone in highly inclined positions, uneon- 
forrnable with the more recent materials composing the slope. At other places the strata appear 
to be conformable. The uplifted strata occur at several points, and are not only exposed in the 
valleys or channels of the streams, but rise above the general level of the slopes. They are 
principally sandstone and conglomerates, composed of the debris of granite or of volcanic rocks. 
Such strata are visible at the Cajon Pass, Williamson’s Pass, Johnson’s River, Cotton-wood 
Creek, San Francisquito Pass, and near the entrance to the Canada de las Uvas. All these 
localities are described in the chapter on the Tertiary formations—Chapter XIII. 
At other points around the slope of the Basin sedimentary strata, similar to these just 
described, are nearly horizontal, and do not appear to have suffered disturbance, but dip from 
the mountains with a gentle inclination, not exceeding five or six degrees, conforming to the 
slope of the surface. This is seen in the channel which leads out from the ravine of the Tejon 
Pass, and was also observed in some of the dry ravines in the slope between Johnson’s river 
and the Cajon. 
The materials which form the slopes are generally very different in their appearance from 
those of the strata, although, in many places, they are regularly stratified. They are, in 
general, coarser and unconsolidated, and appear to be formed from the fragments of the adjoining 
ridges. Thus, for a long distance from one of the isolated granite ridges, containing large 
veins of red feldspar, the surface was of the same color, owing to the distribution of the frag¬ 
ments. The gravel was chiefly composed of this feldspar at the distance of ever two miles. So, 
also, the gravel of the slope around the ridges of porphyry was principally porphyritic ; and, in 
fact, the nature of the rocks of the ridges of that part of the Basin can be determined by the 
debris of their slopes. In general, the upper portions of the slopes are composed of coarse 
materials, the finer and lighter debris being carried lower down before it is deposited. These 
detrital accumulations are believed to attain, at some points, a thickness of over five hundred 
feet; they thus constitute an important feature in the geology of the Basin. 
There is much danger of confounding the thick beds of drift, when in regular strata, with the 
older stratified deposits of similar materials. The absence of good natural sections along the 
undisturbed parts of the Basin-slope often left me in much perplexity to distinguish between 
the drift and older strata. For example, at the entrance to the Cajon Pass, the sediments of 
the slope form a bluff, turned towards the Pass, in which the nearly horizontal layers are 
exposed to view for a depth of about seven hundred feet. 
They do not differ very greatly in lithological characters from the more compact and consoli¬ 
dated strata in the Pass that are highly tilted, hut appear to rest upon their upturned edges. 
Similar upraised strata were found along Cotton-wood Creek ; there, the drift rests directly upon 
the edges, as shown in the little section, page 66, Chapter VII. Whether the strata of the slope 
of the Cajon, for the whole thickness of the exposure at the bluff, are composed of detritus alone, 
or in part of older sandstone, is not easily decided. More extended observations would probably 
remove the doubt. It is probable that the detritus agrees very nearly in its lithological char- 
