142 GEOLOGY. 
by long slopes, which, by their intersection, produce elevated valleys or basins. It appears to 
be a combination of these ridges and included slopes which forms the eastern boundary of the 
Basin, rather than one or more prominent and well-defined ranges. 
According to the observations of Captain Whipple, who crossed the mountains between the 
Colorado and the Soda Lake, at the end of the Mojave, the line of elevation consists of a com- 
bination of ridges and valleys, so that its surface is exceedingly irregular. The elevation of 
the summit at the point of crossing was found to be about 4,900 feet above the Colorado, and 
5,300 feet above the sea.! The general elevation of the peaks does not much exceed this; they 
are probably not more than 600 feet higher. The barometrical observations of Captain Whipple 
show that the two slopes of the range are nearly alike in their inclination. The direction 
or trend of the whole range, or its general axis, at that point, is nearly north and south, or 
inclining slightly to the northeast; but it is very probable that most of the ridges are oblique 
to the general trend of the range, and, by their overlapping, produce the deflection of the main 
axis to the northeast, this being the direction of the line of water-shed between the Basin and 
the Colorado from that point northwards. | ۱ 
South of the Pai Ute range, and between the Colorado and the Peninsula Sierra, we find a 
succession of ridges, which bound or form the northeastern side of the Colorado Desert. When 
seen from the inland base of the Peninsula Sierra these ridges appear to form one continuous 
range, trending nearly northwest and southeast from the northern base of San Bernardino to 
the mouth of the Gila. In traversing the Desert, however, the mountains are found to rise in 
succession in long projecting ridges above the horizon, thus showing the broken or composite 
character of the range, and that the ridges overlap. This structure is more clearly visible from 
the extreme south western point of the range, a few miles northwest of Pilot Knob, near Fort 
Yuma, at the mouth of the Gila. It is most probable that the ridges are separated by long, 
narrow valleys, trending nearly northwest and southeast. ‘The distance to which the ranges 
extend towards the north is not yet known, as the region lying between them and the Mojave 
river has not been explored. The existence of a broad, open plain in that direction has been 
reported, and this renders it most probable that the mountains form one line or range from the 
Gila to San Bernardino. They are so represented on the map. The same chain appears pro- 
longed, southeast of the mouth of the Gila, into the Mexican State, Sonora. "The ranges there 
have the same general trend, are in the same line, and form the first chain of mountains east 
of the shore of the Gulf, from which it is separated by a nearly level and desert-plain—the 
Sonora Desert. These ranges are all rocky, and, to all appearance, entirely barren. Their 
elevation, within the limits of California, probably does not exceed 3,000 feet. s 
The sky-outline of the mountains, north and northwest of Fort Yuma, is exceedingly varied 
and picturesque. In some places they rise in sharp peaks or pinnacles, with vertical sides like 
chimneys, and in others form domes, so regular in curvature that they seem like works of art. 
PLAINS, VALLEYS, AND SLOPES. 
The valleys of California may, for convenience of description, be grouped in four divisions: 
First, the low and broad valleys, which by their extent become pläins ; second, the mountain 
valleys, (generally elevated ;) third, the elevated valleys of the Great Basin ; fourth, the river. 
valleys or caiions. poe ۱ 
A brief, general description of the valleys of the Coast Mountains has already been presented, 
* Report of Captain A. W. Whipple, U. S. Top. Engineers. See also the Report on the Geology of the route, P- 5- 
