216 GEOLOGY. 
descending it by the old Spanish trail; but, on arriving at the river, we found that we were 
2,012 feet lower than the entrance to the Cajon at the upper margin of the slope. This descent 
of 2,012 feet in 19 miles is an average of 105 feet to the mile. 
The descent, however, for a short distance from the summit is the most abrupt, being 497 
feet in a little over two miles. Below that point the grades vary from 93 to 86 feet to the mile, 
and this is the most regular part of the slope; the descent in a distance of 17 miles being but 
little more than 1,500 feet. "This amount of inclination is by no means constant for all the 
slopes of the Basin. Many parts of the northern slope of the Bernardino Sierra have a greater 
angle of descent, and this is especially true of many of the shorter slopes around the isolated 
ranges or Lost Mountains. 
The meeting of opposite slopes necessarily forms a series of valleys, or basin-shaped depres- 
sions, between the bounding mountains and the Lost Mountains; also, between the latter. 
These are very numerous ; in fact, the Great Basin must be regarded as made up of a series of 
smaller and local basins between the ranges. These conditions of the surface are made manifest 
along the whole eastern base of the Sierra Nevada, or wherever there is sufficient precipitation 
of water to form lakes or pools. The chain of lakes extending from Oregon south into Cali- 
fornia is found in a series of basins, or local valleys without outlets to the sea, or to the lowest 
parts of the Basin. They are independent basins of limited extent, and such valleys form the 
Great Basin by being grouped together. E 
In an attempt, therefore, to determine the average elevation of the surface of the southern 
part of the Basin it becomes necessary to consider not only the elevation of the upper margins 
of the slopes, but the altitude of the intersections of their lowest parts. The lowest point of 
the slope of the Sierra Nevada at the Tejon and Cañada de las Uvas, at its intersection with the 
adjoining slopes of the Bernardino Sierra and Lost Mountains opposite, was ascertained to 
be 2,380 feet; this being at the level of the dry lake, or level expase of clay generally found at 
the bottom of the local basins, when there is not a sufficient supply of water on the adjoining 
heights to keep the depression filled. The surface of this dry lake was seen to extend far 
towards the north. This dry lake bed may be regarded as the greatest depression of the 
southern part of the Basin, except the valley of the Mojave. This river appears to connect à 
descending series of local basins, and extends far to the north and east of its sources, and finally 
 terminates in a dry lake covered with an incrustration of soda and salt. This dry lake has - 
elevation of only 1,137 feet, and is probably the lowest point in the whole extent of the Basin. 
Itisa much greater depression than was supposed to exist, but itis possible that there are 
other very low points among the unexplored mountains and ridges between it and the Sierra 
Nevada. i 
The upper margins or limits of the slopes—their intersection with the ridges of the bounding 
mountains—is well defined, and to an observer on the surface of the Basin appear to be at 
nearly the same elevation at all points. A difference in the altitude is, however, shown by the 
instrumental results; but these differences are so slight that they may be disregarded in 2 
general conception of the configuration of the surface of the Basin. The elevation of the geveral 
points from Walker's Pass around the margin of the Basin to San Bernardino may be thus 
stated :! 
£ I have in some cases added to the elevations as given in Lieutenant Williamson's tables, the observations having been made 
in the channels or valleys leading from the passes, and thus below the general surface. In some instances, also, the stations 
8 
represented in the tables as at the foot of the mountains and at the surface of the Basin were at the lower end of the valley 
excavated in the slope, and thus below its upper margin. 
