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MOJAVE VALLEY—BLACK MOUNTAINS. 31 
through a distinct mountain range, one of several (four or five) which compose the chain. This 
range is much higher west than east of the river, and borders the Mojave valley on the south- 
west and west. Its principal component rocks are gray or greenish and serpentinoid granite, 
containing a large proportion of hornblende, and traversed by many quartz veins. The greenish 
granite is scarcely distinguishable from much of that in Monument cafion. North of these 
granitic mountains are hills of conglomerate, &c., similar to those near the southern entrance 
to the cafion. 
MOJAVE VALLEY. 
This is one of the largest of the basin-like areas to which I have so frequently referred. 
It is elliptical in form; some thirty miles long and fifteen wide, and is bounded on all sides by 
ranges of rugged and picturesque mountains. From the bases of these mountains long slopes, 
composed of their debris, extend toward the river. These slopes terminate in gravel mesas 
similar in character to those which have been described as occurring below, and which here 
form four and in some places five distinct terraces. Forty feet below the lowest of these ter- 
races the alluvial bottom lands, forming a narrow belt on either side of the river, follow it in 
its sinuous course. “i 
_The mountains are entirely bare of vegetation, and their geological structure is visible at 
the distance of many miles. The slopes and gravel mesas contain a few clumps of cactus 
Larrea and other desert-loving shrubs, but are generally exceedingly arid and sterile. The 
bottom lands, though frequently highly charged with alkalies, are usually fertile, the better 
portions sustaining a luxuriant growth of willows, cottonwoods, “ mezquite,’’ (Algarobia gland- 
ulosa,) and ‘‘tornilla,’’ (Strombocarpa pubescens,) the more saline surfaces being covered by 
dense thickets of arrow-wood (Tessaria borealis) and Obione canescens. 
A section from the river westward to the mountains bordering the valley is: Ist. Alluvial 
bottom lands bordering the stream, of which the general level is about fifteen feet above its 
bed, and which are usually covered by the annual midsummer inundation; 2d. First terrace, 40 
feet, gravel and clay; 3d. Second terrace, 25 feet, gravel; 4th. Third terrace, 25 feet, gravel, 
with angular fragments of granite; 5th. Fourth terrace, angular fragments washed down from . 
the mountain above; 6th. Mountain of white granite, with strata of clay, mica, and hornblende 
slates. The granite, which is generally white and compact, contains a few quartz veins with 
copper. 
The mountains which surround the Mojave valley exhibit considerable diversity of geological 
structure and trend, and while they should all, perhaps, be classed as belonging to one system, 
(Sierra Nevada,) a part of them exhibit a tendency to depart from the normal trend of that 
system and approach that of the Wahsatch and Rocky mountains. Of these ranges the most 
continuous and important is that of the 
BLACK MOUNTAINS. 
This range seems to take its rise opposite the southern extremity of the Mojave valley, twenty 
miles east of the river. From the Mojave mountains, which it would cross if prolonged, it is 
separated by an interval of plain connected with an extensive valley lying east of this range; a 
valley.which seems to be drained through a broad arroyo, terminating near the northern 
entrance to the Mojave cafion. From their southern point of origin the Black mountains extend 
without break or interruption for more than one hundred miles, with a trend a little west of 
north. They constantly approach nearer to the Colorado, and cross it below the mouth of the 
Virgen, forming the walls of the Black cafion. 
This range of mountains is of peculiar interest, from the difference which its trend exhibits 
from that of most of the mountain chains crossing the Colorado below. Like the Cerbat moun- 
tains, a range parallel to and eastward of this, it approaches quite as nearly to the trend of the 
