DETAILS OF HIGHLANDS. 25 
Rock masses. — The Hualpai Mountains, so far as observed, are com- 
posed mainly of coarse-grained granitic rock, becoming gneissoid at 
the southern end of the range. They are highly mineralized in places, 
and dikes and quartz veins are numerous. At Cedar, near the sum- 
mit of the range, mining operations have been successfully carried on 
for a number of years. The lavas capping the foothills to the west 
are outlying portions of rhyolites and andesites of the Kingman area 
(see p. 16), and the small masses in the Peacock Mountains are prob- 
ably parts of the lavas covering the Truxton Plateau that have been 
let down to their present position by faulting. The detrital material 
of the eastern slope is part of the detrital accumulation described 
under the caption "Big Sandy Valley." 
Structure. — The Hualpai Range was apparently formed by the 
upheaval and tilting eastward of a large crust block, the movement 
taking place along faults west of the range and at the Cottonwood and 
Aquarius cliffs. The evidence of block tilting in addition to that 
given of faulting along the cliffs rests on the steep, scarplike western 
face of the range, the high angles at which the mountain slope meets 
the surface of the Sacramento Valley, and the gently inclined debris 
slope of the eastern face. 
AUBREY HILLS. 
Location. — South of the Hualpai Mountains and lying between the 
Sacramento and Big Sandy valleys is a small group known as Aubrey 
Hills. 
Topographic features. — The Aubrey Hills are low, rising but a few 
hundred feet above the plains on either side. They are separated 
from the Hualpai Mountains by a low debris-filled pass connecting 
the Big Sandy and the Sacramento valleys, and from the Buckskin 
Mountains by Williams Canyon. 
Rock masses. — The rock at the northern end of the hills is granitic 
gneiss. In Williams Canyon occurs similar gneiss overlain by quartz- 
ite, argillite, and black metamorphic limestone. From a distance 
sheets of effusive rock were noted east of the hills, and dark-colored 
lava, probably basalt, was seen on their western flanks. 
BLACK MOUNTAINS. 
Location. — The Black Mountains, located in the extreme western 
part of Arizona, extend from the northern border of the Territory 
southward to Union Pass. 
Topographic features. — The mountains are formed of three more or 
less distinct groups. The northernmost group, extending from 
Boulder Canyon southward to Eldorado Pass, a distance of about 15 
miles, attains a maximum altitude of about 5,500 feet in Mount Wil- 
son. From this- peak the altitude diminishes northward to about 
