26 RECONNAISSANCE OF PART OF WESTERN ARIZONA. 
3,000 feet at Boulder Canyon, and southward to about 2,000 feet at 
Eldorado Pass. 
Mount Perkins, 5,500 feet high, is the center of the second group, 
which consists of a north-south ridge about 30 miles long. The 
third or southernmost group, about 4,500 feet in maximum altitude, 
consists of many small irregular hills extending eastward for a con- 
siderable distance into the Detrital-Sacramento Valley. The three 
groups form a continuous range, the apparent separation being due 
to accumulation of detritus, which fills the old valleys and covers the 
low-lying passes. 
Rock masses. — The core of the Black Mountains consists of granitic 
rock. In Boulder Canyon coarse-grained crystalline rock occurs, as 
described by Gilbert/' overlain on either side by gneiss. In Pyramid 
Canyon, farther to the south, and in the hills east of this canyon the 
rock is granitic gneiss. 
The older crystalline rocks of the range are overlain by extensive 
masses of effusive rock representing several periods of eruption. The 
oldest of the effusives, the andesite of the Gold Roads region, was not 
observed in Black and Boulder canyons, but in many places rhyolites 
and light-colored andesite similar to those overlying the dark-colored 
andesite of Gold Roads (see p. 27), rest upon the granites and 
gneisses. These are especially prominent in Black Canyon, where 
they are many hundreds of feet thick. They were observed near 
the river at the head of the Cottonwood Valley (section K, fig. 6), at 
the head of Pyramid Canyon, and at many places in the Black 
Mountains up to an altitude of 3,000 feet or more. 
The next younger effusive rocks of the Black Mountains are basalts. 
They occur in dikes cutting the older rocks and in sheets overlying 
the rhyolites and younger andesites. North of Union Pass they 
were outpoured in the Detrital-Sacramento Valley, which had pre- 
viously been eroded through the rhyolites and younger andesites. 
BLACK MESA. 
Location. — Black Mesa, about 35 miles long and 5 miles or more 
in width, is situated between the Colorado and the Sacramento val- 
leys, in the central western part of the area described. 
Topographic features. — The mesa is a remnant of a once extensive 
plateau having a general altitude of about 4,000 feet and a maximum 
at Mount Nutt of about 5,000 feet. A large part of the plateau has 
been eroded away and the remaining part deeply dissected by erosion. 
The bordering cliffs, although deeply incised, form a comparatively 
regular escarpment at the eastern margin of the mesa, rising 1,000 
feet or more above the floor of the Sacramento Valley. The western 
margin is much more irregular owing to the greater erosion caused by 
a Gilbert, G. K,. Final Rept. U. S. Geol. Surv. W. 100th Mer., vol. 3, 1875, pt. 2, p. 35. 
