24 RECONNAISSANCE OF PAET OF WESTERN ARIZONA. 
These effusives are megascopically and microscopally the same as 
the rhyolites and younger andesites of Black Mesa to the west, and 
form a part of the lava sheet supposed to have once extended con- 
tinuously across the Detrital-Sacramento Valley. At the northern 
end of the range sheets of olivine basalt occur at the crest of the ridge, 
tilted eastward. (See section Q-Q' ', PI. V.) In several places along the 
eastern base similar sheets occur, dipping steeply to the east and 
passing beneath the detrital filling of the Hualpai Valley. 
Structure. — The eastward inclination of the lava sheets along the 
eastern base of the Cerbat Mountains suggests an eastward tilting of 
the surface similar to that shown in Iceberg Canyon to the north and 
in the Hualpai Mountains to the south (fig. 2). 
HUALPAI MOUNTAINS. 
Location. — The Hualpai Mountains are situated between the Sac- 
ramento Valley on the west and the Big Sandy Valley on the east, in 
the central part of the area described. 
Horizontal scale 
ioooo feet 
Fig. 2.— East-west section across Big Sandy Valley, showing the tilted Hualpai block and the uncon- 
solidated gravel beds inclined toward the face of the Aquarius Cliffs. 1, Chemehuevis (?) gravel; 
2, granite; 3, Temple Bar (?) conglomerate; 4, andesite. 
Topographic features. — The range is about 35 miles long, with a 
general altitude of about 7,000 feet and a maximum altitude of 8,266 
feet, attained in Hualpai Peak. The western slope is very precipitous 
down to an altitude of about 3,000 feet, where it meets the detrital 
plain of the Sacramento Valley at high angles. Along its base occur 
small flat-topped lava-covered hills, rising a few hundred feet above 
the surrounding plain. These are most numerous at the north, near 
the lava fields of the Kingman area, but extend southward nearly to 
the southern end of the range. Detrital material has accumulated 
in alluvial fans and slopes to some extent, but in general the steep 
mountain face meets the plain at high angles. 
The eastern slope of the mountains is not so precipitous as the 
western. At altitudes of nearly 6,000 feet the crystalline rocks begin 
to disappear beneath a corrugated detrital slope of comparatively low 
gradient, which extends eastward to Big Sandy Wash, a distance of 
10 to 15 miles. In this distance the descent is about the same as that 
attained in the western slope in a much less distance. (See fig. 2.) 
At the southern end of the range the mountains cease rather 
abruptly, giving place to low isolated hills separated by broad passes 
occupied by the detrital accumulation of the plains. To the east are 
several comparatively small spurs and detached mountain groups, 
such as Owens Peak, Peacock Mountains, etc. 
