DETAILS OF HIGHLANDS. 27 
the downcutting of Colorado River. On this side are many more or. 
less isolated outlying remnants of erosion, well shown on The Needles 
special map of the United States Geological Survey, which includes 
a small part of the southern end of Black Mesa (there called Ute 
Mountains). The most conspicuous of these outlying masses is 
Boundary Cone, 3,429 feet high — a sharp pinnacle rising 2,500 feet 
above the graded detrital slope which stretches from its base west- 
ward to Colorado River. 
The rock of Black Mesa varies greatly in hardness, and erosion has 
carved out a great variety of topographic forms. In places where 
the mesa is composed of moderately uniform layers it is bordered by 
prominent cliffs of regular outline 1,000 feet or more in height. 
Where the rock varies in hardness within short distances, as at 
Union Pass, it is eroded into a great variety of castellated forms. 
(See PL VI, A.) 
Bock masses. — The granite forming the core of the Black Moun- 
tains extends southward underneath Black Mesa, but it is exposed in 
few places, the mesa to a depth of 2,000 to 3,000 feet being composed 
mainly of effusive rocks. Granite porphyry was found at the base 
of Boundary Cone at the western edge of the mesa, and dark-colored 
andesite at the base of the cliffs at the northeastern extremity; the 
latter is best exposed in the vicinity of Gold Roads mining camp. 
The andesite rests upon the granite and is apparently overlain by 
rhyolite. Wherever observed, however, Black Mesa is composed 
mainly of rhyolitic tuff, breccia, and flow. The rhyolite is in turn 
cut by dikes of olivine diabase and overlain to some extent by sheets 
of basalt. 
The granite underlying the rhyolite is penetrated by the well at 
Yucca and exposed at the base of the hills extending eastward into 
the Detrital-Sacramento Valley at the northern end of Black Mesa. 
The thickness of the effusive rock as determined at these points is 
about 3,000 feet. 
MOHAVE MOUNTAINS. 
Location. — The Mohave Mountains are located south of Black 
Mesa, between the Colorado and the Sacramento valleys. They con- 
sist of a massive central group, with a spur, known as The Needles, 
extending westward to Colorado River, and low unnamed hills 
extending southward to Williams River. 
Topographic features. — The central group is subcircular in outline 
and about 5,000 feet high. It is deeply scored by erosion, and is 
apparently the remnant of a once much more extensive mountain 
mass. 
The Needles are irregular pinnacles resulting from the erosion of 
the mass of effusive rock which formerly extended across Colorado 
River and was later dissected by that liver in cutting its present 
