30 GEOLOGY. 
thick beds of gravel, sand, and clay, which form bluffs at the margin of the bottom lands, and 
extend to the bases of the surrounding mountains. 
MOJAVE MOUNTAINS. 
This important chain bounds Chemehuevis valley on the east. Where it crosses the Colorado 
its picturesque pinnacles were called by Captain Whipple “The Needles.’’ Though continuous, 
these mountains exhibit considerable variety of structure. The most southern portion observed 
by us is composed of porplhyries and trachytes, purple, blue, pink, gray, and green in color. 
The principal mass of the range east of the Chemehuevis valley is light gray or white granite, 
traversed by numerous veins of quartz. This is skirted by a range of foot hills, composed of 
highly colored porphyries and trachytes, similar to those which predominate in the more 
southern portion of the range. Below these, and rising from the base of the chain, is a line 
of rounded hills, composed of black scoria, which have thrown up and changed the Tertiary 
rocks, and seem to be more recent than the granites and porphyries. This line of hills diverges 
from the general trend of the Mojave mountains, and runs down to connect with the erupted 
masses of which I have spoken as bordering the Colorado. Between the mountains and the 
river is a broad sloping mesa, composed of the debris of the adjacent ranges, and of transported 
pebbles. Among these pebbles I found masses of Carboniferous limestone, with characteristic 
fossils. : 
MOJAVE CANON. 
Toward the upper part of the Chemehuevis valley the gravel hills gradually encroach upon 
the bottom lands until they form precipitous bluffs bordering the river. These bluffs seem to 
compose two terraces, of which the surface of the lower is about 75 feet, and the upper 120 
feet above the river bed. Near the entrance to the cafion, in a westerly bend of the river, is 
exposed a large mass of erupted rock, evidently quite recent. By this the beds of sand and 
gravel are much disturbed and metamorphosed, but only locally. 
The foot-hills of the mountains are composed of trap, with Tertiary sandstones and einen 
rates, the latter highly metamorphosed and thrown about in the greatest confusion, exhibiting 
a variety of colors quite as vivid and as strongly contrasted as those of similar strata at the 
southern entrance to Monument caifion. The colors are here, however, not precisely the same, 
the most conspicuous being bright red, green, and white of the stratified, and brown and purple 
of the erupted rocks. 
The cajion itself presented the most varied and interesting scenery we had met with, its 
walls being particularly ragged and picturesque in outline, and the colors such as to produce 
the most unusual and surprising effects. The southern part of the cafion is cut through the 
trap and metamorphic rocks just mentioned; the gateway by which we entered it being com- 
posed of trap and a massive and highly metamorphosed conglomerate, which varies in color 
from umber brown to blood red. 
The most elevated portion of the chain ‘Aiconnk which the cafion passes is composed of 
granite and porphyry, and is apparently older than the hills of trap and metamorphosed Tertiary 
rocks which skirt it on either side. The Needles themselves are formed of purple porphyry 
and trachyte. Our Camp 40 was located midway of the cafion. The highest summits in the 
vicinity are composed, on the north: and west, of granite; on the south and east, of porphyry. 
The lower hills, immediately adjacent to camp, consist of metamorphic conglomerate, or rather 
— sontaining firmly cemente: | angular fragments of granite, similar to that of the neigh- 
bably derived from them, with intruding masses of purple and brown 
Se et on the left bank. Some of the trachytes contain numerous 
, (phlogo: vite,) and masses of black oxide of manganese. 
“That part of the caiion which lies north of Camp 40 is formed by the passage of the river 
‘materials, with green volcanic tufa, form the picturesque 
