218 GEOLOGY. 
rock, piled together in such wild confusion as to become impassable. These accumulations 
somewhat resemble the heavy talus at the base of the mural faces of trap in Connecticut, but 
the blocks of granite are all large, and no small fragments or earth is seen. The ridge or bluff 
from which they appear to be broken is but little higher than the pile of ruins. 
Several of the isolated ridges a few miles north of the south end of the Dry Lake, and many 
miles west cf the Mojave, were also found to be composed of gray granite, traversed by veins or 
dykes of red feldspar and quartz. These feldspathic veins were so enormous and abundant that 
the whole slope around the elevation was colored red by the fragments. The gray granite was 
found to be rapidly disintegrating, and large blocks of it were found resting upon the surface. 
It is doubtless the fact that erupted rocks, trap, porphyry, and the like, have played an 
important part in modifying the topography of the Great Basin. Two ridges of porphyry were 
found between the granite just described and the Sierra Nevada. The principal outcrop has a 
chocolate-brown or reddish color, and includes small white crystals. The rock is also colored 
in stripes or belts, like jasper, and is very hard. Erupted rocks also occur along the base, and 
in the passes of the Bernardino Sierra, but none were observed in the Sierra Nevada at the 
Tejon. Itis probable that many other outcrops will eventually be found within the southern 
part of the Basin and north of the ridges mentioned. 
It is probable, also, that extinct volcanoes and overflows of lava will be discovered at various 
points, on making detailed explorations. During Lieutenant Williamson's examination of the 
lower part of the Mojave River, he passed a conical butte of volcanic rock, which, from its nature 
and peculiar symetrical appearance, he was disposed to regard as a small volcano. He has 
described it in his report in the following words: 
** About midway between the peak and the camp was a singular isolated hill, about 300 feet 
high. It was composed of very black volcanic rock, and its form that of a very symmetrical. 
truncated cone, surrounded at its báse by a circular horizontal bed of the same rock—the cone 
being in the centre. This bed was between two and three miles in diameter; its edges well 
marked, and rising from two to six feet above the surrounding gravelly plain.’’! 
This volcanic butte was about twenty-five miles northeasterly from the Depot Camp on the 
Mojave, and I have indicated its position on the General Map. It is worthy of the attention of 
explorers who pass that way, and being near the Mormon road may be readily visited. 
The observations on the isolated mountains, although limited, were sufficient to sh 
they are chiefly composed of granitic rocks. The compact granite appears to predominate, 
and no true gneiss or mica slate was seen. The rocks appear to be eruptive rather than meta- 
morphic. The outcrop at the Mojave was the only locality at which metamorphic rock was 
seen. It is, however, found further north in the Pai-Ute Range. The general distribution of 
the outcgops of granite, and the frequent occurrence of a slight knob of rock, hardly perceptible 
at a short distance, led me to conclude that the subjacent granite was not far below the gener 
surface of the Basin, and that its underground outline was exceedingly irregular, being cu 
into a series of valleys and ridges. 
ow that 
STRATIFIED FORMATIONS—SLOPES. 
The sedimentary accumulations, resting upon the granitic rocks of the Basin, appear to be m 
paratively modern, or of the period of the Tertiary. These strata are nearly covered from ۴ 
1 Report of Lieutenant R. S. Williamson, 8vo. edition, page 28. 
