52 
GEOLOGICAL REPORT—THIRTY-FIFTH PARALLEL. 
In ascending this range, the dry valley of a stream was followed in a northwest direction, 
which being the same as the trend of the ridges, the rocks were not fully exposed to view. Mr. 
Marcou notes of the range that it is four miles wide. Trap and metamorphic conglomerate 
was observed in crossing, and an extended inclined plain composed chiefly of the debris of 
the surrounding granitoid rocks. Beyond this range Mr. Marcou notes a second, which, 
between Camps 136 and 131, was composed of red metamorphic conglomerate, covered by black 
and shining phonolitic and basaltic trap. A great development of trap-rock was seen in 
crossing the chain, and beyond it a broad and elevated plateau was found to extend to the base 
of the third range. This plateau is composed of the debris of erupted rocks, and in several 
places is covered with a whitish brecciated limestone. This is in horizontal strata, and was 
considered Tertiary by Mr. Marcou. 
The third chain, according to Mr. Marcou’s notes, is five or six miles wide, and is composed 
of a very hard hornblendic granite, and of a whitish dioritic rock. Amphibolic granite was 
noted on the eastern base. A specimen from this range is in the collection, (No. 54 ;) it may 
be called a quartz porphyry. This range is separated from another—the fourth—by an inclined 
plain, as before, which was composed of the debris of granite. The fourth range appeared to be 
composed, at the base, of a hornblendic granite, similar to that in the third. In crossing it, or 
from Camp 140 to 141, quartziferous granite was found to outcrop for twelve miles, and was 
succeeded by a large exposure of trap and greenstone. Mr. Marcou also records the presence 
of a great body of drift, forming a bank eight hundred feet thick. This consisted of sand and 
large blocks but little rolled. I judge that this accumulation is similar to those found around 
the bases of the mountains of the Great Basin, especially on those forming its southern 
boundary. In crossing the range Mr. Marcou saw seven or eight cones, of a red color, about 
ten miles northward, and he believed them to be volcanic. This fourth ridge, or range, bordered 
the valley of the Mojave on the east. The Soda lake, into which the Mojave sometimes flows, 
is directly at its western base. We have thus crossed, in our descriptions, from the Colorado 
river to the Great Basin, within which the Soda lake is found. 
Although in the descriptions four distinct ranges of mountains have been mentioned, they 
may, for the purposes of geology, be considered as forming one great chain of a composite 
character. The rocks in each are similar, and they are connected by the same formations, 
underneath the superficial covering of tertiary and post-tertiary sedimentary accumulations, 
which fill the valleys and flank all the ridges. If this slight covering of earth and of local 
detritus and debris were subjected to a powerful denuding current for a short time, they would 
all be removed, and expose a continuous but rugged granite surface, traversed, of course, as we 
have already seen, by numerous dykes and veins of intrusive rocks—trap, porphyry, and the 
like. Having myself explored the rocks along the Mojave and of the southern end of the Great 
Basin between the Mojave and the Sierra Nevada, I am enabled to be more positive and general 
in the description of this region. From all the information I can obtain, both from Captain 
Whipple and Mr. Campbell, I conclude that the system of parallel ranges, the geological 
structure of which has been given in a general way, corresponds very nearly with the ranges 
found throughout the Great Basin, and which have generally received the title of Lost Mountains , 
from the fact that they do not extend continuously for very great distances, hut may be avoided 
by a slight detour either north or south. This structure is exceedingly interesting not only to 
the civil engineer who seeks practicable grades, but to the geologist who reads the former exist¬ 
ence of stratified formations, now plicated and folded in great waves, and traversed by intrusive 
masses of granite. 
This broken character of the mountains, together with their sharp, ragged outlines, render 
the scenery of that region highly peculiar. 
The effect is heightened by the purity of the air and the deep coloring which distant mountains 
assume and change each hour in the day. The absence of vegetation, the nakedness of the 
