46 GEOLOGICAL REPORT——THIRTY-FIFTH PARALLEL. 
called magnesian limestone by Mr. Marcou; but he found in it the fossils characteristic of the 
mountain limestone or lower Carboniferous. 
Granite.—Near Camp 90, just at the southern base of the volcano, an outcrop of granitic 
rock was found. This, according to Mr. Marcou, was ‘‘ a great mountain of red sienitic rock ;”’ 
and Captain Whipple states, page 80, that the volcano terminates at the southeast ina gigantic 
mass of granite. I have, therefore, represented an outcrop of this rock on the map and section. 
The magnesian limestone was found in horizontal beds a short distance below; hence I infer 
that the granite isolder than it. Itisvery probable that Mount Kendrick and Mount Sitgreaves, 
on the west of the volcano, are of granite also, and connected with the first-mentioned outcrop. 
According to Mr. Campbell, Mount Kendrick is composed of chocolate-colored porphyry. I 
regret that there are no specimens from these rocks in the collection. 
Extinct volcano, called San Francisco Mountain.—In the preceding observations on the outcrops 
of limestone and granite, we passed rapidly from the banks of the Colorado Chiquito to a point 
several miles west of the great volcano, now extinct. This cone rises north of the line followed 
by the expedition, and is on the left bank of the Colorado Chiquito. It is unfortunate that this 
lofty and imposing peak cannot be distinguished by a name which will not lead to endless mis- 
apprehension of its position. Nearly all who are not aware of its existence, will, on seeing its 
name, conceive it to be near the great city of the same name on the Pacific. If the Indian 
name can be ascertained, it should be adopted. The trail of the expedition passes directly at 
the southern base of the mountain, over the limestone and granite already described, and in 
many places over the streams of congealed lava. 
The peak is a prominent land-mark to the traveller in passing westward over the plains beyond 
the Sierra Madre. It was seen by Captain Whipple, for a long distance, and was used as a 
guiding pillar for the survey. Its elevation was estimated as about 5,000 feet above the plain, 
or 12,000 feet above the sea. In form or outline it is not remarkably regular, but yet has a 
general conical shape. The flanks are covered with congealed lava streams, which extend far 
from its base and cover a wide area of the surrounding plains. The lineof survey first reached 
this lava between Camps 86 and 87, where the limestone was left, and the ascent upon the 
surface of the lava commenced. From this point onward, until the mountain was passed, the 
road led over a succession of lava streams or broad beds which covered the underlying strata 
from view. Before the base of the principal mountain was reached, numerous secondary cones 
or outlying vents for the lava were found. These were near Camp 87, and, according to Captain 
Whipple, are beautifully regular in form, and one of them appears to have broken away at one 
side, permitting a stream of lava to flow from it in a serpentine course. This has hardened in its 
bed, and now reveals the motions to which it was subjected, almost as distinctly as if it had flowed 
down but yesterday. This lava is of a black color, and very hard, but in some places is decom- 
posed op the surface so as to form a soil, which appears to be particularly favorable for the 
growth of the nutritious grama grass. A portion of the lava is cellular and porous, and 
between Camps 88,and 89 volcanic ashes were found. This was also abundant towards Camp 
90, almost directly south of the great cone. Mr. Marcou also records the presence of numerous 
basaltic dykes and reddish lava. Amygdaloidal trap was found at Leroux’s spring, and beyond. 
The principal flow of lava from this volcano, and its numerous cones, appears to have been 
towards the east, where it has spread out even to the banks of the Colorado Chiquito at the Cas- 
cades, several miles north of the point where it was crossed by the survey. The flow in this 
direction is in accordance with the present relief of the surface, and shows that at the period of 
eruption the valley had very nearly its present form. 
Sandstone.—Mr. Campbell informs me that the limestone south of the volcano is covered in 
many places by mounds or table-hills of a light-colored sandstone in horizontal beds. This is 
not described by Mr. Marcou. It may be the continuation of the lower beds seen at the crossing 
of the Colorado Chiquito. 
Bill Williams’ Mountain.— This peak is probably another extinct volcano of great extent. It 
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