86 
GEOLOGICAL REPORT—THIRTY-FIFTH PARALLEL. 
In addition to these there are many streams of basaltic lava covering broad areas, and which 
cannot he directly traced to either of the above enumerated sources. Among them the vertical 
walls of lava near the mouth of the Hawilhamoolc, on the Colorado and between the Colorado 
and the Mojave, may he mentioned. The most important of these localities are the mountains 
called San Mateo, San Francisco, Bill Williams’, and Cygnus mountain. Observations on these 
volcanoes have already been made in the chapters devoted to the description of the geology of 
the route, and some general remarks only need to he added. 
The outpourings of lava from San Mateo and San Francisco appear to have proceeded from a 
number of side cones or vents much below the principal crater. At San Mateo there is a cone or 
crater several miles west and a little north, directly on the east border of the branch of the San 
Jose river, which heads in Campbell’s Pass. This crater, although much lower than San Ma¬ 
teo, and hardly visible a few miles distant, furnished a stream of lava which coursed down the 
valley of the river until it mingled with the streams from the great cone, and perhaps extended 
beyond them to Covero and Laguna. This crater may perhaps be regarded as entirely distinct 
from San Mateo, as it is about twenty miles distant from the peak. The eastern flank of San 
Francisco volcano appears to be sprinkled with cones of various sizes, from which lava has 
poured out and flowed down the slope to the east. These cones were seen all along the route 
from a short distance west of the Colorado Chiquito to the base of the principal peak. Beyond 
the peak no cones appear to have been seen—they are confined to the eastern and southern 
slope of the mountain. Mr. Marcou mentions seeing some basaltic buttes on the east hank of 
the river. These are in the line of prolongation from the principal crater eastward through the 
lesser vents. Secondary cones do not appear to have been found around Bill Williams’ or 
Cygnus mountain. 
The probable volcanic character of the high conical mountain seen in the southeast, after 
crossing the Sierra Madre, was indicated by a lava stream which had flowed from that direction 
and which extended across the line of survey. Mr. Marcou estimated its elevation to be ten 
thousand feet. The high peak called Lantern or Abajo mountain, seen far to the northwest of 
San Francisco and Bill Williams’ mountain, is supposed to be volcanic from its isolation and 
conical shape. It is probably beyond the great Colorado, and in the broad plains southwest ot 
the lower extremity of the Wasatch range. 
The source of the broad areas of lava cut through by the Colorado and Hawilhamook, near the 
mouth of the latter, is not known ; but it is probably in the mountains further south. The 
horizontal layers of lava near Camp 137 are on the same line of trend, although far northward, 
and their source is probably north of the route. 
The cones seen at the north of the line when near the Soda lake, and a small crater found by 
Lieutenant Williamson west of the Mojave, show that the volcanic forces have acted in the 
Great Basin, although not with the power and effect which is exhibited east of the Colorado. 
No volcanoes or lava are found in the Bernardino Sierra, and we may thus consider the region 
of volcanoes to commence at the west, in the Pai-Ute range, between the Mojave and the Colo¬ 
rado. It is, however, important to observe, that volcanic phenomena are abundantly presented 
in the Sierra Nevada, north of the parallel of 36°—broad fields of basaltic lava being found at 
the western base of these mountains, along the head-waters of the San Joaquin, Merced, Stan¬ 
islaus, and Mokelumne. 1 
On the east, it is worthy of note that the broad Atlantic slope appears to be without any vol¬ 
canoes, the firstjvolcanic cones found being on the western side of the Santa Fe mountains. A 
great contrast is thus presented between the plains on the east and those on the west of the moun¬ 
tains, which are probably of the same age, and which we would naturally expect to be similarly 
covered by volcanoes. The region of greatest volcanic activity in former times thus appears to 
have been between the mountains along the Rio Grande and those on the Colorado. The peaks 
See the author’s Report on the Geology of the route in California surveyed, in 1853, by Lieutenant R. S. Williamson. 
