24 
GEOLOGY 
storms, that covered the ground with an immense white sheet during nearly the whole time of our 
exploration. 
I will only say , in general , that there are four or five large extinct volcanoes over this space , 
the largest being that of San Francisco, which is 12,500 feet above the level of the sea. In pla- 
ces where the lava does not entirely cover the ground, we Magnesian Limestone, the Sandstone 
of the Coal-measures and the Carboniferous Limestone (between our camp N° 95, Lava creek, and 
N° 96, Cedar creek) - the last containing fossils in abundance, the principal ones being the Pro- 
ductus semi-reticulatus , P. Cora var. Mogoyoni and P. costatus; Spirifer striatus and the Terebratula suhtihta'). 
These Stratified rocks are upheaved , and dip generally to the north-north-east, following se- 
veral lines of dislocation which belong to the chain of mountains called Sierra de Mogoyon, oi 
Sierra Blanca. In several places, and especially at Pueblo Creek (Latitude 34°, 55', 41 , longi- 
tude 112°, 49', 08".), beds of Old Red Sandstone are seen below the Lower Carboniferous, and 
in contact with the gneiss and granite, similar to the Old Red of the Catskill mountains, iSevv-York. 
The system of dislocation of the Sierra de Mogoyon, the direction of which is east-east-south 
and west-west-south , is anterior to the apparition of the Rocky Mountains and the Sierra Madre , 
and 1 put it at the end of the Triassic period, and before the deposite of the .lurassic rocks. 
From Cactus Pass to the junction of Bill William Fork with the Rio Colorado (Latitude 34 , 
17', 46"; longitude 114°, 05', 29".), we cross successively three or four chains of mountains run- 
ning from North to South, and crossing the chains of the Mogoyon System. These mountains, 
which belong to the System of the Sierra Nevada, and which we called Cerhats Mountains, are 
formed entirely of eruptive and metamorphic rocks , with some beds of conglomerate and red clay be- 
longing to the Tertiary epoch. 1 have recognised along the conrse of Bill William Fork several veins 
of argentiferous lead — an indication that silver is common in these mountains. 
From the Rio Colorado to Monte, we cross a country of mountains formed almost wholly of 
Granitic rocks , with the exception of three plateaux , which are occupied by sandstones or lime- 
stones, and Modern sand. From the point where we quit Mojave river to Cajon Pass, there is a 
plateau formed by a white conglomerate sandstone of diffuse stratification , and much upheav ed by 
the Sierra Nevada. This Sandstone is evidently Tertiary and posterior to the Eoedne. 
From Monte to Los Angeles, and at San Pedro the road is constantly over Modern alluvium, 
which probably conceals beds of the Tertiary epoch. 
1) On the borders of the Rio Colorado Chiquito (Latitude 34°, 53', 01"; longitude 109°, 59', 57".) 
1 found several fossils of the Mountain Limestone formation, such as: Productus semi-reticulatus, P. cos- 
tatus and Terebratula subtilita, much rolled and worn, and brought, without doubt, fi'om the gorges of 
the Sierra de Mogoyon or Blanca, where the Colorado Chiquito has its sources. These fossils are of 
a red or rose color, and resemble Agate or Jasper. My friends, the geologist Dr. Randall, President 
of the California Academy of Natural Sciences, and the celebrated voyageur F. X. Aubry, who have both 
crossed the Sierra Blanca in following the banks of the Rio Colorado Chiquito and Rio Prieto, gave to 
me at San Francisco, in April 1854, some of these fossils found in situ, in a very hard marble lime- 
stone of rose, almost red color, and which crops out on several points of the Sierra. Fmplly Lieu- 
tenant A. W. Whipple, who has passed several years on the borders of the Rio Gila, occupied in run- 
ning the boundary line between the United States and Mexico, collected in place and not rolled , at the junc- 
tion of the Rio San Pedro with the Rio Gila, the same Mountain Limestone of a grayish rose color, 
containing the following fossils: Productus semi-reticulatus and Terebratula subtilita. 
