RESUMK OF GEOLOGY OF MOJAVE VALLEY. 103 



KECAPITULATION. 



The Mojave valley is composed of beds of conglomerate and sandstone, of very loose texture 

 and easy of degradation, through which the river and streams have worn their way, exposing 

 in some jilaces 100 feet in thickness of these beds reposing unconformably upon the igneous 

 rock. 



Beds of clay, sand, and gravel, 20 to 150 feet in thickness, are deposited over these sandstones, 

 and in many cases appear to be formed out of these materials. 



A few shells of anodonta, similar to those found near New river, at Alamo Mocho, on the 

 Colorado desert, were found west of the range bounding Soda lake and on the alluvium within 

 100 yards of the river. 



The upheaving plutonic rock is chit fly felspathic, red quartzose, porphyry, red amygdaloid, 

 with the cavities filled with chalcedony, and occasionally serpentine, epidote and trap. 



Felspathic (orthose) granite was not found ; albitic granite formed the mountain range east of 

 Navajo camp ; porphyritic rock, resembling it, constitutes the hills separating the Mojave from 

 the Colorado ; the axial rock is thus assimilated in mineral character to that of the Sierra Nevada. 



The conglomerate and solidified sandstones may be looked on as tertiary and of the same age 

 as those at Carrizo creek, of the Colorado desert, perhaps Eocene. The clay and gravel beds 

 belong to a mucli later period. It is these loose beds which give the peculiar and characteristic 

 features to this region east of the sierra ; everywhere the mountain ranges are approached from 

 either side by a gentle slope which runs direct up to the edge of the hill, so that it appears as 

 if the level of the valley and the slope of the mountain could be defined by an exact line, the 

 local drift bed running close up. Thus every valley presents a central depression, rarely filled 

 by a river bed ; thence on each side a gentle slope until the mountain base is suddenly reached. 

 This appearance, which is very striking, is also observed in the western Alps, (Europe,) where 

 an individual may set one foot on the plain, and the other on the mountain slope — so decided 

 is the line of demarcation. These loose and deep beds are related to those west of the sierra, 

 and to those east of the Colorado and on the Sonoranian desert south of the Gila. They are, 

 no doubt, of Quaternary age, and to some extent of local origin. 



In parallel 34° short ranges of hills, running northwest and northeast, divide the country 

 between the Sierra Nevada and the Colorado into a series of ridges and plains, or valleys, which 

 character continues as far as 35° 30', or 20 miles north of Soda lake. This mountain belt of 

 country is much more elevated than the Colorado desert further south, its lowest level. Soda 

 lake, being 1,116 feet above the sea. The Colorado level (from which it is separated by a lofty 

 though short range, over 4,900 feet high,) being, almost in the same parallel, 350.4 feet. 



Assuming the Colorado as lying at the lowest point of the district between the Cerbat range 

 and the Sierra Nevada, the country to the west would consist of a series of ranges of albitic 

 granite and porphyry hills running N. 60° W., occasionally interlocking, forming isolated 

 valleys, but more usually unconnected ones, giving rise to extensive plains passing round the 

 edges of the ridges and connecting. As many as five of these i)arallel ridges lie between the 

 Cajon pass of the sierra and the eastern boundary of Soda lake, and Lieutenant Whipple 

 describes as many as nine between the Colorado and the sierra. — (H. Doc. 129, p. 29.) 



The intervals between these chains vary from 5 to 15 miles ; are in places composed of loose 

 drifting sand, which cover the surface and check vegetation, or in places of a hard sandstone 

 pebble formation, and occasionally clay beds. The river bottom (Mojave) is alluvium. On 



