2i8 JOHN S. BROWN 



northeast and southwest, reaching their highest elevation in the 

 vicinity of San Gorgonio Pass, where the basin is narrowest. The 

 mountains on the southwest are generally known as the Peninsular 

 ranges. They extend down into Mexico, and form the crest of 

 the peninsula of Lower CaHfornia. They include the San Jacinto, 

 Santa Rosa, Vallecito, and Laguna ranges, and their crest separates 

 the drainage of Salton Basin from that of the Pacific Slope. The 

 summit of the Peninsular Mountains is well watered and generally 

 timbered. Along it there are numerous tracts of gently rolling 

 land sculptured into only a moderate relief. Such are Pinyon Flat, 

 Montezuma Flat, and the region near Jacumba. These tracts are 

 doubtless remnants of the surface of the region before it was uplifted 

 to form the present mountain chain. The eastern slope of the 

 Peninsular ranges, however, is precipitous, and its dissection deep. 

 The San Bernardino Mountains and other ranges form the 

 northeast wall of the basin. To the southeast these break up into 

 a series of low, disconnected ranges. All of them are barren rock 

 masses, at whose feet are gathered great debris fans. 



THE BASIN FLOOR 



A great part of the basin floor is a monotonous plain formed of 

 alluvium brought in from every side. In the center the alluvium 

 is fine silt, but at the mountain borders it is a coarse fanglomerate. 

 Over some of the basin floor there is a considerable deposit of sand 

 in the form of dunes. About the borders there are, at various 

 places, exposures of soft, folded, sedimentary beds, which have 

 been carved into picturesque badlands. Since the basin is prac- 

 tically rainless, vegetation is scant, and restricted to desert plants, 

 such as creosote, cacti, ironwood, and, at places, mesquite. 



One other feature of the basin floor deserves mention. Near 

 its center, at the southeast end of Salton Sea, is a large group of 

 active mud volcanoes, associated with small exposures of recent 

 lava (obsidian). 



STRATIGRAPHY 



The rocks of Salton Basin have been studied in detail at only a 

 few places. For the purposes of this paper, however, they fall 

 naturally into three classes. The first class comprises pre-Tertiary 



