SCIENTIFIC BACKGROUND 173 



and twenty miles towards the Mexican boundary near 

 Yuma. Its actual course is very conspicuous for ap- 

 proximately one hundred and fifty miles of this dis- 

 tance, to near the east end of the San Gorgonio Pass 

 (Morongo Indian Reservation), near Banning, where it 

 branches and beyond which the identity of its main 

 stem is lost in the structural complexities of the pass 

 itself and beneath the sand covered floor of the Colo- 

 rado Depression.^ Regardless of publications to the 

 contrary, the best guess is that it passes through the 

 lowest points of the Coachella arm of the desert and 

 the Salton Sink, near the mud volcanos, and on toward 

 Yuma. The definite trace of the western half, from 

 south of Maricopa to Cajon Pass, the section between 

 Cajon Pass and Whitewater, and the indefinite trace of 

 the eastern half of this southern section of the San 

 Andreas Rift are herein termed the Valyermo, the San 

 Bernardino and the Desert segments respectively. 



I have reconoitered the entire length of the Southern 

 Section from its west end to Yuma and studied some 

 parts in close detail. Likewise, I have had the benefit 

 of some of Noble's observations of former years and 

 his recent publications. As he has shown, it is by no 

 means a simple fault line feature, but in places it is a 

 complex zone of narrow parallel blocks some of which 

 have been elevated and others depressed. Further- 

 more, great branches like the Mill Creek and San 

 Jacinto Rifts lead divergently away from its rnain 

 trace. 



That portion of the San Andreas Rift between the 

 "bend," south of Maricopa and Cajon Pass, above 



'Brown, John F. Fault Features cf the Saltcn Basin. Journal of Geol- 

 ogy, 1922. 



