2 20 JOHN S. BROWN 



rift, a notable fault along which occurred the displacement that 

 caused the San Francisco earthquake of 1906. The CaHfornia 

 Earthquake Commission traced this fault continuously from San 

 Francisco southeastward to the very tip of Salton Basin. In its 

 report' the commission suggests that the rift probably continues 

 southeast, and is connected in some way with the formation of the 

 basin. In the Peninsular Mountains, southwest of the basin, there 

 are also several well-recognized faults, portions of which have been 

 mapped, but these have generally not been traced eastward to 

 their visible terminations. These faults and their relation to the 

 faults described here are shown in Figure i. 



The evidence of faulting in this region is as a rule, derived from 

 the topography, and can only rarely be established by stratigraphic 

 criteria. Igneous and metamorphic rocks, such as compose the 

 mass of the mountain ranges, are so homogeneous in nature that it 

 is often impossible to determine differences in the rocks exposed on 

 either side of a fault, while the Quaternary material which fills 

 most of the valleys has been deposited since the faulting occurred 

 and obscures it in many places. 



THE INDIO FAULT 



At the base of the crystalline ranges (Little San Bernardino, 

 Orocopia, etc.) bordering the upper part of the northeast side of the 

 basin, Hes a conspicuous chain of low, badland hills in which are 

 exposed the late Tertiary sedimentary beds, and occasionally small 

 patches of the underlying bedrock. This chain of hills is separated 

 by a slight break into two ranges known as the Indio Hills and 

 Mecca Hills. The Indio Hills have a total length of 20 miles, and 

 a width of 2 or 3 miles. The Mecca Hills are also about 20 miles 

 long, and at some places are 4 or 5 miles wide. The general slope 

 of the hills is southwest, where they dip beneath the basin floor. 

 They are separated from the crystalline ranges northeast by a 

 trough from i to 3 miles in width filled with coarse alluvial debris. 

 At some places the canyons that head in the main mountains cut 

 entirely across the hills, as at Shaver Canyon and the Thousand 



* California Earthquake Commission Report, Carnegie Inst, of Wash., 19 10. 



