22 2 JOHN S. BROWN 



Palms. At other places they are deflected by the foothills into 

 long, longitudinal valleys between the crystalHne and Tertiary rocks. 



The Indio and Mecca Hills have undoubtedly been lifted from 

 the basin floor by a fault for which the name of Indio fault is sug- 

 gested. This fault passes along the northeast border of the hills 

 formed by the Tertiary rocks. Near Indio there is for several 

 miles a steep and very perfect scarp, generally not more than loo 

 to 300 feet in height. The broken Tertiary beds dip southwest from 

 the present face of this escarpment at angles of about 45^. Along 

 the northeast side of the Mecca Hills the scarp is less perfect, but 

 still constitutes a very well-defined ridge. West of Dos Palmas it 

 is a low bluff along the east base of a point of clay hills. 



Although the Indio fault as here described was observed only at 

 a few scattered points, it appears reasonably certain that it extends 

 throughout the length of the Indio and Mecca hills. The most 

 critical point at which it should be examined is the contact with the 

 crystalline bedrock in the Orocopia Mountains, where there appears 

 to be no marked separation of the crystalline range from the bor- 

 dering sedimentary foothills. The fault doubtless continues at 

 least as far south as an isolated hill of Tertiary material near 

 Durmid, which has probably been elevated in the same manner as 

 the hills farther north. Its total length, then, is about 50 miles, 

 and its form an arc of gentle curvature. Its northwestern end has 

 a trend nearly west, and lies in the direct line of probable extension 

 of the San Andreas rift, from which the actual separation is only a 

 few miles in length. The conclusion seems justified that the Indio 

 fault represents the extension of the San Andreas rift. At the 

 southeastern end the fault-trend is more nearly south, and points 

 suggestively toward the mud volcanoes. 



It should be noted that the throw of this fault is opposite to 

 that of the San Andreas rift farther northwest, and also to the 

 original displacement which must have formed the Salton Basin. 

 Such reversals of throw along prominent fault lines are, however, 

 neither impossible nor uncommon,^ and occur at several places 

 along the course of San Andreas rift farther northwest. 



' The Bright-Angel fault in the Grand Canyon is a case in point. Science, 

 April 24, 1908, p. 667. 



