406 University of California Publications in Geology [Vol. 13 



west, passes through Two Hole Spring. Another passes through Old 

 Woman Springs and southeastward beyond Rock Corral. Approxi- 

 mately parallel to this are several others: one extending from Box S 

 Springs to Rabbit Springs ; another along the southwest side of Fry 

 Mountain ; another extending northwestward from a point one and 

 one-half miles southwest of Means Wells; one just north, and three 

 about two miles east of Negro Butte. 



The San Andreas fault skirts the south flank of the main mountain 

 mass, passing in a southeasterly direction along Potato Canon to Pine 

 Bench, and then on to the mouth of Lion Canon, thence nearly due 

 east, beyond Whitewater River. South of this fault, and striking 

 nearly east-west, there is another bordering the lowermost foothills. 

 Between these two faults are several of less importance. North of the 

 San Andreas fault, and approximately parallel to it, is the Mission 

 Creek fault extending entirely across the area mapped. The eastern 

 portion borders the south flank of the mountains and is of some 

 importance, while immediately to the south the San Andreas ends. 

 Thus it may be that the Mission Creek fault, as it continues eastward, 

 marks the same line of weakness as does the San Andreas to the north- 

 west. Between the two are smaller faults probably representing 

 adjustments. 



The Santa Ana Canon was, in part, formed by faulting, the exact 

 nature of which has not been fully worked out. The lines mapped 

 merely represent the general effect, but the actual faulting was prob- 

 ably somewhat complex. The outlines of the structure are indicated 

 in the stereogram, figure 12. 



Another important feature is a general uplift, or doming, of the 

 whole region. This is best evidenced by the youthful aggressiveness 

 with which erosion is removing the floor of the San Gorgonio Pass at 

 the heads of several small canons west and south of Beaumont. 



Geological History 



In pre-Cambrian time there was a great invasion of granites in 

 the country now occupied by the San Bernardino Mountains and the 

 portion of the Mojave Desert immediately to the north. Erosion 

 removed nearly all the overlying rocks and in lower Cambrian time 

 the ocean advanced over a granite floor upon which were deposited 

 sands now represented by the quartzite in the Iron Mountains and 

 near Oro Grande. The Arrastre quartzites in the San Bernardino 

 Mountains are believed to be of the same age. 



