1922] Vaughan: Geology of San Bernardino Mountains 333 



platform ends rather abruptly against the hills west of Negro Butte. 

 This sort of thing is common in the desert and will be considered 

 further on another page. The important point to bring out at this 

 time is that the surface between Saddlerock Spring and Rock Corral 

 was probably evolved under desert conditions, and by further com- 

 parison it seems likely that this is also true of the Bluff Lake region. 

 This is also indicated by the presence of several inclosed basins, such 

 as are characteristic of the desert. 



The ridge from Granite Peak eastward is a remnant of an old sur- 

 face covered with huge boulders pitted and scarred by wind erosion. 

 Although the country rock is a uniform granite, there are to be found 

 here many angular fragments of polygenetic rocks, quartzite, schist, 

 aplite, etc. Usually these are very hard, and quartzite is the most 

 prominent. They prove that the surface was once extensive, for only 

 under such conditions could these fragments be brought here. The 

 ridge to Tiptop Mountain is part of the same surface, but it is some- 

 what different toward the east due to the nature of the country rock, 

 which is limestone. It is rather even and rounded at the crest. Tip- 

 top Mountain itself is not a simple peak, but consists of three rounded 

 hills rising above the ridge. This area is at about the same elevation 

 as some of the ridges around Bear Valley, and the whole may well 

 have been continuous at one time. 



Other remnants of the second and third cycles are probably to be 

 found in these mountains, but their correlation is even less certain than 

 some of those already described. 



The Fourth Cycle 



The mountain mass as a whole is now undergoing rejuvenation by 

 xiplift and the surfaces of the older cycles are rapidly disappearing. 

 Bear Creek plunges down a youthful canon which is gradually eating 

 back into Bear Valley. All the streams along the north front of the 

 range from Silver Creek to Arrastre Creek are cutting back to the 

 south. Arrastre Creek itself has a rather uniform and not precipitous 

 grade from the very head down to Smart's Ranch ; but below this point 

 it is precipitous. Rattlesnake Creek flows in a narrow canon with steep 

 sides often approaching the vertical. It has cut down to grade south- 

 ward to the bend two miles southeast of the Rose Mine. Above this it 

 is very steep. 



