1922] Vaughan: Geology of San Bernardino Mountains 373 



intergrowths of quartz and orthoclase, small quantities of deep green 

 hornblende, rather large crystals of titanite, small flakes of ilmenite, 

 and stout prisms of zircon. This is cut by pegmatite and aplite dikes. 

 One of the latter was examined under the microscope, but was not 

 found to differ greatly from others in the region. It consists of equal 

 amounts of orthoclase and quartz, some being in micrographic inter- 

 growth, very little biotite, some oligoclase, and a few grains of mag- 

 netite. 



In the Mojave Desert north of the mountains numerous hills rise 

 above the desert debris, and these are in most cases composed of 

 granitic rocks. As in the mountains, two periods have been recog- 

 nized. All heterogeneous masses consisting of several granitic rocks 

 have been considered as older, while large uniform masses of light- 

 colored granite containing equal amounts of orthoclase and quartz and 

 rather little biotite have been considered younger. Wherever the two 

 were found together the contact relations justified this interpretation. 



The rocks north of Means Wells are varied in character and with 

 them are schists, but on the ridge to the west the younger granite has 

 intruded the older and extends northwestward off the quadrangle in 

 a somewhat broken ridge. At the highest part of the crest there is a 

 remnant of the older granites which greatly resembles those on the 

 plateau north of Saddlerock Spring. 



Fry Mountain is a complex mass cut by numerous pegmatite dikes 

 and also considerably epidotized in some portions. Younger granites 

 form the hills on either side of Negro Butte. The granites west of 

 Rabbit Springs are cut by numerous coarse pegmatite dikes contain- 

 ing quartz far in excess of orthoclase. Because of the weathering of 

 the granite they are particularly prominent and appear to constitute 

 a significant part of the mass. 



The ages of the various granitic intrusions are somewhat conjec- 

 tural. The abundance of pegmatite dikes in the vicinity of Burns 

 Canon has already been referred to as indicative of pre-Cambrian 

 time. It is a significant fact that granites belonging to that period 

 in the earths history occur at Cadiz, about sixty miles to the east. 

 Darton 18 says: 



The Iron Mountains present a considerable variety of rocks, including pre- 

 Cambrian granite The granite .... is overlain .... by rocks of Cam- 

 brian age, consisting of a basal quartzite with a thick body of overlying lime- 

 stone and shale The granite has a wave-worn surface, and the beds were 



deposited on this surface when it was a sea bottom, a fact which establishes the 

 age of the granite as pre-Cambrian. 



is Darton, N. EL, Santa Fe Eoute, U. S. G. S., Bull. 613, Part C, 1915. 



