1922] Vaughan: Geology of San Bernardino Mountains 



365 



cutting the whole mass rises the granite of Granite Peak. It extends 

 northward to Grapevine Creek and thence westward. The rock is a 

 medium coarse-grained granite whose general appearance is pinkish 

 to yellowish grey. A specimen from one mile northeast of Cactus Flat 

 was examined in thin section and its principal constituents found to 

 be nearly equal amounts of quartz and pink orthoclase with Carlsbad 

 twinning. Part of the quartz and orthoclase occurs as numerous 

 rounded areas of vermieulate, radiate intergrowths. Some albite-oligo- 

 clase is present. The principal accessory is biotite, but there is also a 

 small amount of muscovite. Many small prisms of apatite are scat- 

 tered through the rock as well as a few grains of magnetite and 

 rounded colorless crystals with high index of refraction and low bire- 

 fringence. The latter may be due to strain, for both the orthoclase 

 and quartz show undulatory extinction. In that case the mineral may 

 be garnet. Because of its prominence in the vicinity of Cactus Flat 

 granite of this period will be referred to as the Cactus granite. 



A small area of dark medium-grained granite cuts the limestone 

 near the point where the above specimen was collected, but is itself 

 intruded by the larger granitic mass. It consists of albite-oligoclase, 

 and a very little orthoclase and even less quartz, with biotite and dark 

 green hornblende in about equal amounts and in sufficient quantity to 

 give the rock its dark color. Titanite is unusually abundant and some 

 rather large crystals are to be seen. A little augite, a large number 

 of small apatite prisms, and a few grains of magnetite were also found. 

 A very small amount of quartz and orthoclase are in micrographic 

 intergrowth. Decomposition is under way, the feldspar going over to 

 muscovite and kaolin. This rock is not strictly a granite, but is really 

 a biotite-hornblende diorite. 



North of Baldwin Lake there is an area of granite which seems to 

 differ somewhat from the larger part of the mass and yet both seem 

 closely related in the field and look somewhat alike. This rock con- 

 tains equal amounts of quartz and orthoclase with Carlsbad twinning, 

 but also considerable oligoclase twinned on the albite, pericline, and 

 Carlsbad laws. Biotite is the most important accessory and along 

 with it is a little muscovite. A few grains of magnetite and titanite, 

 small prisms of apatite, and also rounded crystals, similar to those 

 believed to be garnet in the main mass, are also present. 



The main mass of granite ends on the surface about a mile west of 

 Cactus Flat, but rises through the limestone and quartzite farther 

 west in Van Dusen Canon, at Union Flat, and also northwest of Union 



