190 ARNOLD AND STRONG — CRYSTALLINE ROCKS OF SAN GABRIEL 



character of its rocks is considered. Similar conditions, however, prevail 

 in large areas of the Sierra Nevada, so that possibly this is the normal 

 condition for the ranges of this part of the continent. The most char- 

 acteristic granite area is in the vicinity of mount Waterman, and three 

 rocks of this type from that locality will be described. 



A mass of biotite-granite occurs in Chillao canyon, several miles west 

 of mount Waterman. Megascopically this rock (A. M. S. number 15) 

 is light brown, fine grained, and is distinguishable from most of the rocks 

 of the region by the paucity of the ferromagnesian minerals. ' Its pecu- 

 liar rusty color is caused by the weathering of small amounts of the 

 biotite. Although the rock is unusually fresh, thin sections show it to 

 be much crushed. It is hypidiomorphic granular in structure, and con- 

 sists essentially of orthoclase and quartz, although well developed crystals 

 of plagioclase are not uncommon. Some of the orthoclase crystals are 

 much broken and show weathering slightly along the joint planes. 

 Biotite is found sparingly throughout the rock and a little muscovite is 

 also present as a secondary product in some of the orthoclases. 



Biotite-granite, much richer in biotite than the one just described, is 

 found at Fern camp, in Buckhorn canyon, near mount Waterman. The 

 rock at this camp appears to be finer grained and less weathered than in 

 most other places along the canyon, but in other respects is quite similar 

 to the common facies. Hand specimens of this granite (A. M. S. num- 

 ber 19) are dark gray in appearance and show feldspar, quartz, and much 

 biotite. Microscopically the rock is interesting on account of the micro- 

 cline which is characteristically developed in it. Intergrowths of this 

 mineral and quartz were noted. The orthoclase and microcline together 

 are in excess of the plagioclase, although there is nearly as much plagio- 

 clase as orthoclase alone. One crystal of plagioclase shows every alternate 

 albite twin completely kaolinized, while the intervening ones are unaltered. 

 Epidote is the principal weathering product of the feldspars. The biotite, 

 which is present in considerable quantities, shows brown to greenish 

 pleochroism. Magnetite also occurs sparingly. 



Another large mass of granite is found on the northeastern face of 

 mount Waterman. This rock (A. M. S. number 22) is medium grained, 

 gray, and shows feldspar, quartz, biotite, and hornblende. In thin sec- 

 tions the rock is seen to be hypidiomorphic granular. Orthoclase, 

 apparently the dominant mineral, occurs in prominent xenomorphic 

 crystals, through which pass parallel microscopic veinlets of kaolin and 

 possibly muscovite. Zonal structure is common, the weathering follow- 

 ing the zonal lines and producing kaolin, muscovite, and epidote. Occa- 

 sional crystals of microcline are also present. The plagioclases are next 



