194 ARNOLD AND STRONG CRYSTALLINE ROCKS OF SAN GABRIEL 



orthoclase, and plagioclase. Quartz is a somewhat less important con- 

 stituent than the orthoclase, is present in small grains, and is free from 

 inclusions so far as noted. Biotite occurs quite commonly in small 

 grains, which show the characteristic fine cleavage lines and brown to 

 green pleochroism. Muscovite is also present in small amounts, but 

 probably only as a secondary product. A few grains of magnetite, occa- 

 sional small green hornblendes, and some chlorite are also present. A 

 characteristic diamond-shaped crystal of titanite occurs in one of the 

 sections. 



The San Rafael hills, west of Pasadena, are largely composed of biotite- 

 granodiorite, which in most places is much jointed and weathered. 

 Specimens of this rock (R. A. numbers 8 and 31) appear dark gray in 

 the hand, and are seen in thin sections to be composed principally of 

 plagioclase, biotite, orthoclase, and quartz, named in order of relative 

 abundance. 



It is not unusual in the granodiorite areas of the Sierra Madre range 

 to find portions of the rock mass in which the ferromagnesian minerals 

 are relatively more abundant than in the typical facies. A specimen 

 (R. A. number 24) from one of these segregations at the summit of mount 

 Wilson shows biotite, plagioclase, hornblende, orthoclase,. quartz, and 

 secondary chlorite, occurring in relative abundance in the order named. 

 The minerals are nearly all xenomorphic and their characters similar 

 to those of A. M. S. number 10. Still another granodiorite similar to 

 A. M. S. number 10 and R. A. number 7, but containing a large percent- 

 age of hornblende, is found at the summit of mount Lowe. The min- 

 erals composing this rock are, in order of relative abundance, plagioclase, 

 hornblende, biotite, orthoclase, and quartz, with considerable quantities 

 of chlorite and a little muscovite as secondary products. 



An example of the second or coarser grained facies of the granodiorite 

 is found at Pine flats, southwest of mount Waterman. It forms a band 

 of rock about a mile in width, which has a northwesterly trend across 

 the divide from the West fork of the San Gabriel to Alder creek. The 

 rock is more resistant to weathering than the adjacent plutonics and 

 forms a terrace across the face of the mountain. Hand specimens of 

 this granodiorite (A. M. S. number 12) are gray in color and are charac- 

 terized by phenocrysts of orthoclase 5 to 18 millimeters in length, which 

 are usually twinned according to the Carlsbad law. The groundmass 

 consists of feldspar, quartz, hornblende, and some biotite, the last some- 

 times being altered to chlorite. Microscopically the rock is seen to be 

 made up principally of xenomorphic crystals of feldspar, hornblende, 

 and quartz. The plagioclases, which appear to be about equally divided 



