192 ARNOLD AND STRONG — CRYSTALLINE ROCKS OF SAN GABRIEL 



at many other localities throughout the mountain area under discussion. 

 The rock consists of quartz, plagioclase (either oligoclase or andesine, or 

 both), orthoclase, hornblende, and biotite, with titanite, zircon, magnet- 

 ite, and apatite as accessories. The quartz is present in relatively small 

 quantities, so small, in fact, as often to place these particular cases very 

 near the true diorites. The plagioclase is always in excess of the ortho- 

 clase, but in some areas gradations toward quartz-monzonite show almost 

 as much orthoclase as plagioclase. Hornblende is the principal ferro- 

 magnesian mineral, although biotite is often present in considerable 

 quantities. The character of the rock varies from medium to very fine 

 grained. In color the granodiorites range from light to dark gray, de- 

 pending on the amount of the ferromagnesian minerals present. In some 

 localities the feldspars are pink and give the rock a reddish cast when 

 viewed from a distance. This latter condition is particularly noticeable 

 in the granodiorites exposed along the line of the Mount Lowe railroad, 

 in Grand canyon, and in the west wall of Eaton canyon. Two facies of 

 the granodiorite are recognizable in these mountains, a fine, almost uni- 

 formly grained type being characteristic of the Sierre Madre or southern 

 border range, while a somewhat coarser grained form containing large 

 crystals of orthoclase occurs in the central mountain mass. 



A typical example of the first class is the granodiorite in the vicinity of 

 Strains camp, on the northern slope of mount Wilson. On the surface 

 this rock is usually badly weathered and decomposed, but in some of 

 the stream beds it is possible to find comparatively fresh exposures. 

 The specimen here described (A. M. S. number 10) is from the lower 

 spring near the camp. In the hand specimen the rock is medium grained 

 and rather dark gray in color, and shows feldspar, quartz, a consider- 

 able amount of hornblende, and a few flakes of biotite. Some of the 

 hornblende is altered to chlorite. Examined in thin sections, the plagio- 

 clases were seen to be in excess of the orthoclase, and, though consider- 

 ably weathered to kaolin and occasionally to epidote, were determined 

 to be mostly oligoclase. The orthoclase occurs in xenomorphic grains 

 which are somewhat larger than those of the plagioclase, and often show 

 zonal structure and occasionally inclusions. Muscovite and kaolin are 

 its common alteration products. The hornblende is abundant in auto- 

 morphic crystals, which show dark green to brown pleochroism. Quartz 

 occurs only sparingly in small grains. Fibrous aggregates of colorless 

 zeolites, microscopic veins and small grains of epidote, and occasional 

 particles of magnetite are also present in the rock. Titanite was noticed 

 in a section of a similar granodiorite (R. A. number 7), which was found 

 near A. M. S. number 10, but was not present in any of the thin sections 

 of the latter. 



