368 University of California Publications in Geology [Vol. 13 



is present. Most of the biotite is bleached green. Small grains of 

 magnetite and numerous rods of apatite are scattered through the 

 rock. A medium-grained porphyritic granite of similar mineralogical 

 composition occurs to the south. Part of the orthoclase is twinned 

 on the pericline law and some of the phenocrysts are more than a 

 centimeter across. No plaglioclase is present. A few stout prisms of 

 apatite occur, but small rods are by far the more numerous. Another 

 rock near this is a medium-grained yellowish grey granite with a 

 "salt and pepper" appearance due to the fineness of the biotite flakes. 

 As is the case with the others, the orthoclase and quartz in this rock 

 are in equal amounts and small areas in micrographic intergrowth are 

 found. A little oligoclase, muscovite, apatite, and magnetite are also 

 present. 



Besides the granites with original structure there are some in this 

 locality which have been subjected to great stresses and rendered 

 gneissie. In some cases the one grades directly into the other. To 

 add to the complexity, other granites have cut the whole mass, sending 

 out many apophyses, large and small, into the older rock. The rock 

 on the ridge just below the south knob of San Gorgonio Mountain is a 

 medium-grained granite with the biotite in schistose arrangement, the 

 individual streaks, however, not being continuous for more than a few 

 centimeters. Orthoclase is very prominent, many of the crystals being 

 more than five millimeters across. Quartz is somewhat less abundant 

 than orthoclase and only an occasional grain of albite or albite-oligo- 

 clase occurs. A small amount of quartz and feldspar is present in 

 micrographic intergrowth. Some muscovite occurs with the biotite and 

 both often have bent cleavages. Another evidence of strain is the 

 undulatory extinction of the quartz. A few colorless garnets are 

 associated with the biotite ; also grains of magnetite and titanite. 



The most prominent rock of San Gorgonio Mountain and the one 

 which seems to intrude all others extends to the southwest from the 

 peak. It is a medium-grained granite with equal amounts of ortho- 

 clase and quartz, a few large crystals of oligoclase, and green biotite. 

 Scattered throughout are smaller amounts of muscovite, magnetite, 

 and small rods of apatite. At the head of the gully just east of this 

 ridge there is a granite which differs from those described above in 

 that it contains a large amount of dark green hornblende and only a 

 little biotite. Titanite is also somewhat more abundant, although only 

 as small grains. The structure differs in that the quartz and ortho- 

 clase have the same degree of automorphism and present a mosaic 



