202 ARNOLD AND STRONG — CRYSTALLINE ROCKS OF SAN GABRIEL 



and characteristic spotted appearance. Still farther up the trail from 

 No. 8, and grading imperceptibly into it, is the quartz-monzonite de- 

 scribed as A. M. S. number 8. Transitions similar to the one just 

 described are rather common throughout the range, and often take place 

 within relatively short distances. 



A hornblende-gneiss (R. A. number 9), having about equal amounts 

 of orthoclase, plagioclase, and hornblende, makes up a large area on the 

 Sierra Madre-Mount Wilson trail, a short distance above the Half-way 

 house. The orthoclase in this rock shows zonal structure and, in sev- 

 eral cases, twinning according to the Carlsbad law. Quartz, epidote, 

 chlorite, magnetite, and hematite are also present in relative abundance 

 in the order named. Nearly all of the minerals, especially the feldspars, 

 are clouded by rows of minute inclusions. 



Biotite-granite-gneiss. — The walls of the canyon leading back from Al- 

 pine tavern toward the summit of mount Lowe are composed of an intri- 

 cately contorted banded gneiss. Megascopically this rock (R. A. number 

 4) is rather dark colored, the darker bands having a somewhat glossy 

 appearance along the planes of schistosity, caused by the minute flakes 

 of biotite, which megascopically appear to be the dominant mineral. 

 Under the microscope, however, the principal constituent of the rock is 

 seen to be orthoclase feldspar instead of biotite. This feldspar occurs in 

 xertomorphic grains, some of which are quite large. Rows of inclusions 

 parallel to the lines of schistosity pass through the feldspars and occa- 

 sionally through the quartzes. No twinning is noticeable in the ortho- 

 clase, but occasional examples of zonal structure are seen. Biotite is 

 next in order of abundance to the orthoclase, although, as previously 

 mentioned, it appears in the hand specimen to be the dominant min- 

 eral. It occurs in small reddish brown flakes scattered among the feld- 

 spars and shows pleochroism from yellow to green. One case of twin- 

 ning was noticeable in the biotite. Following the biotite in relative 

 abundance is quartz, which occurs in small xenomorphic grains. A 

 small amount of plagioclase (oligoclase), a few elongated crystals of 

 hornblende containing rows of inclusions, some epidote, magnetite, zir- 

 con, and a little chlorite are also present. The sections are crossed by 

 minute cracks approximately parallel to each other, but perpendicular 

 to the banding. The feldspars are noticeably more kaolinized along 

 these cracks, but the latter appear to have no relation to the cleavage, as 

 the same crack may extend through a feldspar and an adjacent quartz or 

 hornblende crystal. These cracks are probably due to stresses which 

 acted perpendicular in direction to those producing the original banding 

 of the gneiss, and which took place after the crystals had become com- 

 pletely formed. 



