PETROGRAPHY OF METAMORPHIC ROCKS 203 



Hornblende-schist. — Hornblende-schist makes up a considerable area 

 at the foot of the Mount Wilson trail, at Sierra Madre. It occurs in thick 

 layers and is dark green in color. Hand specimens of the rock (R. A. 

 number 28) show it to be fine grained, but with no apparent lines of 

 schistosity, although the lines are easily distinguishable in large masses 

 in place. The schist is weathered on the surface to chlorite, which also 

 coats the faces of the numerous joint cracks ; but small crystals of feld- 

 spar are abundant enough in places to give it a decided^ speckled ap- 

 pearance. Under the microscope hornblende is seen to be the prin- 

 cipal constituent. This mineral is present in small grains, elongated 

 fibrous blades, and elongated prisms with frayed ends (shown in sec- 

 tions parallel to the long axis). Pleochroism is from light green to 

 blue, and light yellowish brown to blue and green. Most of the crys- 

 tals show the characteristic cleavage lines, but a few were noticed which 

 showed no cleavage whatever. The usual alteration products are pres- 

 ent in abundance. Orthoclase is not uncommon, occurring in small 

 xenomorphic grains, which are often clouded by minute inclusions. 

 Crystals of plagioclase are also occasionally present. 



Garnetiferous schist. — A large boulder of a dark-gray garnet-bearing 

 schist was found in the Eaton Canyon wash about a mile below the 

 mouth of the canyon. The garnets, which were abundant throughout 

 the rock, ranged in size from minute dots to five-eighths of an inch (16 

 millimeters) in diameter. No sections of this rock were made. Garnets 

 an inch in diameter are reported as having been found in rock in place 

 in Eaton canyon, but, with the exception of the boulder noted above, 

 no garnetiferous rocks were found in this territory by the writers. 



Summary 



The San Gabriel mountains, comprising an area of about 1,200 square 

 miles, extend for approximately 60 miles in a west-northwesterly direc- 

 tion from Cajon pass, in San Bernardino county, to the Santa Clara 

 river, in Los Angeles county. Considerable divergence of opinion re- 

 garding the age of the chain has prevailed among previous writers, but 

 it is probable that it received at least the greater part of its elevation 

 during late Eocene or Oligocene time. 



The southern range of the chain, the Sierra Madre, is composed prin- 

 cipally of granodiorite and gneiss, with some associated quartz-monzonite 

 and gabbro and intruded aplite, quartz-hornblende-porphyrite and dia- 

 base porphyry. The central portion of the mountains consists of some- 

 what coarser grained granites and granodiorites, with intruded aplite, 

 micropegmatite, etcetera. 



