GENERAL CHARACTER 189 



zonite, granodiorite, hornblendite, aplite, micropegmatite, quartz-horn- 

 blende-porphyrite, diabase-porphyry, hornblende-diorite-gneiss, biotite- 

 granite-gneiss, hornblende-schist, and garnetiferous schist. 



The Sierra Madre range consists essentially of granodiorites and 

 gneisses, with more acid areas in which the country rock is quartz- 

 monzonite. Large dikes or included masses of hornblendite are present 

 at several localities, notably on the south slopes of mount Lowe, while at 

 other places smaller dikes of quartz-hornblende-porphyrite and diabase 

 porphyry cut the country rock. Aplite dikes and quartz veins are of 

 common occurrence, some of the latter yielding traces of gold and silver. 

 Garnet-bearing and hornblende schists are also found in the southern 

 range. 



The character of the rocks of the mountain area north of the Sierra 

 Madre is considerably different from that of the latter. True biotite- 

 granite and rather coarse grained granodiorite, decidedly different in 

 physical appearance from that of the south range, are found in the 

 northern mass. Aplite and micropegmatite are also found in the latter 

 region. 



Taking the San Gabriel mountains as a unit, therefore, it is found that 

 the southern border is composed principally of fine grained granodiorites 

 and gneisses, while the central portion (the extreme northern border of 

 the mountains was not examined by the writers) is composed of rela- 

 tively coarser grained rocks, which are probably somewhat more acid 

 in average composition than the border range types. 



Detailed Petrography 

 plutonic rocks 



General characteristics. — The plutonic rocks found include biotite-gran- 

 ite, quartz-monzonite, granodiorite, and hornblendite. With the excep- 

 tion of the latter, all of the rocks of this class are gray colored and fine 

 grained, and all, without exception, are prone to fall an easy prey to the 

 destructive forces of weathering. The outcrops as a rule show rounded 

 outlines, and in many instances huge detached boulders, the products 

 of weathering, are found on the nude surfaces of the country rock. On 

 account of the susceptibility of these plutonics to the weatherwing 

 process, fresh specimens were hard to obtain, and the determination of 

 the rock, even when examined in thin sections, was often more or less 

 uncertain. 



Biotite-granite— The paucity of true granites in the territory under dis- 

 cussion is rather remarkable when the size of the area and the general 



