PETROGRAPHY OF PLUTONIC ROCKS 193 



A chemical analysis of A. M. S. number 10 follows : 



Analysis of granodiorite (A. M. S. number 10) from mount Wilson, A. M. Strong , 



analyst 



Si0 2 61.38 



Ai 2 3 14.33 



Fe 2 3 7.64* 



FeO 1.02 * 



MnO Trace 



CaO 5.42 



MgO 2.98 



K 2 2.58 



Na 2 4.71 



H 2 0. 0.13 



100.19 



Granodiorites of the same general type as A. M. S. number 10, but still 

 having minor differences, occur at several places within the territory 

 under discussion. One of these (R. A. number 7) may be taken as char- 

 acteristic of the granodiorites of the crest of the Sierra Madre range. 

 Megascopically this rock is very light gray, fine grained, and shows the 

 clear glassy quartz and the glossy cleavage surfaces of the feldspars. 

 Small particles of biotite are scattered quite thickly throughout the mass, 

 but the light colored constituents so predominate as to give it almost the 

 appearance of aplite. A few small crystals of quartz have a greenish 

 yellow cast, and, near the weathered surfaces, small amounts of green 

 chlorite may be detected. When viewed microscopically the structure 

 is seen to be hypidiomorphic granular. Plagioclase, which was deter- 

 mined to be oligoclase and andesine in about equal quantities, is the 

 most abundant constituent. In this mineral the faces parallel to the 

 twinning planes are generally perfect, while the ends of the prisms are 

 irregular in outline. Albite twinning is common, and, in one instance, 

 twinning according to the pericline law was seen. Inclusions of mus- 

 covite, biotite, and magnetite occur in some of the plagioclases. Ortho- 

 clase, next in order of abundance to the plagioclase, occurs usually in 

 irregular grains, but also occasionally in automorphic crystals. Zonal 

 structure is common, and twinning according to the Carlsbad law was 

 noted in several crystals. One orthoclase shows inclusions of biotite, 



♦There was undoubtedly an error in the determination of the iron, the percentage of ferric 

 iron being altogether too high for a rock containing as much hornblende as A. M. S. number 10. 

 Taking the total iron as 8. 60 (this also being too high, as the oxidation of the ferrous iron would 

 add weight), the calculated percentages for each oxide would be about as follows : 



Fe 2 3 2.50 



FeO 6,16 



