PETROGRAPHY OF PLUTONIC ROCKS 195 



between oligoclase and andesine, are the dominating minerals. Peri- 

 cline twinning is common in the oligoclase. The orthoclase, as previously 

 mentioned, occurs principally as large phenocrysts, twinned according 

 to the Carlsbad law. Considerable quantities of quartz are present in 

 xenomorphic grains of various sizes. The hornblende is in irregular 

 masses, shows green to yellowish pleochroism, extinction angles of about 

 20 degrees, and is often altered to epidote around the periphery. Titan- 

 ite is present in small quantities in wedge-shaped crystals. Zircon, 

 apatite, magnetite, and hematite in scattered crystals complete the 

 mineral composition of the rock. The following is an analysis of this 

 granodiorite : 



Analysis of granodiorite (A. M. S. number IS) from Pine flats, A. M. Strong, analyst 



Si0 2 64.45 



Ti0 2 Trace 



A1 2 3 17.18 



Fe 2 3 3.32* 



FeO 0.60* 



MnO 1.62 



CaO 4.31 



MgO 0.75 



K 2 2.98 



NajO 4.24 



H 2 0.59 



P 2 5 Trace 



100.04 



Thegranodiorites of the San Gabriel mountains appear on the average 

 to be finer grained and to contain less quartz, titanite, and zircon than 

 those from the Sierra Nevada range of central California, but otherwise 

 the rocks of this class from the two regions appear to be quite similar- 

 The granodiorites near the center of the southern California chain are 

 more nearly like those of the Sierra Nevada than are the rocks of the 

 same class from the outer or Sierra Madre range. This statement holds 

 true not only of the grain and color of the rocks, but also of their chem- 

 ical composition. 



A comparison of the anatyses given in the following table will show 

 the chemical relations of the granodiorites from the San Gabriel and 

 Sierra Nevada mountains. 



♦See note under previous analysis concerning error in determination of iron oxides. In this 

 case the Fe^ should be about 1.31 and the FeO 2.61. 



