CLASSIFICATION OF IGNEOUS ROCKS 683 



Butte granite from other localities in the vicinity of Butte show 

 slightly more ferromagnesian minerals. Weed 1 has called atten- 

 tion to the generally uniform composition of this great body of 

 granite as indicated by five chemical analyses, one of which is given 

 in the tables of mica and hornblende. The specimen from near the 

 Parrot vein will be made the basis of the following discussion. 



Optical investigation of the plagioclase by the Michel-Levy 

 method with Carlsbad twins of albite twins showed that the cen- 

 tral portion of each crystal has approximately the composition 

 of Ab 1 An 1 , but the marginal zones are more sodic, having in 

 some crystals the composition of oligoclase as indicated by the 

 optical orientation and the relative index of refraction as com- 

 pared with quartz by the Becke method. Without making the 

 necessary observations for estimating the relative proportions of 

 the several zones, and the average composition of the plagio- 

 clase as a whole, it was seen that the greater part of each crys- 

 tal is about Ab 1 An,, and only a narrow margin is oligoclase, 

 but the optical orientation of the outer portions shifts gradually 

 in most cases from the central part to the margin. The average 

 composition of the plagioclase is probably andesine somewhat 

 more sodic than Ab, An, . If Ab 3 An 3 , it would have the compo- 

 sition of the mixture of albite and anorthite calculated from the 

 chemical analysis of the Butte granite from Walkerville Station 

 already cited, after developing biotite and hornblende. In this 

 case the 42.47 per cent, of plagioclase would yield by molecular 

 calculation 24.71 per cent, albite and 17.76 percent, anorthite. 

 In the second rock there would be 24.29 per cent, albite and 

 17.18 per cent, anorthite. The orthoclase may be considered as 

 wholly potassic and reckoned as pure orthoclase. 



In order to obtain the salic and femic components of the biotite, 

 hornblende, and pyroxene, since the other minerals are standard, 

 we may take the analysis of Butte biotite from Table XIV and 

 multiply it by 0.0977, the percentage weight of biotite, and take 

 the analysis of Butte hornblende from Table XIII and multiply 

 it by 0.0681, the combined percentage weight of hornblende and 



1 Weed, W. H., Jour. Geol., Vol. VII, pp. 737-50. 



