CLASSIFICATION OF IGNEOUS ROCKS 663 



A less precise method may be used, which is sufficiently 

 accurate considering the inaccuracy of the approximation to the 

 average composition of zonally built feldspars, namely, to apply 

 the ratio Ab/An obtained optically directly to the percentage 

 weights of plagioclase. Thus 50 per cent, of plagioclase, whose 

 average composition has been estimated at Ab 3 An 2 , may be 

 separated into 30 per cent, albite and 20 per cent, anorthite 

 with approximate correctness. 



In the case of hornblende, augite, and mica, the color and 

 optical properties are undoubtedly equally characteristic, but 

 they have not received sufficient attention to permit of the same 

 direct application. They are, however, guides to the choice of 

 typical formulas or chemical analyses, which may be used tem- 

 porarily in place of more exact methods. 



For this purpose tables have been arranged giving the 

 chemical analyses of certain rock-making alferric minerals, and 

 also the analyses of the rocks in which these minerals occur. 

 From these it is possible to select cases corresponding more or 

 less closely to those in the rock whose norm is to be deter- 

 mined. For it appears from data already at hand that the 

 chemical composition of each mineral in a rock bears such a 

 relation to the chemical composition of the whole rock, that 

 minerals of the same kind, for example the hornblendes, when 

 they occur in similar rocks have very nearly the same composi- 

 tion. The compositions of the hornblendes and micas in the 

 granodiorite of the Sierra Nevada, California, and of those in 

 the very similar quartz-monzonite of Butte, Montana, are nearly 

 the same. 



For minerals with variable composition, then, select from 

 Tables of Analyses the analysis of the mineral corresponding 

 most closely to the one in the rock in question, considering 

 both its optical characters and the general character of the rock 

 in which it occurs, and reduce its several chemical constituents 

 to the proper amount by multiplying by the percentage of the 

 mineral as determined from the rock. 



Having reduced each mineral to its chemical constituents 



