242 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



DIFFERENCES OF COMPOSITION EXPRESSED BY DIAGRAMS. 



The four analyses may be represented by the Brogger diagram (fig. 75), in 

 the same manner as employed for the earlier andesite. 



The diagrams show the principal elements of fresh rocks, and fulfill all 



Scale:. 01 (quotient figure)-^ inch 



KEY 



Flo. 75. .Diagram showing changes in composition during alteration of the later andesite. 



ordinary purposes for these, but in altered rocks the altering agents have fre- 

 quently entered into the rock and constitute an important part of its bulk. To 

 take cognizance of three of the most important of these agents in this case water, 



