244 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



quantity results in an impracticable length for this radius, it is represented on 

 half the ordinary scale. Water was so abundant in some of the analyses that it 

 has been represented in the diagrams on the same scale as silica for a similar 

 reason. Only the water given off above 106 C. has been represented, that being 

 chemically combined, while that given off below this point is mostly hygroscopic. 

 Otherwise the scale used is the same as for the preceding diagram. 



COMPARISON OF LATER ANDESITE WITH WASHOE AND EUREKA ROCKS. 



The first two analyses of nearly fresh rocks are similar to analyses of pyroxene- 

 andesites from the Comstock region and from Eureka, as shown in the following 

 table. Nos. 1 and 2 in the preceding table are here called A and B. 



Analyses of andesites. 



Loss on ignition. 



A. Mizpah Extension shaft, Tonopah, Nevada. 



B. Halifax shaft, Tonopah, Nevada. 



C. Granular pyroxene-andesite, Eldorado, outcrop, Washoe, Nev." 



D. Pyroxene-andesite, Sutro tunnel, Washoe, Nev. 



E. Pyroxene-andesite, Richmond Mountain, Eureka, Nev. * 



DEGREE OF ALTERATION OF FRESHEST TONOPAH LATER ANDESITE. 



The freshest Tonopah specimens (A and B) show, not only under the microscope 

 but by the analyses, the beginnings of alteration more than do the Eureka and 

 Washoe rocks with which they are compared. The presence of carbonates, of a 

 greater amount of water, and of a small quantity of pyrite indicates that the 

 former have been somewhat attacked by waters containing oxygen, carbonic acid, 



a Hague, A., Mon. t". 8. Oeol. Survey, vol. 'X. p. >. 



6 Op. eit., p. '264. 



