60 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



Rocks of different mineralogical but similar chemical composition are riot dis- 

 tinguished, therefore the classification is one of magmas, and is especially valuable 

 in discussions of the relation of magmas. 



The Tonopah dacite and rhyolite analyses (the last six in the table on p. 57) 

 were classified according to this system. The results are as follows: 



Position of Tonopah rhyolites and (Incites in the quantitative classification. 



Thus it is seen that all the Brougher dacite falls under one subrang, toscanose; 

 the rhyolite falls under a quite distinct order, rang, and subrang, magdeburgose; 

 while the Tonopah rhyolite-dacite is in the same order as the rhyolite (though nearly 

 in the same order as the Brougher dacite) and otherwise like the Brougher dacite: 

 so that it falls into the subrang tehamose. 



These divisions correspond to the natural divisions; and the classification is 

 evidently in this case a suitable one. 



It may be added that the dacite from Washoe, Nev. (analysis No. 2 in table on 

 page 58), is classified by Washington" as toscanose, like the Tonopah dacites, and 

 rhyolite from Eureka, Nev. (analysis No. 7, p. 58), as mihalose (near dellenose).* 

 It is of the same order and rang as the Tonopah rhyolite-dacite of Tonopah, but 

 of a dopotassic subrang, like the Tonopah rhyolite, and is, therefore, intermediate 

 between these two Tonopah rocks. 



Varying composition of lavas in different vents. The transition phases of the 

 dacite-rhyolite are not limited to small areas, but are represented by large masses; 

 so that there is no fixed point, either theoretical!}' or in the field, where one can be 

 separated from the other. Each vent, now represented by a more or less separated 

 and isolated volcanic plug, seems to have ejected nearly homogeneous lavas that 

 differed slightly in composition from the lavas from neighboring vents. Thus the 

 wilica content in the dacite-rhyolite series was least in the Brougher Mountain vent, 

 and increased successive!}' in Butler Mountain, Golden Mountain, Rushton Hill, and 

 Mount Oddie. The difference between the lava of Brougher Mountain and that of 

 Mount Oddie is very considerable. When compared with the Brougher Mountain 



nOp. clt., p. 167. &0p. cit., p. 181. 



