COMPLEMENTARY NATURE OF DACITES AND ANDESITES. 65 



Mean composition of Tonopah daciteg and later andesites compared with composition of early andesite. 



To compare these results with known rocks, the nearest analyses of Washoe 

 and Eureka rocks are also given above. No. 3 is mica-andesite from Washoe," 

 already twice referred to; No. 4 is andesitic pearlite from Eureka. These two 

 rocks from Eureka and Washoe are among those which are regarded (p. 219) as 

 closely similar to the earlier andesite of Tonopah. 



By comparison of the different analyses it is seen that the dacite and the 

 later andesite of Tonopah added together produce an andesite of intermediate 

 composition, such as is usually a hornblende-andesite or a hornblende-mica- 

 andesite. Moreover, the amounts of silica, lime, soda, and potash in this average 

 are strikingly like those in the average of the partial analyses of basalt and 

 rhyolite, as is shown by the following table: 



Analyses of siliceous andesite compared with mean analysis of rhyolite and basalt and mean analysis of dacite 



and later andesite. 



1. Average of rhyolite and basalt (Nos. 376 and 168, p. 64). 



2. Averages of later andesite and dacite (see table above). 



3. Earlier andesite, Tonopah (p. 216). 



4. Mica-andesite, Washoe. 



Further averages of the silica, lime, soda, and potash of the dacite and later 

 andesite may be had by combining with the andesite analyses the partial analyses 

 of the dacite (No. 368) from the east end of Butler Mountain and of the dacite 



"Mon. U. S. Geol. Survey, vol. 20, p. 282. 



16843 No. 4205- 



