64 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



two parts of an original andesitic magma. The average of the anatyses of these 

 rocks resembles the analysis of the type of hornblende-mica-andesite, taken as a 

 standard in default of any fresh andesite of this kind in Tonopah (p. 217). 



Comparison of the means of the analyzes of rhyolltic and basaltic rocks of Tonopah with those of andesitic rocks. 



1. Siliceous rhyolite, Belinont shaft. 



2. Basalt, Mount Siebert. 



3. Average of 1 and 2. 



4. Average type of andesite. 



5. Andesitic pearlite, Eureka." 



6. Mica-andesite, Washoe. '' 



COMPLEMENTARY NATURE OF DACITES AND LATET ANDESITES. 



The fact that the rhyolite and basalt of the district were nearly contemporaneous 

 and probably complementary, and were perhaps derived from an original magma 

 like that of the earlier andesite, suggests that the later andesites and dacites, whose 

 eruptions in a general way intervened c between those of the earlier andesite and 

 of the rhyolite-basalt, may also be complementary and represent an earlier stage 

 in the differentiation. 



There is available a single complete analysis of the dacite made from a typical 

 specimen of the Brougher dacite '' of Brougher Mountain (No. 359). There are, as 

 before stated, two complete analyses of the fresh later andesites (Nos. 225 and 349, 

 p. 57). To determine how far the dacite and later andesite may be complementary, 

 these analyses have been added together and halved. 



The average of No. 349, perhaps the freshest specimen of later andesite, and of 

 No. 359 (dacite) is given in column 1 of the following table. The average of two 

 analyses of fresh later andesite (Nos. 349 and 225) was averaged with the dan to 

 analysis. The result is given in column 2. 



o Won. V. S. Oeol. Survey, vol. 20, p. 264. 



' Ibid., p. 282. 



This applies lo the Heller daelte, the Fraction dacite breccia, and the Tonopah glassy rhyolite-dacile. The Brougher 

 dacite In an exception, Immediately succeeding the basalt eruption of Mount Siebert, and being probably nearly contem- 

 poraneous with the Odillc rhyalite. 



rfSlnce this part of the re|x>rt was written, an analysis was made of the glassy Tonopah rhyolite-daeite (No. 661) 

 north of the King Tonopah shaft, as given on p. 57. This analysis has not been introduced into these calculations, sine-i- 

 ll offers no new but only corroboratory evidence concerning conclusions here set forth. This will appear from thu 

 following average of the glassy Tonopah rhyollle-dacite (So. 661) with fresh later andesite (No. 849). 



HIO,. 64.28: AI,O,. 14.98; Fe,O a . 3.55; FeO, 0.71; MgO, 1.67; CaO. 3.07: Na-O. 2.95; K;O, 4.04. 



