CHEMICAL COMPOSITION OF LAVAS. 



57 



A rather characteristic difference between the dacites and the rhyolites is the 

 predominance of potash over soda in the latter; and in this particular the inter- 

 mediate character of the Tonopah rhyolite-dacite is also seen. 



Analyses of Tonopah 



a Analyses 1, 7, 8, 9. and 10 are by Dr. E. T. Allen; analysis 2 by Dr. \V. F. Hillebraiul: analyses 3, 5, and 6 by Mr. 

 George Steiger. 



1. Basalt, Siebert Mountain (specimen 168). This basalt is not typical chemically, containing 

 only 2.37 less silica than the andesite, analysis Xo. 2. It appears to fall, more accurately 

 considered, into the group intermediate between the basalts and the andesites, for which the 

 writer has proposed the name tileutite. For the same reasons that are given later for not using the 

 term latite, however, the name basalt will be retained. 



2. Augite-biotite-andesite (later andesite), Halifax shaft (specimen 349). 



3. Augite-biotite-andesite (later andesite), Mizpah Extension shaft (specimen 225). 



4. Hornblende-biotite-andesite (earlier andesite). (See p. 217.) 



5. Mountain dacite, Brougher Mountain (specimen 359). 



6. Glassy Tonopah rhyolite-dacite, 2,700 feet north of King Tonopah shaft (specimen 661). 



7. Mountain dacite, Butler Mountain (specimen 368). 



8. Mountain dacite, Golden Mountain (specimen 388). 



9. Rhyolite, Belmont shaft, Rushton Hill (specimen 376). 

 10. Rhyolite, G. & H. tunnel, Mount Oddie (specimen 337). 



CHEMICAL COMPOSITION OF THE DAC1TE-RHYOLITE SERIES. 



Differences and relations. The volcanic rocks which have been described as 

 dacites and rhyolites often differ markedl}' in composition as well as in age. For 

 example, the rock of Brougher and Butler mountains is quite different from that 

 of Mount Oddie, as is evident to every one, be he geologist or not. Yet the two 

 rocks are closely related and there are transitions between them, as represented, 

 for example, in the rock in parts of Golden Mountain. 



