C H A T» T E R III. 



RHYOLITE. 



There are three distinct varieties of rhyolite iu the Eureka District, more notice- 

 ably distinct in the hand spet-imen than in thin section, siiuie their essential constitu- 

 ents are the same throughout. The difiference arises from a change in the relative 

 proportion of the phenocrysts and in the nature of the groundmass. That from 

 about Pinto Peak which covers the greatest area has a light colored groundmass, 

 for the most part white, also gray and purplish gray, partly vitreous and partly 

 crystalline in appearance, with numerous porphyritical crystals of quartz and feld- 

 spar and a few scattered bits of mica. A second variety, from Rescue Canyon, has a 

 reddish purple, vitreous groundmass, crowded with large crystals of quartz and bril- 

 liantly reflecting sanidine; and the third, from south of Carbon Ridge, has a dense, 

 reddish purple groundmass, often finely banded, having few phenocrysts except those 

 of copper-colored mica. Upon a su]>erflcial examination of these rocks in the field it 

 would seem natural to separate the three varieties into the classes suggested by Von 

 Richthofen in 1867.' That from Rescue Canyon has all the appearance "at a dis- 

 tance" of granite, and might be said to be "granite like," while that from Pinto Peak 

 is certainly " porphyry-like," and the variety fi'om south of Carbon Ridge, being 

 quite poor in macroscopic crystals and having a beautifully banded structure, answers 

 to the description of rhyolite proper; but under the microscope the granite-like vai'iety 

 is found to have an almost wholly glass groundmass, and to correspond, therefore, 

 more or less closely to quart/.porphyry. The groundmass of the porphyry-like kind, 

 on the contrary, is found to be microcrystalline in most cases, or niicrogranitic, and 

 the third to vary from a quite glassy to an entirely crystalline rock. Hence no sys- 

 tematic classification has been undertaken, the varieties re(>eiving local designations 

 sufficient for the purposes of the present report. - 



Pinto Peak Rhyoiite.-Under the microscopc thin sections from a great number of speci- 

 mens of tbis variety present an extremely monotonous appearance; a finegrained, 

 more or less wholly crystalline groundmass rich in large crystals and fragments of 



' Von Richthofen. Natural System of Volcanic Eocks, San Francisco, 1867, ]). 16. 



^Sincethis was written a study of the rhvolites of the Great Basin leil to more detinite conclusions regarding von 

 Richthofen's classification of rhyolites, which were expressed in a paper on the volcanic rocks of the Great Basin by Arnold 

 Haiue and .T. P. Iddinis. Am. Jnnr. Sci, vol. xxvri 1S84. p. 461. 

 374 



