CHAPTER III. 



RHYOLITE. 



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

 ably distinct iii the hand specimen than in thin section, since their essential constitu- 

 ents are the same throughout. The difference 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 quart/ 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 quart/ and bril- 

 liantly reflecting sauidine; 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 superficial 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 from 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 variety 

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

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

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

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

 tematic classification has been undertaken, the varieties receiving local designations 

 sufficient for the purposes of the present report. 2 



pinto Peat Rhyolite. Under the microscope thin sections from a great number of speci- 

 mens of this variety present an extremely monotonous appearance ; a fine grained, 

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



1 Von Richthofen Natural System of Volcanic Bocks, San Francisco. 1867, p. 16. 



2 Since this was written u study of the rhvolites of the Great Basin led to more definite conclusions regarding von 

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

 Hague and J. P. Iddings. Am. Jour. Sri., vol. xxvn 1884, p. 461. 



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