36 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



KHYOLITES AND DACITES. 



INTERRELATION OF RHYOLITES AND DACITES. 



The rhyolites and dacites at Tonopah are closely bound together in every 

 way in chemical and mineralogical composition, in areal distribution, and in 

 manner and time of eruption. In fact, they can be best understood if considered 

 as portions of the same great magma, split up, as the author would like to assume, 

 b} 7 internal segregation or magmatic differentiation. These lavas constitute tran- 

 sitions between the two types (rhyolite and dacite) named above, and the dacite 

 itself is a very siliceous" one, barely deserving distinction from the rhyolites 

 were it not necessary to emphasize the distinction between it and the still more 

 highly siliceous rhyolite which forms some of the hills of the region, such as 

 Oddie and Ararat. Moreover, although the rocks of Butler, Brougher, Siebert, 

 and Golden mountains are distinctly of the dacitic type, and so fairly classed 

 together and distinguished from the rhyolite, yet different hills (being denuded 

 volcanic necks and so representing separate vents) show different phases. Golden 

 Mountain, for example, is made up of a lava which, both in the field and under 

 the microscope, seems to be more closely allied to the near-by rhyolite than to 

 the dacite of the more distant eminences in the lower or southwestern half of 

 the mapped area, such as Brougher Mountain. Chemical tests bear out this 

 impression in large measure. The fine-grained border facies of this Golden 

 Mountain intrusion, being glassy with sparser feldspar phenocrysts than the 

 normal type, is indistinguishable, without chemical analysis, from similar rhyolite. 

 The glassy dikes which extend from the main mass are of the same character. 

 Many of the small dacite-rhyolite flows, erupted at an earlier period than the 

 volcanic necks, are similarly tine grained, and difficult to classify exactly as dacite 

 or rhyolite without numerous and altogether useless chemical tests. It is prac- 

 tically certain that many of these are transitions between the two extreme but 

 closely related types. 



SIMULTANEOUS ERUPTION'S. 



The eruption of dacite and rhyolite, which succeeded that of the andesite, 

 extended over a long period and was characterized by many variations in the 

 rhyolite and dacite. The observed phenomena favor the conclusion that different 

 vents were in a state of eruption nearly or quite simultaneously, each one 

 contributing its characteristic rock, and that the notable alternation^ of different 

 kinds of lava are due rather to the temporary inactivity of some of the vents 

 than to any real change in the character of the magma in the supply basins. 



In order to describe better the geologic history and the economic geology a 

 number of subdivisions have been made in the dacite-rhyolite series. 



Fur the use of the term "dacite" set 1 pp. 58-59. 



