DACITE. 21)7 



both, in much the same way as the more crystalline dacites occur 

 as transition products between the crystalline varieties of hornblende-mica- 

 andesite and rhyolite. Near the entrance to Sierra Canyon, 011 botli 

 sides of the road, there is exposed a characteristic variety of gray 

 dacite, and at Dry Lake and South Hill they occur with andesitic eruptions, 

 but only in obscure low ridges and knolls, breaking through the Nevada 

 limestone of the Devonian. Again some varieties of dacite are closely 

 associated geologically with rhyolitic pumices and tuffs, but differ from 

 them petrographically in having a predominance of triclinic instead of mono- 

 clinic feldspars. 



Rhyolite.- The essential components of this natural group are restricted 

 to orthoclase and quartz. Usually they carry more or less triclinic feld- 

 spars, in some cases almost equaling the monoclinic form, but they are 

 rarely developed in as large individuals as the orthoclase. Biotite as an 

 accessory mineral may be present in varying amounts, but is quite as likely 

 to be wholly wanting. In chemical composition they form the most acid 

 of all natural groups into which the lavas have been divided. In color and 

 texture no rock surpasses the rhyolite in the endless modifications which it 

 undergoes even within very limited areas. In crystalline structure it may 

 vary from a rock possessing a holocrystalline groundmass, with or without 

 large macroscopic secretions of the essential minerals, to one almost wholly 

 made up of glass. Whether the rhyolite is crystalline or in large degree 

 composed of glass, the sanidines occur in well developed crystals, frequently 

 presenting the brilliant iridescent hues so often observed elsewhere through- 

 out the Great Basin: The quartz occurs both as dihexahedral crystals and 

 dark gray and black angular grains which stand out in strong contrast to 

 the prevailing light tints of the inclosing groundmass. 



At Eureka, where acid lavas are singularly well developed, among 

 the many extrusions of rhyolite occur two principal varieties which cover 

 large areas and embrace the greater part of the outbursts, and for the pur- 

 poses of the present chapter may be designated by local names: one, the 

 Rescue Canyon rhyolite, the other, the Pinto Peak rhyolite. In mineral 

 and chemical composition they are closely allied. The Rescue Canyon 

 rhyolite when fresh has a decidedly red color due to a considerable amount 



