DACITE. 237 



iHitli. ill luucli tho same way as the more crystalline dacites dccur 

 as transition ])r(i(luets between the crystalline varieties of hornblende-niica- 

 anflesitc and rhyolite. Near the entrance to Sierra Canyon, on both 

 sides of the road, there is exj)osi'd a cliaracteristic variety of Ljra}' 

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

 hut onh' in ol)scure low ridg'es and knolls, breaking through the Nevada 

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

 associated geologically with rhyolitic pumices and turts, l)ut ditfer from 

 them petrographically in havuig a predominance of triclinic instead of mouo- 

 clinic feldspars. 



Rhyoiite.-The essoiitial components of this natm'al group are restricted 

 to orthoclase and quartz. I'sually they carry more or less triclinic feld- 

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

 rarelv develoj)ed in as large individuals as the orthoclase. Biotite as an 

 accessor\' mineral niay be present in varying amounts, but is (][uite as likely 

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

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

 texture no rock sm-passes 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 holocrystalHne groundmass, Avith 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 dihexaliedral crystals and 

 dark gray and black angular grains Avhich stand out in sti'ong contrast to 

 the prevailing light tints of the inclosing groundmass. 



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

 the many extrusions of rhyolite occur two principal varieties which cover 

 large areas and emljrace the greater part of the outbursts, and for the jjur- 

 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 thev are closely allied. The Rescue Canyon 

 rhyolite when fresh has a decidedl} red color due to a consideral)le amount 



