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CLASSIFICATION OF RHYOLITES. 



crystals. The first of these is the rarest, and is usually known as 

 granitic rhyolite or nevadite. The second type is perhaps the most 

 typical, being common wherever rhy elites occur. It corresponds to 

 the old quartz porphyry and felsite. The third type comprises the 

 hyaline rhyolites, which include perlite, obsidian, and pumice, and 

 includes the old felsitic pitchstones. No region has yielded so 

 many varieties of rhyolite as that examined by the Fortieth Parallel 

 Survey in North America, where Zirkel finds every type of rhyolitic 

 structure hitherto recognised in other parts of the world, and groups 

 the typical rhyolites into fifteen varieties. 



Granitic Rhyolite. The granitic rhyolite, which is always rare, 

 is distinguished by being entirely crystalline. The ground mass 

 of this Nevadite consists of well-defined microscopic grains of quartz 

 and felspar, through which larger crystals are scattered. 



Zirkel's Classification of Khy elites. We briefly summarise 

 Zirkel's classification of the typical rhyolites. The commonest type 

 presents a microfelsitic structure, sometimes becoming more or less 

 granular, and usually characterised by imperfectly formed sphserolites. 

 It generally contains ferrite and opacite. 



A second variety consists chiefly of microfelsite, with some 

 polarising particles and single dark axiolites ; or the microfelsite may 

 be traversed by fibrous strings arranged axially, with a distinct middle 

 division ; or it may consist of a network of such strings, enclosing 

 radially fibrous and concentric spherolites in its meshes ; or the meshes 

 may include more or less distinct aggregations of a crystalline granular 



1 Justus Roth : " Beitrage Petrograph der Pluton." Gesteine, 1873, taf. xiii.-xiv. 



