272 



MINERAL CONSTITUENTS OF RHYOLITE. 



and on both, Zirkel expresses doubts as to the grains being originally 

 generated in rhyolite. Tridymite occurs in the ground mass in 

 uiinute crystals ; and it also occurs in well-marked aggregates of 

 hexagonal plates, but is only abundant when the proportion of quartz 

 is small ; and it is absent from the glassy varieties of the rock. 



Felspar in Rhyolite. The predominant felspar is sanidine, clear 

 as water, and sometimes in twin crystals of the Carlsbad type. 

 Occasionally lamellae of plagioclase occur in the sanidine. The felspar 

 is richer in gas cavities than the quartz. The glass inclusions may be 

 either clear or coloured. They are ovoid or many-sided, or ledge- 

 like forms, with angular indentations. Their microliths are isolated, 

 as are the plates of biotite. The felspar in the granitic rhyolite of 

 the Kotorua Lake in New Zealand is opaque. 



Plagioclase occurs in small, clear, long, isolated crystals. Their 

 principal cleavage-plane agrees with that of albite and oligoclase. 

 The amount of plagioclase, however, is less than might be expected 

 from the percentage of soda in the analyses, so that soda is probably 

 diffused in the ground mass. 



Mica, Hornblende, and other Constituents in Rhyolite. The 

 magnesian mica is of a brown tint, rarely green. Its films are often 

 bent by fluxion structure. It frequently occurs in hexagonal plates, 

 which are usually visible, but may be microscopic. 



Hornblende is sometimes associated with the mica, sometimes 

 separate. It always occurs in brown plates or needles. 



Augite is commonly seen in small grains or microscopic crystals, 

 but in some of the American rhyolites the ground mass is an almost 

 wholly crystalline aggregate of comparatively large grains of felspathic 

 quartz and augite. 



Contrary to an otherwise general rule, augite in these rocks is 



1 U. S. GeoL Surv., Fortieth ParaUel, vol. i. p. 652 ; Table XI. 



