PETROGKWIMIV — IMIVOLITKiS AND TRACUVTRS. 485 



with a gradual passage between one another." Occasionally, the rock sjjlits 

 easily into flat slate-like plates. These a))pearances were observed in the 

 Black Hills rocks as [127], [13;")], &c. This groundmass has again a. 

 resemblance to burnt clay, with broad, reddish bands in a light-yellowish 

 mass of a porous and rough texture. The groundmass has also a lialf- 

 glassy or glassy structure, with the character of obsidian, pearl ite, &c , 

 with but few imbedded crystals. This form of rhyolite is called half-glassy 

 or h3-aline-rhyolite by Zirkel, and includes obsidian, pumice-stone, pearlite, 

 &c. which belong geologically to the rhyolites. 



A determination of the percentage of silica is necessary, when the 

 rhyolite is of a very compact, felsitic texture with but few crystallized 

 minerals, such as quartz and sanidin, to explain its nature. In such a case, 

 the amount of silica, the microscopic behavior of the thin section, and the 

 geological association must all be called into requisition. 



A rough, decomposed, porous structure with numerous holes and 

 cavities is frequently the character of the groundmass. The most usual, 

 however, is a fin^i-grained or micro- crystalline texture, generally of a light- 

 gray color, appearing almost compact; a close examination being required 

 to see the minute crystalline points. The microscope shows the mass to 

 consist of small sanidin crystals, quite rough or ragged in form, and closely 

 interlaced. Of this kind are the Black Hills rhyolites [135], [147] and 

 [152]. The imbedded crystals are not very abundant, sometimes entirely 

 wanting, and are sanidin and occasionally biotite. 



Spherulites, often present in the felsitic groundmass, are small, round, 

 fibrous masses of a distinctly radiated structure. They vary greatly in 

 size, from very minute to quite large, and are not very sharply separated 

 from the groundmass. When the fibers are arranged on a line instead of 

 around a center, the structure thus produced is very peculiar and interest- 

 ing ; it is called axiolite by Zirkel, and is stated to belong chiefly to 

 rhyolites. 



As rhyolite is the acid variety of trachyte, owing to the presence of 

 quartz, the silica percentage of the groundmass is always much higher 

 than that of sanidin-trachyte. In the Black Hills trachytes it varies from 

 67 to nearly 80 per cent., while Zirkel gives the analysis of a rhyolite 



