484 GEOLOGY OF THE BLACK HILLS. 



the class to which they belong, as is shown by a chemical determination of 

 the silica and composition. 



Of the varieties of rocks mentioned above, there are represented in the 

 specimens from the Black Hills thus far examined rhyolite, sanidin-trachyte, 

 sanidin-oligoclase-trachyte, and phonolite; no andesite was observed. 



Rhyolite. — Rhyolite is quite different in general appearance from 

 sanidin-trachyte, in that it is much more compact and fine-grained or fel- 

 sitic in structure. It consists of a groundmass, in which are visible in most 

 cases crystals of sanidin and biotite or hornblende, with small grains of 

 quartz. Its most predominant colors are white, yellowish and greenish- 

 white, light-gray, pearl gray, greenish-gray, and sometimes pale-red. In 

 many cases the quartz can only be detected in the thin section with the aid 

 of the microscope. The groundmass varies greatly in structure and appear- 

 ance from a crystalline, or micro-granitic, and micro-felsitic to a felsitic, 

 spherulitic, half-glassy and glassy character * 



The first or coarsely crystalline structure is somewhat granitic in 

 appearance, so much so that the geological occurrence must be relied upon 

 to a great extent to distinguish the rhyolite from granite-porphyry or feld- 

 spar-porphyry. This form of the groundmass does not occur very fre- 

 quently, but there is an example of it in the rocks [101] and [103] from 

 Terry Peak, which, according to the occurrence given, pass gradually into 

 well defined rhyolite or trachytic rock 



A characteristic peculiarity of the compact groundmass of rhyolite is 

 a banded appearance, caused macroscopically by an alternation of two 

 colors, generally dark-gray and pinkish- violet, in parallel, narrow bands. 

 These colors, in a great number of shades, are often in layers as thin as mere 

 lines, so that the rock has somewhat the appearance of banded agate, the 

 most delicate markings being scarcely visible to the naked eye. From 

 examination under the microscope by Zirkel, these bandings and also the 

 wavy, fluidal appearance are produced "by a different amount of coloring 

 particles (needles and grains of ferrite and opacite) alternating in layers, 

 and by a band-like alternation of different varieties of structure, generally 



* For a minute and detailed classification of the various microscopical types of 1he groundmass 

 see Zirkel's Microscopical Petrography of the Survey of the Fortieth Parallel, p. 203. 



