488 GEOLOGY OF THE BLACK HILLS. 



Biotite occurs in many of the rhyolites and is characterized by its 

 brown color and distinctly marked lines of lamination. It is strongly 

 dichroitic when the sections are rectangular, or cut about parallel to the 

 prismatic faces, and not at all so when the sections are hexagonal in outline, 

 or parallel to the basal pinacoid. Biotite crystals often have a border of 

 magnetite grains, which is quite characteristic, as in the section figured in 

 Plate I, Fig. 2. 



Magnetite, in small black grains generally quite uniformly distributed 

 through the groundmass, with an occasional larger mass, is of frequent 

 occurrence and is recognized by its submetallic luster in reflected light. 



The groundmass, even when macroscopically compact and of uniform 

 texture, in the thin section under the microscope, is seen to consist of small 

 feldspar microlites or crystals, interwoven and interlaced so as to form a 

 solid mass. In some cases, these crystals are of larger size, but they 

 always have ragged or forked terminations. This formation of the ground- 

 mass can be best seen with crossed nicols, when a portion of the small 

 crystals are light, the others being dark. By then turning the slide on its 

 center the dark crystals become light and those previously light, dark, thus 

 proving that the entire groundmass is an aggregation of microlitic feldspar 

 crystals. In a few cases, a more uniform or glassy groundmass and on the 

 other hand a more crystalline appearance were observed and are mentioned 

 in the detailed description of the thin sections. The majority of the Black 

 Hills rhyolites have a crypto-crystalline or felsitic groundmass, uniform in 

 texture and often colorless; but no truly glassy groundmass occurs, the 

 crystalline prevailing. 



Trachyte. — The trachytes proper are distinguished from the rhyolites 

 and quartz-trachytes by the absence of quartz, having consequently a much 

 smaller percentage of silica, and by the preponderance of crystallized 

 sanidin, either alone or with oligoclase. 



These minerals are imbedded in a rough, porous groundmass, which is 

 more or less crystalline. It generally has a dull luster and light, white, or 

 light-gray colors, though dark varieties sometimes occur. The variation in 

 the color of trachyte is due to the groundmass, the petrographic compo- 

 sition being always identical. Hornblende, magnetite, and biotite are the 



