508 



GEOLOGY OF THE BLACK HILLS. 



Ksmidiii, but lutt (jiiito so .ibundant. The biotite is very dark and much 

 decomposed, but still shows the characteristic, deep furrows and a strong 

 dicliroisin. The sections are very abundant and quite large. Magnetite is 

 thickly scattered through the groundmass in small grains and sometimes 

 in large masses, while there are also some very opaque and thick films of 

 oxide of iron of a dark reddish-brown color, in which the large magnetite 

 grains are imbedded 



The groundmass is made up of small irregularly shaped feldspar 

 crystals, of very indistinct outlines, and but feebly polarizing, mixed with 

 minute grains of magnetite and brown biotite. It has the peculiar tra- 

 ch}tic appearance, which cannot easily be mistaken, and a partly fibrous- 

 microlitic structure. This rock borders closely upon the rhyolites, but its 

 nuicroscopical rough, ragged and porphyritic appearance and structure 

 differ so widely from the fine-grained and uniform light-gray characteristics 

 of the former as to place it among the trachytes proper. The silica amounts 

 to 6F}AH per cent, and 10.09 per cent, is soluble in hydrochloric acid. 



The rhyolite [147] from the north peak of Inyan Kara is a compact, 

 fine-grained and very light-gray rock, with only an occasional white sanidin 

 crystal and black biotite needle visible. Under the microscope, the section 

 appears to consist of a great quantity of small, bright sanidin crystals in a 

 groundmass, with biotite and magnetite. The silica present is 73.18 per 

 cent. 



The similarity of the small sanidins to large ones is very easily seen 

 ill their clearly defined outlines and numerous twinned crystals. They 

 have in(^lusions of microlites. There is one group of large and very trans- 

 parent cr3'stals in the slide, which polarize in very beautiful and brilliant 

 colors showing some banding, all of which, however, is not that of plagio- 

 clase. The bands of color, except in one case, are not exactly parallel and 

 resemble more the varying colors of sanidin. Biotite is in brownish masses 

 and leaves of small size, and also in very minute black needles, which are 

 partly altered to magnetite, a few being still dichroitic. Magnetite grains, 

 as in almost every one of these rocks, are plentiful. Some quartz in rounded 

 grains and polarizing in bright colors is present. 



