506 



GEOLOGY OF THE BLACK HILLS. 



sluirply defined terminations and angles. They are also dicbroitic and scat- 

 tered ])lontifiill}' through the rock. 



As in [171] there seems to be some transparent glass mass, \yhich is 

 perfectly isotrope and of a very different texture from the sanidin. The 

 yellowish spots in the hand specimen are seen in the section to be of very 

 irregular shape, but ti-ansparent. An occasional yellowish, opaque and 

 hexagonal crystal may be decomposed mica or feldspar, but its exact nature 

 could not be determined. Some magnetite in large masses is present, but 

 sparingly. The groundmass, as mentioned before, is entirely feldspathic, 

 clear and cloudy in irregular masses, and showing, perhaps, traces of a 

 slight fluid-like structure. The percentage of silica is 58.53, and the 

 amount dissolved in hydrochloric acid 18.59 per cent. 



The rock [141, 142] from Crow Peak is a rhyolite containing plagio- 

 clase, and is much more crystalline than the preceding, having macroscop- 

 ical sanidin crystals plainly imbedded in a groundmass. They are white 

 and very transparent. There are also some black crystals of hornblende 

 and empty cavities, which were formerly filled with the latter mineral. In 

 the section, the microscope shows the rock to consist of large, clear crystals 

 of sanidin and plagioclase in a crystalline groundmass, also containing 

 broken biotite crystals and some quartz in grains and crystals. 



The sanidin in this rock is ver}^ beautiful, being unusually large and 

 transparent. The crystals are sharp in ' outline and sometimes terminated 

 on both ends, showing numerous faintly marked zones parallel to the edges. 

 Many of them are twinned, the different colors of the two parts being very 

 conspicuous. Numerous microlites are included, some of them tolerably 

 large, colunniar in form and quite long. Others again are in short, stout 

 crystals, distinctly showing prismatic and terminal faces. The former may 

 be apatite and the latter hornblende. Magnetite and small biotite folia are 

 also among the inclusions of the sanidin. The colors in polarized light are 

 very brilliant, but are in irregular masses or blotches in the same crystal ; 

 near one edge a bright red predominating, then yellow, turning gradually 

 to a greenish-yellow, with spots of red and blue. All these colors are 

 interlaced, often penetrating from the edges in lines and points one into 

 the other. 



