510 



GEOLOCxY OF THE BLACK RILLS. 



tills ; their brownisli color shows them to be probabl}' glass inclusions. 

 There are a few dark-green hornblende crystals rather prismatic in shape, but 

 also tabular and irregular ones, which are strongly dichroitic. Some of them 

 are altered into a dark reddish-l)rown substance, notably two quite large 

 hexagonal sections. The brown, resultant mineral does not polarize nor is 

 it at all dichroitic. There are also a few plagioclases present, and the single 

 grain of magnetite discovered has an unusually brilliant luster. In some of 

 the sanidin crystals and also filling up fissures is a granular mineral, which 

 polarizes faintly like sanidin, but with a rougher surface. Its exact nature 

 could not be determined, but it is probably zeolitic. The radiated, trans- 

 parent mineral already mentioned is quite remarkable and conspicuous, as 

 it gives bright blue, yellow, and red colors in polarized light. It is a zeo- 

 lite, presumably natrolite, and gives a phonolitic character to the rock, shown 

 by the very large percentage of the soluble portion. 



The groundmass is very fibrous in character, being composed of a 

 great quantity of long, capillary, yellowish-green needles closely interlaced 

 and grown together. They may be hornblendic. Hie silica present is 

 59.06, and 29.25 per cent, is soluble in hydrochloric acid, but no nephelite 

 could be found in the rock or thin section. 



The rock [169] occurring at the contact between the trachyte and 

 sandstone at the base of Little Missouri Buttes has a very porous, spongy 

 structure, full of large, rounded cavities,- so that it is much like pumice to 

 the touch. In the mass are seen quartz grains, white, decomposed crystals, 

 dark-colored spots, and some transparent sanidins. It has an earthy, dirty- 

 yellow color, but is hard and resonant. In the section, it was observed to 

 consist of large grains of quartz, sanidin crystals and a little plagioclase, 

 with fragments of another rock, a rhyolite, imbedded in a rather opaque 

 cementing mass of volcanic character. 



Many of the sanidin crystals are so much decomposed as to be nearly 

 opaque and whitish in color, while others are very clear and transparent, 

 having inclusions of microlites. Some of the smaller feldspars are banded 

 like plagioclase. The quartz grains are rounded, polarizing, of course, in 

 brilliant colors, and are easily recognized. They contain long, hair-like 

 microlites, besides an abundance of cavities and some prismatic-shaped 



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