472 GEOLOGY OF THE BLACK HILLS. 



crystalline and have rather a splintery fracture. There is some mica pres- 

 ent in equally distributed crystals, but not at all grouped in layers or rows. 



The quartzite from French Creek [208] possesses the rounded grains, 

 but without cementing material, while in those from Amphibious Creek 

 [205], Terry Peak [206], Red Canon Creek [207], Box Elder Creek 

 [200] and [201 ], the rounded grains are cemented together by some material 

 which has the appearance of having once been in solution. It is sometimes 

 silica, and at other times of an argillaceous nature. In the latter case the 

 rocks are sandstones rather than quartzites 



The secondary minerals often occurring in quartzite are, besides the 

 mica already mentioned, principally orthoclase, hornblende, magnetite, 

 cyanite and garnet. With the exception of cyanite, all were observed to be 

 present in these rocks. 



The quartzite from the Stockade, French Creek [17] is a grayish, fine- 

 grained rock of quite dark color, containing numerous small, dark, long- 

 shaped crystals having a high luster. An examination of the thin section 

 under the microscope shows that it is composed of crystalline quartz in 

 small grains full of cavities, black particles of magnetite and microlites, 

 the latter probably hornblende. Scattered through the mass are also 

 larger, irregular, broken crystals of hornblende, strongly dichroitic, and in 

 some cases still showing the characteristic cleavage lines of the angle of 

 124°. With crossed nicols they are of a fine dark-green color, changing, 

 upon turning the polarizer, to a lighter yellowish-green Some rather 

 opaque, non-polarizing masses are probably decomposed feldspar. The 

 magnet readily attracts particles of magnetite from the pulverized rock. 



The quartzite from Hall's Cut-off [38] is quite different from the above. 

 It is of a lighter color and contains mica in small spangles, abundantly 

 scattered through the mass. The microscope shows the quartz to be very 

 clear, containing some cavities, highly crystalline and giving beautiful 

 colors with the polariscope. The mica seems to be somewhat decomposed, 

 although it occasionally shows the laminations peculiar to its structure. 

 Large, isotropic crystals of garnet are present and are easily recognized. 

 There are also black, opaque masses of magnetite, submetallic in reflected 

 light. 



