478 GEOLOGY OF THE BLACK HILLS. 



dichroitic that they change from brown to green and red with the analyzer 

 alone. They are also abundant in the rock. The brown mica occurs in 

 large masses, irregular in shape and sometimes striated very distinctly. It 

 polarizes only from brown to very dark-brown, being also dichroitic. Some 

 long, transparent crystals of white mica, similar to those in [1], are not at 

 all dichroitic, and show very little change of color even with crossed nicols. 

 There are also a few crystals of orthoclase present, quite large and toler- 

 ably well defined in form. A large, banded crystal may be cyanite. This 

 rock is a mica-schist, but approaching gneiss in character. 



The rock [24] from the Stockade, French Creek, has a general greenish 

 color, caused by large crystals of hornblende imbedded in a whitish felds- 

 pathic mass. Under the microscope, large, green, strongly dichroitic 

 crystals of hornblende are at once remarked. The lines of cleavage are 

 very distinct, and the sections showing these lines give the most marked 

 change of color with the analyzer alone from yellowish-brown to dark- 

 green. Quartz is also quite abundantly present in grains, having cavities 

 and microlites. Orthoclase makes up with the quartz the groundmass of the 

 rock. It is partially decomposed, so that it is somewhat cloudy and opaque, 

 but it occasionally shows colors in polarized light and also cleavage lines. 

 There is a little magnetite present. This rock is a hornblende-gneiss, 

 although it scarcely shows any lines or marks of the stratification peculiar 

 to gneiss, owing to its very crystalline structure. 



The mica-schist [27] from Amphibious Creek is gray in color, with red 

 spots in it of oxide of iron, and has a crumbling, weathered appearance. 

 In the section, the rock appears to be made up of quartz, transparent, crys- 

 talline and in large masses, and mica. The former contains unusually 

 large and fine cavities, microlites, and also some short, stout, dichroitic 

 hornblende crystals. The mica is quite dark and nearly opaque, but still 

 changes to a darker brown when observed with the analyzer alone. It is 

 scattered through the rock in large, irregular masses. A small piece of a 

 granular and polarizing mineral noted is no doubt a fragment of orthoclase, 

 not being nearly as transparent as the quartz. 



The rock from French Creek [40] has a crystalline luster, but a 

 very slight schistose character. It is very dark-green, the macroscopical 



