482 GEOLOGY OF THE BLACK HILLS. 



and quite soft. In the section under the microscope, there appears to be 

 principally quartz, with irregularly shaped particles of a brown, partially 

 altered mica arranged in rows as if stratified. This mica is not generally 

 dichroitic, but an occasional long prismatic section is quite strongly so. 

 Between the quartz grains microlitic, needle-like crystals, pointing in one 

 direction, probably occasion the schistose character of the rock, and are mica 

 crystals. It is a mica-schist of similar character to the former ones [69]. 



No. [86], from the same locality as the preceding, is much lighter in 

 color, being brownish- white and quite hard. On the cross fracture it shows 

 very plainly a schistose structure. An examination of the section under the 

 microscope showed considerable white, but rather opaque, mica, and par- 

 tially decomposed orthoclase in a fibrous groundmass, strongly recalling a 

 similar appearance of the mica-slates previously described. There is also 

 a little quartz in small transparent grains, but subordinate in quantity. 

 Some plagioclase was also noted; it is weathered, but still shows the colored 

 banding of the twinned crystals quite plainly. The orthoclase is rather 

 muddy, giving, however, considerable color in the fresher portions, where 

 traces of the cleavage were also quite well marked. These minerals are 

 confused and ragged in outline, so as to render the examination somewhat 

 unsatisfactory. The fibrous groundmass and preponderance of the ortho- 

 clase over the quartz makes the rock a feldspathic mica-slate, or a clay- 

 slate partially metamorphosed and intermediate between that rock and 

 gneiss. 



The dark-green rock [97] from Box Elder Creek has a slightly 

 greas}' feel, and is Aveathered on the surface to a light-yellowish crust. 

 Under the microscope, it shows in polarized light with crossed nicols a dark 

 ground, with evenly distributed white spots of irregular shape, and occa- 

 sionally some yellowish crystals almost entirely altered to an opaque, black 

 mineral or magnetite. This mineral is also present in great abundance, 

 sometimes having an indistinct crystalline form. The section in ordinary 

 transmitted light has a pale-green color. The whitish particles have a 

 needle-like, crystalline form, with a fibrous structure, while the yellowish 

 crystals, being much broken, resemble olivine in process of alteration into 

 magnetite and opaque, black serpentine-mass. The fibrous, feathery and 



