52 GEOLOGY OF THE BLACK TIILLS. 



iferous inicu schists, the fine material removed and deposited in the creek- 

 beds consists Uirgely of g-arnets and mica, tlie latter showing so brightly on 

 the creek bottoms beneath the water as often to be mistaken for flakes 

 of gold. In panning the gravel of upper French Creek and Castle Creek, 

 when the pan of dirt has been nearly washed, the residue is found to be 

 composed almost entirely of garnet crystals more or less rounded, beneath 

 whicli the j)articles of gold are collected in the heavier iron sand, while the 

 light mica is completely washed off. Farther down the streams the mica is 

 found still finer, while by the abrasive action of the current the garnets have 

 been worn round and smooth. 



The mica schist changes sometimes almost impercej)tibly, becoming 

 finer and containing less and less mica, and passes into a talcose or hydrous- 

 mica schist, frequently highly garnetiferous, presenting the usual soapy 

 feeling and general character of talcose schist The talcose schist being 

 more coherent than the very micaceous rocks is more prominent in the 

 topography, and forms harder and rougher ridges. 



By a similar gradation the mica schists pass into silicious schists con- 

 taining but little mica, and then by a more abrupt change into ledges or 

 strata of liard, dense, and tough quartzite. The quartzites, which are less 

 developed in the western than in the eastern series, are among the most 

 durable rocks and in the Hills generally stand in abrupt, dike-like 

 ridges, running with the stratification of the rocks. They vary from almost 

 white to a light or dark gr^ y color, and are bright and glassy on the 

 fracture, which is usually conchoidaJ. They commonl}' contain mica in 

 small quantities and sometimes are like ver}' silicious mica schists in which 

 the mica is subordinate and the quartz tough and glassy. They never 

 in their structure or relations indicate an intrusive origin, but are true 

 metamorphic strata, differing in character from the associated schists because 

 of an aboriginal difference in their composition — the latter having been 

 originally mixed deposits, while the former were more or less purely sili- 

 cious. They are similar in character to the quartzites of the eastern series. 



Sometimes the mica schist passes by almost imperceptible gradations 

 into true chloritic schist, in which the mica seems to have been replaced by 

 chlorite. This chloritic schist is usually soft and easily decomposable, of a 



