PETROGRAPHY— MICA-SCHISTS AND SLATES. 481 



abundance, probably hematite as a resultant of decomposition, or it may 

 be only weathered mica. The fibrous structure mentioned before arises in 

 all likelihood from the interlacing of very small mica crystals This rock 

 is a mica-slate. 



The slate from Rapid Creek [74] is almost black in color, and of a 

 uniform, homogeneous structure, cleaving easily into long columnar frag- 

 ments. It resembles much more a true slate than any of the preceding, a 

 fact which the microscopical examination undoubtedly proves. The struc- 

 ture of the rock was observed to be very fine-grained indeed, apparently 

 consisting of minute crystals arranged in parallel rows, with an occasional 

 dark line between them There is but little quartz present, and that in very 

 small grains ; also in lines parallel to the stratification. The small crystals 

 are a little dichroitic, this being observed in some of larger size, which 

 might determine them to be mica. Magnetite is present, particularly in one 

 large mass surrounded by quartz. The homogeneous character of the rock 

 macroscopically and also as seen under the microscope, together with the 

 absence of much quartz, decides this rock to be a clay-slate, although it 

 is not quite as uniform as true roofing slate. 



The schist from Box Elder Creek [81] is of a dark-green color, with 

 veins of white quartz running through it, the mica being greatly in excess. 

 Under the microscope, the mica was observed to make up most of the rock 

 and to have a fine brownish-green color. It is but slightly dichroitic when 

 cut parallel to the lamination, but in the cross sections of prismatic shape 

 it is strongly so. The fine green color in polarized light at first indicated 

 hornblende, but there was no satisfactory evidence found of the character- 

 istic cleavage lines, which could hardly fail to be present in such a crystal- 

 line rock had the mineral been hornblende. It was determined, conse- 

 quently, to be magnesia-mica, of which the dark-green color and strong 

 dichroism are usual peculiarities. There is considerable quartz present, 

 quite crystalline, and having interspersed through it small, transparent, 

 green crystals of mica. This rock is a mica-schist, the mica predominat- 

 ing, but of a different character from any of the preceding, being perfectly 

 fresh and crystalline, without any traces of weathering or decomposition. 



The rock from Iron Ledge, Box Elder Creek [85], is lighter in color 

 31 B H 



