PETKOGEAPHY— MICA-SCHISTS AKD SLATES.. 477 



The true clay-slate consists also of quartz and mica, but never crystalline, 

 being so homogeneous and amorphous in structure that Zirkel defines it as 

 made up of "microscopic mica scales and dust-like quartz grains." This 

 distinction is quite apparent under the microscope, when none can be made 

 between the two hand specimens. Note the resemblance between [64], 

 [65] and [74]; while the two former are quite micaceous, the latter [74] is 

 nearly a true clay-slate, showing scarcely a trace of crystalline structure. 

 By the dwindling in size, then, of its constituent minerals, mica-schist 

 graduates into mica-slate and argillaceous mica-slate as it comes nearer to 

 clay-slate in character. 



I would also note that the variation in these rocks from French Creek 

 and Rapid Creek may be due to metamorphic influences of different degrees. 



The mica-schist from the Stockade, French Creek [1], has a gray 

 color, inclining to greenish, and is evidently very rich in mica. Under 

 the microscope, quartz in abundance was observed containing numerous 

 cavities, some quite large, and also microlites. Occasionally, there are 

 large, pellucid masses of this crystalline quartz. The mica is of two 

 kinds, one greenish-brown and dichroitic, the other being in the form 

 of long white crystals, sharply defined and clear, but not possessing any 

 dichroism. These crystals generally point in the same direction, and are 

 sometimes bent and curved around a quartz grain and also imbedded in 

 the leaves of brown mica. They show longitudinal lines of lamination, 

 polarize brightly, giving red and green colors, and are undoubtedly white, 

 transparent muscovite. The brown, dichroitic biotite is quite irregular in 

 shape, and in large plates and leaves, and changes in polarized light only 

 to a darker greenish-brown. An abundant black mineral, sometimes having 

 a hexagonal section, although of minute size, and a submetallic luster, is 

 considered to be in part magnetite and in part small crystals of mica. The 

 structure of the rock is crystalline. 



Mica-schist from the same locality [13] is quite different from the pre- 

 ceding, it being much coarser in structure and colored reddish by oxide of 

 iron. In the section, the quartz is seen to be predominant and in large 

 clear masses, containing cavities and small, well defined, dichroitic horn- 

 blende crystals. The latter are sometimes of large size and so strongly 



