PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 319 



Flint. Slate.] 



Meg. A dark gray* rather coarse-grained slate or schist, containing cubes of 

 pyrite. It is rather soft and not decidedly siliceous. 



Mic. The section shows a coarse band lying between two finer ones. The 

 coarse band has numerous rough rhombs of siderite, a few angular quartz grains and 

 a little pyrite in a finely fibrous groundmass. This groundmass is made up largely 

 of minute greenish fibres or flakes, which have very little influence on polarized 

 light, much of the section remaining practically dark between crossed nicols; these 

 greenish flakes are regarded as chlorite. There are a few brightly polarizing flakes, 

 probably muscovite, and a few minute quartz grains. The finer bands lack the siderite 

 and angular quartzes, and are practically similar, although somewhat finer grained, 

 to the groundmass of the coarser band. Throughout the section is much black dust- 

 like material, and also specks, which are opaque, black in transmitted light and gray 

 in reflected light. 



One section. 



Age. Archean (Keewatin). u. s. G. 



No. 327. FLINT. 



Knife lake end of the portage between Knife lake and Maple Leaf lake; in Ontario. 



Ref. Annual Report, ix, page 86. Compare rock No. 1429; also No. 973 (A. W.), Annual Report, xvi, 

 page 210. 



Meg. Flint, nearly black, but weathering light, with conchoidal fracture and 

 sharp edges which gave name to Knife lake, about whose shores it is common. It is 

 only local or in beds, or sometimes in ridges. 



Mic. The rock is a very fine-grained clastic, of the nature of Nos. 322 and 324. 

 With a high power, numerous angular fragments of quartz can be seen lying in a 

 matrix, which, between crossed nicols, is rather dark, and which consists, probably, 

 of much decayed feldspathic debris, in which, however, are sprinkled a few scales 

 of muscovite. 



One section. 



Age. Archean (Lower Keewatin). N. H. w. 



No. 328. SLATE. 



Ontario. Maple Leaf lake end of the portage between Maple Leaf and Carp '(Pseudomesser) lakes. 

 Ref. Annual Report, ix, pages 86, 87. 



Meg. From the roofing slate series of Knife lake. The sample shows alternations 

 of fine and coarser-grained rock. 



Mic. The section, evidently made from the coarser part of the specimen, 

 embraces conspicuous angular quartzes and a few striated feldspars, lying in a matrix 

 similar to the matrix of several already mentioned (Nos. 324, 325, 326) which is 

 semi-isotropic between crossed nicols, the exact nature of which it is difficult to decide 

 but which may have been derived from a devitrification of volcanic glass. It is 



