CLIMATE AND PHYSICAL CONDITIONS OF KEEWATIN g 



being removed. They are often seen resting on Laurentian gneiss, 

 but the latter was not the source of the sand of which they were 

 formed, since the Laurentian is everywhere in eruptive relation- 

 ships with the Couchiching and hence is of later age. The gneiss 

 penetrates the overlying schist and has often broken off slices 

 which have been floated away by the molten flood. 



As mapped by Lawson, Couchiching schists are widely distrib- 

 uted on Rainy Lake, which must be looked on as the type locality. 

 In my field work many outcrops of these rocks have been studied 

 near Rice Bay, Grassy Portage, Gash Point, Goose Island, Sand 

 Point Island, and at other places on the way eastward toward 

 Bear's Passage; and I can confirm Lawson's description of them. 



Near Grassy Portage and Nickel Lake they include iron range 

 rocks of a somewhat unusual variety, in which pyrite and pyr- 

 rhotite largely replace iron oxides; and some. miles to the west 

 on Rainy River, below Fort Frances, they are found with sand- 

 stone-like silica almost free from iron. 



In general, however, the Couchiching schists occur in large 

 areas by themselves, always dipping at high angles (6o°to 8o°), 

 often having widths across the strike of hundreds of yards, some- 

 times of a mile or more. They may have various relations to the 

 green Keewatin schists, sometimes underlying them and at others 

 appearing to be interbedded with them. Near Shoal Lake there 

 are, however, schists resembling the Couchiching which lie above 

 the basal Huronian conglomerate and are evidently of much later 

 age. 



Lawson maps the Couchiching as extending from west to east 

 across almost the whole Rainy Lake sheet, a distance of more than 

 60 miles; and the Hunter's Island and Seine River sheets, to the 

 southeast and east respectively, contain large areas also, as mapped 

 by Lawson, W. H. Smith, and Mclnnes. The whole length shown 

 is about 90 miles, and the breadth 24 miles. 



Lawson estimates the thickness of the Couchiching at about 

 25,000 feet, but in such ancient rocks, now folded in mountain 

 structures, it is possible and perhaps probable that this thickness 

 is excessive. The real thickness may be repeated many times by 

 folding, but it can hardly be less than some thousands of feet. 



