202 C. K. LEITH 



distance of 200 miles, with a width north of Rainy Lake of 120 miles. 

 The Huronian, including the Keewatin and Couchiching rocks, is in 

 general an immense series of water-worn sediments, in the upper part 

 mixed with eruptives, perhaps largely later injections, but partly pyro- 

 clastic. The Keewatin is largely of eruptive origin, though it contains 

 important sedimentary members ; the Couchiching is entirely sedi- 

 mentary. 



The Keewatin, and in the southern part of the region the under- 

 lying Couchiching, form sharp synclines, curving as wide meshes 

 around the areas of Laurentian, which vary from less than a mile to 

 fifty miles in diameter. 



Diabase and porphyry eruptives form an important part of the 

 Keewatin. These are in large part surface flows, represented by ash 

 rocks, agglomerates, etc., but many of them are probably laccolitic sills. 

 The water-formed elastics of the Keewatin, include limestones, slates, 

 quartzites, grits, graywackes, breccias, and pebble and bowlder con- 

 glomerates. The limestones are of limited extent, being found in any 

 thickness only at Steep Rock Lake. The slate on analysis yields 7.44 

 per cent, of carbon, pointing perhaps to the presence of life. The 

 conglomerates are in places schistose. Near Shoal Lake the most 

 common pebbles are quartz-porphyry and porphyrite, felsite, and 

 green schists indistinguishable from the adjoining Keewatin schists; 

 black and red quartzite, white pulverulent sandstone, vein quartz, and 

 anorthosite. No gneiss or granite pebbles have been found. Most of 

 these pebbles are easily matched by Keewatin rocks, sometimes, how- 

 ever, many miles distant ; a few are evidently Couchiching : and none 

 are Laurentian. 9 



The break represented by this conglomerate comes high up in the 

 Keewatin, instead of at its base, just above the Couchiching, as held 

 by Lawson. Striking evidence that the break is not at the base of the 

 Keewatin is found at Shoal Lake, where a few bowlders of the coarse- 

 grained anorthosite found in the schist-conglomerate are exactly like 

 portions of a boss of anorthosite two miles away. As this anorthosite 

 area contains masses and strips of characteristic Keewatin schist, swept 

 off during its eruption, it is evident that an immense lapse of time 

 separates the conglomerate and the underlying Keewatin. It is 

 probable that the conglomerates represent an important interval of 

 erosion, perhaps equivalent to the one shown by Van Hise and others 

 between the Upper and Lower Huronian in the states to the south. 



