350 THE VERMILION IRON-BEARING DISTRICT. 



shown on tlie outcrops seems to point conclusively to the fact that these 

 changes are due to an increasing metamorj)hism of these sediments, corre- 

 sponding to our approach to the main Giants Range granite mass lying far 

 to the south. In this western portion of the area the center of intrusion 

 lies in the Griants range, a number of miles south of the area described in 

 this monograph. 



The metamorphism of these Knife Lake slates can be seen to advantage 

 in the vicinity of Snowbank Lake. On the north shore of Snowbank Lake, 

 at a number of places, mica-schists may be found which are cut through and 

 through by dikes derived from the Snowbank granite. These schists, when 

 followed inland for a considerable distance, are found to grade into the less 

 and less altered sediments, until eventually the normal Knife Lake sedi- 

 ments are reached. Somewhat similar metamorphosed mica-schists can be 

 observed upon the portage between Round Lake and Disappointment Lake. 

 These have been metamorphosed by the gabbro which lies only a short 

 distance to the south, although in all jDrobability they were first affected by 

 the intrusion of the Snowbank granite. Here the cleavage is parallel to 

 the bedding. This mica-schist resembles in a remarkable degree the mica- 

 schist " which occurs as the upper part of the Upper Slate division of the 

 sediments at English Lake, near Penokee Gap. The most crystalline part 

 of this Upper Slate member runs from Penokee Gap westward, and here the 

 basal member of the Keweenawan, lying north of it, is a great mass of 

 gabbro. The lower members of the Upper Slate at Penokee Gap, although 

 at 'lower horizons, and therefore presiimably more deeply buried, and 

 moreover containing unquestionable intrusions of the diabase, are never- 

 theless composed of comparatively little metamorphosed black slate. It 

 seems conclusive that these mica-schists in the vicinity of Snowbank Lake 

 and those occurring in Penokee Gap both owe their present characters to 

 the alteration of an original slate by the gabbro. 



At a point only a few miles west of Montreal River, and again at the 

 top of the Upper Slate member, near the gabbro, the rock is a gray, coarse, 

 strongly micaceous graywacke, the only recognizable clastic material in the 

 rock being the coarse quartz and feldspar. In Michigan, on the Penokee- 

 Gogebic range, east of Montreal River, the bottom members of the 

 Keweenawan are comparatively thin-bedded lava flows, dolerites, and 



fflMon. U. S. Geol. Survey Vol. XIX, 1892, pp. 302-308. 



