373 



is made to include two classes of rock, friction breccias or 

 autoclastic rocks and volcanic breccias formed where a 

 dark lava has intruded the older lavas. The friction 

 breccia is the common intermediate stage in the develop- 

 ment of the sericite schists. These schists with the breccia 

 agglomerate are shown on the unnamed island west of 

 Queer island, also upon Slate island in the vicinity of a 

 Keweenawan dyke. 



A marked feature of these schists is the prevalence of 

 ferro-dolomite or ankerite, which in some cases forms vein- 

 like masses as much as 20 feet (6 m.) in width. As a rule 

 this material is not pure, but contains streaks of sericite 

 or chlorite and some quartz. Its weathered surface is 

 ochre yellow and of striking appearance. Good examples 

 of ferro-dolomite can be seen on a small island east of 

 Pipestone point and north of Square island, on the mainland 

 east of Square island, and on an island east of Whiskey 

 island. Another conspicuous band of this material is 

 shown on the west side of Middle island. 



Clay Slates, etc. — This subdivision consists principally 

 of highly altered hornblende and biotite schists which may 

 or may not contain garnet. In some instances true slate 

 has been found, but this is only in small quantity. In 

 regard to making a distinct division of the Keewatin to 

 include these rocks there is considerable diversity of 

 opinion. It may be said, however, that they have been 

 found principally in close proximity to Laurentian masses 

 or to the later (?) granites and there seems to be no objec- 

 tion to considering them merely as highly altered phases 

 of the ordinary Keewatin traps. In numerous instances 

 this rock contains large veins of pyrrhotite which have been 

 prospected for gold, but as a rule these deposits have been 

 found to be of no economic value. 



In certain places on West Hawk lake these highly altered 

 rocks (7, p. 202) seem to be of sedimentary origin, but so 

 far as partial analyses of rock from Lake of the Woods show 

 (7, p. 179), the rocks of this subdivision found there are 

 probably of igneous origin. 



This highly altered rock outcrops near Keewatin and 

 Norman. It underlies the town of Kenora and continues 

 thence in a northeasterly direction for about six miles 

 (9-6 km.). 



Agglomerate. — Under this title are grouped fragmental 

 rocks of extremely varied texture and origin. The more 

 35069— 7|b 



