336 THE VERMILION IRON-BEARING DISTRICT. 



PETROGKAPHIC CHARACTERS. 



Macroscopic characters. — In mapping the Knife Lake slates it has been 

 found desirable to include with them numbers of beds of grit and fine- 

 grained conglomerates which belong structurally with them. These, how- 

 ever, are so unimportant relative to the great mass of the slates that they 

 will not be considered in the further description unless they possess especial 

 characters that warrant reference to them. The slates vary from those 

 which are exceedingly fine grained and aphanitic to grits. As a rule, the 

 exceedingly fine-grained forms predominate. They are for the most part not 

 earthy clay slates, but are flinty, break with a ringing sound, have con- 

 choidal fracture, and form fragments with sharp, cutting edges. The normal 

 clay slates are in decidedly smaller quantity than the above-mentioned 

 flinty forms. ' The color of the slates is in general rather dark on fresh frac- 

 tm-e, varying from dark gray and olive green to bluish black. Associated 

 with these dark slates are light-grayish and greenish-colored slates and 

 graywackes. Occasional bands of white to gray and purplish black cherts 

 occur with the slates. On weathered surfaces the normal slates have a light 

 gray to light-brownish color. The flinty slates, however, weather with an 

 almost snow-white crust, showing macroscopically by this weathering that 

 they consist to a very considerable extent of silica in an exceedinglj- fine 

 state of division. This is in strong contrast to their invariable dark bluish- 

 black color on fresh fracture. 



In general, there is a difference in the slates in different portions of the 

 district, due primarily to the character of the rocks from which they are 

 derived. As a rule, where the Knife Lake slates and graywackes lie next to 

 the Archean greenstones, without large masses of Ogishke conglomerate 

 between them, they are green, and grade into the normal gray and blue 

 Knife Lake slates, made up in large proportion of granitic ddbris only at 

 considerable distance from the greenstones. Slates of this greenish color 

 are especially noticeable on Birch, Cai-p, and Ensign lakes, and in the area 

 southwest of Emerald Lake. From Moose Lake east to Ensign Lake the 

 slates range from the greenish ones, looking much like extremely fissile 

 green schists, to sericitic, gray, fissile slates, which predominate. Northeast 

 of Ensign Lake the slates again grade into the green ones, and in Bass 

 Lake the normal Knife Lake slates are at some places very flinty, and 



