328 THE VERMILION IRON-BEARING DISTRICT. 



either of distinct outcrops or of marked topographic depressions indicating 

 their continuation. The lake here Hes in a synchne in the sediments, and 

 on the opposite shore of the lake — that is, upon the northwest shore — there 

 ai'e carbonate-bearing rocks overlying conglomerates which seem to repre- 

 sent the northwest limb of this syncline and to be a repetition of the car- 

 bonates on the southeastern shore. In the rocks on this northwest shore the 

 carbonate-bearing character is not nearly so marked as in those on the 

 southeast limb of the syncline. The high cliffs of the carbonate on the 

 southeastern shore have been noted by previous observers, and have been 

 referred to by Winchell as a limestone. This rock is pure enough at some 

 of the exposures on the southeast shore of Ogishke Muncie Lake to be called 

 a limestone. Microscopic examination shows that it consists chiefly of a 

 carbonate with a small amount of fine-grained cherty silica and some ciabes 

 of iron pyrites. The carbonate is decidedly ferruginous, as is shown by 

 the marked vellow, ocherous weathered crust. It passes down into a 

 carbonate-bearing slate and then into the Ogishke conglomerate. In the 

 other direction, upward, it passes into the Knife Lake slates. 



This carbonate-beai'ing horizon is assumed to be the representative of 

 the cherty iron carbonate rocks from which, it is presumed, the iron-bearing 

 rocks at the same horizon at othei' places were derived. 



The second facies of the iron-bearing formation of the Lower Huro- 

 nian of the Vermilion district is better suited to bear this name than tlie 

 carbonate-bearing rocks just described, which contain but little iron. This 

 second facies consists of chert, jasper, iron oxide, and slate interbanded. 

 The iron oxide is chiefly jnagnetite with very little hematite. The chert 

 varies from white to gray and even darker when it has more magnetite mixed 

 with it, becomes red when it contains hematite, and thus passes over into 

 the brilliant-red jasper. This jasper is relatively rare in this iron formation. 

 The slates are the normal gray to bluish and greenish slate carrying more 

 or less ferruginous carbonate, like the above-mentioned cherts. The 

 slates are very much changed, and their clastic characters are not recog"- 

 nizable. They are now very fine-grained, fissile, well-banded slates, con- 

 sisting of small chlorite flakes, grains of quartz, some ferruginous calcite in 

 rhombohedra, and crystals of magnetite. These rocks show nothing of 

 especial interest, being essentially the same as similar rocks forming the 

 iron-bearing Soudan formation of the Archean and hence are not described 

 here in detail. 



