996 THE GEOLOGY OF MINNESOTA. 



[Additional foot*. 



the rocks Nos. 1322 and 1319, mentioned last above. In both cases there has been 

 an intergranular later introduction of interlocking quartz, cementing the rock into 

 a dense taconyte or taconitic quartzyte. 



7. In several instances this older rhyolitic rock has been seen in the vicinity of 

 Gunflint lake. It is identical with that already mentioned at Prairie River falls (No. 

 1529), and is illustrated by No. 435H, and by several of those of No. 720W. This 

 always lies near or directly upon the Archean. It is a jaspilyte, identical in all respects 

 with the normal jaspilyte of the Vermilion Iron range, but varying to felsyte, and 

 finally to less siliceous slate or flint, and to a dark rock which has not been carefully 

 studied. This rock is never taconitic (globular), but streamed and striped like a 

 fine-grained rhyolyte, showing white and red bandings, which are accompanied by 

 thin sheetings of magnetite or hematite. It is this which has furnished the silicified 

 pebbles of the globular taconyte, and it is the original (glassy) condition of this which 

 furnished the non-silicified globules (or glauconite) of the granular taconyte. 



The most remarkable exhibition of this rock is to be seen at a point on the shore 

 of Black Fly bay, which is an appendage of Gunflint lake on the Canadian side, where 

 it can be seen lying immediately on the Archean granite, presenting beautifully 

 colored, sharp, fluidal contortions. A bluff which occurs a little further north, illus- 

 trated by rock No. 720W, consists largely of this rock in a brecciated condition, some- 

 what mingled with sedimentary debris of rock like itself, and in those places showing 

 the characteristic globular composition. This bluff is thus described by A. Winchell:* 



" S. E. ^ sec. 18, T. 65-3, as of Minnesota, just south of the cape. A remarkable display. In a rounded, 

 naked bluff, fifteen feet high, is seen the aspect of a conglomerate with many whitish constituents. Examination 

 shows it to be a portion of the slate formation [i. e., of the Animikie] contorted in a striking manner. The 

 laminations are still preserved and serve to evince the disturbance. There are some quartzose layers, and some 

 quartz veins. Much of the slate has assumed a flinty confititution, and some laminae are of red jasper. There 

 are patches of what I have called oolitic magnetite, and areas in which the spherules are sparsely scattered in a 

 somewhat homogeneous matrix of undetermined character. In some places the crystalline magnetite sparkles 

 brilliantly, and there are others in which it has been oxidized by water and burnings [of the forest] into a 

 crumbling ferruginous mass, like the waste of a hematite mine." 



The specimens collected, fourteen in number, show all these features. 



8. There is a curious iron-bearing rock (Nos. 1896 and 879G) north from the 

 line of strike of the recognized Animikie, near the late workings of the Gunflint 

 Lake Iron company, lying in or on the Keewatin greenstone. Whether this belongs 

 with the Keewatin or with the Animikie is immaterial to the point to be mentioned, 

 since it is plain that the ores of the two formations had an identical manner of 

 origin. This ore, while presenting some of the taconitic characters of the Mesabi 

 ore, yet embraces igneous minerals, such as pyroxene. As an Animikie rock it may 

 have been a lava which locally came into contact with volcanic glass sand (and 

 perhaps other kinds) which it incorporated into its mass. On silicification the 

 pyroxene was exempted from the usual change. As a Keewatin rock it may have 

 been a detritus of jaspilitic sand and greenstone debris which, on metamorphisna, 



" (Hxteenth Annual Report, p. 341. 



