﻿Iron Ores of the Lake Superior region. 263 



clear chert and chalcedony, or to jasper — which is simply a 

 finely crystalline silica stained with red oxide of iron — or to 

 the magnetitic slates. In the latter case there is supposed to 

 have been a reaction between the carbonate and the silica as a 

 result of which the iron of the decomposed carbonate, having 

 been partially oxidized, has separated out as magnetite, while 

 the magnesia, demonstrated by analysis to be present in the 

 carbonate, entered into union with the silica to form tremolite 

 and actinolite. The excess of silica has separated out as the 

 crystalline to chalcedonic material which everywhere pervades 

 these magnetitic slates. A very fine magnetite dust is found 

 present as an inclusion in the unaltered carbonates, and along 

 with the organic matter which stains certain bands of the car- 

 bonate, but the bulk of the magnetite is certainly secondary. 

 Lest the original character of the carbonate involved should be 

 questioned, I may say that all appearances in the thin section 

 and in the field prove it to be the oldest mineral present. 

 Thin crossing veins of a secondary siderite and dolomite are to 

 be found, but the stratiform carbonate is manifestly original. 



The iron-bearing beds of the Mesabi Kange above alluded to 

 show essentially the same association of materials. Magnetitic 

 and actinolitic cherty substances have there again a very con- 

 siderable development, and have even attracted a good deal of 

 attention from the iron miner, although as yet without satisfac- 

 tory result. 



It should be said that, while these iron-bearing beds of the 

 Animike series have in general the flat-lying position above 

 indicated, there are, nevertheless, points at which they show 

 sudden and somewhat violent disturbances, suggestive of move- 

 ment along the contact line between this formation and the 

 granite to the north of it, and there seems to be a connection 

 between the amount of alteration and the presence of these dis- 

 turbances, the greatest alteration having been noted where the 

 disturbances occur. At the same time I am not as yet pre- 

 pared to say that there is a necessary connection between these 

 facts. 



As already indicated, the strata of the Penokee region on the 

 south side of Lake Superior dip to the northward, the angle 

 being generally quite a high one, more often over, than under, 

 60 degrees. The iron-bearing horizon of this series lies in its 

 lower portion, and near its southern limit,* and is the counter- 

 part, in character and stratigraphical position, of the iron hori- 

 zon of the Animike series. 



*See Geol. Wisconsin, vol. i, pp. 80-95; vol. iii, pp. 100-101. See also this 

 Journal, March, 1885, pp. 236-249. With the latter paper is a small map show- 

 ing in a compact form the position and general relations of the ferruginous 

 horizon. 



Am. Jour. Scl— Third Series, Yol. XXXII, No. 190.— October, 1886. 

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