SUMMARIES OF PRE-CAMBRIAN LITERATURE OF 



NORTH AMERICA FOR 1909, 1910, 191 1, AND 



PART OF 191 2 



EDWARD STEIDTMANN 



University of Wisconsin 



III. LAKE SUPERIOR REGION AND ISOLATED PRE-CAMBRIAN AREAS 

 OF THE MISSISSIPPI VALLEY 



Adams^ states that the formations of the Cuyuna iron-bearing 

 district of the north-central part of Minnesota consists of a series of 

 complexly folded Upper Huronian slates and other elastics inter- 

 stratified with lenses of iron formation, and intruded by basic 

 Keweenawan rocks; the whole trunkated and almost completely 

 covered by patches of Cretaceous conglomerate and a thick mantle 

 of glacial drift, presenting a surface of gentle relief. 



The iron formation occurs as a series of parallel and overlapping 

 lenses or belts, generally associated with magnetic attraction on the 

 limbs of trunkated anticlines, trending northeast-southwest. It 

 presents all gradations between original cherty iron carbonates, 

 and its secondary phases, ferruginous cherts, hard, soft, low-grade 

 hematite ores, and amphibole magnetite rocks. 



Adams follows Van Hise and Leith in his belief that the ores 

 occur in places which have been favorable to the circulation of 

 surface solutions, such as in inclined basins formed by pitching folds 

 superimposed on the major anticlines or by the intersection of dikes 

 with the walls adjacent to the iron formation lenses. Since the ores 

 in such localities grade into ferruginous cherts, and cherty iron 

 carbonates where the circulation of solutions was less favorable, he 

 concludes that the ores have resulted from the oxidation of the 

 cherty iron carbonates to ferruginous cherts and from the latter by 

 the leaching of the silica. The amphibole magnetite rocks probably 

 have developed from the anamorphism of cherty iron carbonates. 



'Francis S. Adams, "The Iron Formation of the Cuyuna Range," Econ. Geol., 

 V, No. 8 (1910), 729-40; ibid., VI, No. i (1911), 60-70; ibid., VI, No. 2 (1911), 

 156-80. 



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