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acmite Aitkin County Animikie anticline Arko Armour Biwabik Biwabik formation body Bureau of Mines cent manganese chloritic correlation cross section B-B Cuyuna district Cuyuna iron Cuyuna Range deposits drill holes east explored fault feet thick ganese Geol glacial granites gray slates graywackes green carbonate slate green slate greenalite Grout Gruner Harder and Johnston hematite igneous intrusives iron formation iron ranges iron-bearing member Keweenawan Lake Superior layers Leith lenses Lower Cherty member Lower Slaty member magnetite Mahnomen manganiferous iron mapped Maroco mation Merritt Mesabi Range metamorphism minerals Minnesota Geological Survey North Cuyuna North Range northeast Northland open pit oxides Pokegama formation probably production quartz quartzite Rabbit Lake reported Sagamore schist sequence SEVi Sec silicates Slaty beds slaty iron formation South Range member southeast southwest stilpnomelane structure syncline taconite Thiel thin tion U.S. Bureau unconformity Upper Cherty member Upper Slaty member Virginia formation Virginia slate Wolff Zapffe