THE PROTEROZOIC ERA. 



189 



dition of the formation, the others being secondary phases. Iron ore (iron oxide) 

 is found in the lower part of the formation, usually just above the Palms for- 

 mation. The Ironwood formation has a thickness of about 850 feet. The upper- 

 most formation of the series is (3) the Tyler slate, locally altered to schists 

 The thickness of this formation exceeds 12,000 feet. The structure of the Animi- 

 kean of this region is monoclinal (dip of 60 D to 70°), but a series of transverse 

 folds has been superposed on the monoclinal structure. 



In the Mesabi region, 1 the Animikean includes three formations. In ascend- 

 ing order these are (1) the Pokegama quartzite, with basal conglomerate, rest- 



Fig. 72a. — Map showing (in black) the position of the iron-producing areas in the 

 Lake Superior region. 1, Michipicoten district; 2, Kamanistquia and Matawin 

 district; 3, Steep Rock Lake and Attikokan district; 4, Vermilion district; 5, 

 Mesabi district; 6, Penokee — Gogebic district; 7, 8, and 9, Marquette, Crystal 

 Falls, and Menominee districts. 



ing unconf ormably on Archean and Huronian, 0-500 feet thick ; (2) the Biwabik 

 (iron-bearing) formation, consisting of cherts, with ferruginous slates, quartzites, 

 and iron ores (iron oxides) . The formation from which these varieties of rock 

 were derived consisted largely of granules of green ferrous silicate,, to which the 

 name Greenalite has been given. The thickness of the Biwabik formation may 

 range from 200 to 2000 feet. Locally it is profoundly metamorphosed by con- 

 tact with the gabbro and granite of the Keweenawan series. Above the Biwabik 



Leith.Mono. XLIII, U. S. Geol. Surv 



