RESUME OF LITERATURE. 121 



The Animikie rocks comprise the Pokegama quartzite, MesabI iron- 

 bearing formation, some limestone and slate, all strictly conformable with 

 one another. The thickness is several hundred feet, sometimes reaching 

 nearly 1,000 feet. The dip of the series is uniformly to the south, 8° to 12°. 



The iron-bearing formation and the Pokegama quartzite constitute the 

 base of the formation. The quartzite in places is beneath the iron forma- 

 tion; in other places it is in the some horizon, and in still others is above 

 the iron formation. Commonly the- base of the Animikie is marked by a 

 conglomerate, containing debris from the underlying Keewatin rocks. 

 This is a narrow horizon which soon graduates upward into a quartzite, 

 known as the Pokegama quartzite, from its typical development near Poke- 

 gama Falls on the Mississippi River. The thickness of the quartzite is not 

 known to exceed 50 feet, and is sometimes less than 26 feet. 



Above the quartzite, or in alternating beds with it, or below it, appears 

 the iron-bearing or taconite member of the Animikie, which contains the 

 iron-oi'e deposits of the Mesabi iron range. The ore is usually hematite in 

 the western part of the range and magnetite in the eastern part. It was 

 previously supposed to have been derived from the alteration of a greenish 

 glauconitic sand rock; but later work has seemed to show that the green- 

 sand is a volcanic sand, and that the so-called taconitic rock itself has 

 resulted from igneous forces. This is accounted for by supposing a chain of 

 active volcanoes to have existed where the Mesabi iron range is now found. 

 These volcanoes yielded flows and ejectamenta to the adjacent waters which 

 have been modified into the vai'ious phases of the iron formation now seen. 

 This volcanic epoch may have a deep-seated connection with the Cabotian 

 or lower division of the Keweenawan (described later). 



Above the ii'on-bearing member is an impure dark colored limestone a 

 few feet in thickness, not exceeding 20. It extends apparently the whole 

 length of the Mesabi range, but has been identified in two places only, sec. 7, 

 T. 58 N., R. 17 VV., and doubtfully on the shores of Gunflint Lake. This 

 limestone may be regarded as the basal horizon of the next overlying rock. 



The black slate is probably several thousand feet in thickness and 

 constitutes the bulk of the Animikie. In the neighborhood of Gunflint 

 Lake it has been divided by Dr. Grant into a lower black-slate division 

 and an upper graywacke-slate division, both of which members are intnided 

 by diabase sills 



