REVIEWS 83 



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 various 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 iron-bearing member is an impure dark colored lime- 

 stone a few feet in thickness, not exceeding twenty. It extends appar- 

 ently the whole length of the Mesabi range, but has been identified in 

 two places only, Sec. 7, T. 58 N., R. 17 W., 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 

 interleaved with diabase sills. 



In the Indian reservation at Grand Portage and at various places 

 along the Grand Portage trail is a graywacke, which is supposed to 

 overlie the black slate member, but its extent and stratigraphical posi- 

 tion have not been satisfactorily established. 



The top of the Animikie has not been identified. The first recog- 

 nizable datum plane after the close of the Animikie is the Puckwunge 

 conglomerate, supposed to be the fragmental base of the Keewee- 

 nawan. 



At one or two places southwestward from Birch Lake, and at Little 

 Falls on the Mississippi River, and in Morrison county, the Animikie 

 has been converted into a mica-schist. 



The age of the Animikie is believed to be Lower Cambrian for the 

 following reasons : It graduates upward into Upper Cambrian rocks as 

 seen on the south side of Lake Superior. The derivation of the iron 

 ores from a glauconitic green-sand indicates that large quantities of 

 foraminiferal organisms once lived in the Animikie ocean, and 

 Matthew has shown the existence of foraminiferal organisms associ- 

 ated with the iron ore in the St. Johns group of New Brunswick. Fur- 

 ther the Animikie has a uniformly low dip, while the lower strata are all 

 highly tilted. There must therefore have been a great lapse of time 

 between the deposition of the two series. 



The Keweenawan. — The Puckwunge conglomerate is taken to be 



