122 THE VERMILION IRON-BEARING DISTRICT. 



In tlie Indian reservation at Grand Portage and at various places along 

 the Grand Portage trail is a graywacke, wliich is supposed to overlie the 

 black-slate member, but its extent and stratigraphic position have not been 

 satisfactorily established. 



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

 able datum j)lane after the close of the Animikie is the Puckwunge 

 conglomerate, supposed to be the fragmental base of the Keweenawan. 



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 greensand indicates that large quantities of foraminiferal 

 organisms once lived in the Animikie ocean, and Matthew has shown the 

 existence of foraminiferal organisms associated with the iron ore in the 

 St. Johns group of New Brunswick. Further, 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 the 

 fragmental base of the Keweenawan, although certain igneous rocks which 

 antedate it, and which, perhaps, are contemporaneous with the upper por- 

 tions of the Animikie, are also called Keweenawan. The conglomerate is 

 found at Grand Portage Island, at Isle Royale, on the Baptism River, at 

 Little Marais, on Manitou River, at the deep well at Short Line Park, near 

 Duluth, and at New Ulm. 



Above this conglomerate are conglomerates and sandstones of Ke- 

 weenawan age which are stratified with lavas of diabasic natin-e. Still 

 higher up the eruptive rocks become less in quantity and the fragmental 

 rock is a sandstone, known as the Hinckley sandstone, quai'ried in the 

 gorge of the Kettle River in Pine County. This in turn grades up 

 into typical Upper Cambrian sandstones of the St. Croix Valley. The 

 term Potsdam is restricted to the Puckwunge conglomei'ate and the 

 hardened quartzites immediately overlying it, represented by the Sioux 

 quartzite, the Baraboo and Barron count}'- quartzites of Wisconsin, the 

 qnartzite at Grand Portage Island, and west of Grand Portage village, 



