UPPER HURONIAN. 387 



varying quantities of magnetite. It is not necessary, therefore, to assume 

 any abnormal conditions otlier than the contact action of the gabbro on 

 beds having the proper composition. Complete recrystallization of properly 

 constituted beds, the process taking place slowly and extending over a long 

 time, would readily account for the existence of these abnormal Gunflint 

 beds. Since we consider this recrystallization of the rocks and production 

 of magnetite, etc., to have taken place as the result of the metamorphism 

 produced by the Duluth gabbro, it is evident that the iron in the rocks 

 must have accumulated prior to Keweenawan time." As the result of the 

 metamorphism the rocks were so changed that no further concentration 

 of iron took place, and consequently we find these deposits in this part 

 of the district differing both in petrographic character and in size from the 

 great deposits of the western Mesabi or Mesabi proper, whose conceiatra- 

 tion was not seriously interfered with except locally during Keweenawan 

 times, but has continued right on up to the present. 



RELATIONS TO OTHER FORMATIONS. 



The peculiar Gunflint formation, found at the base of the Upper 

 Huronian, rests upon rocks of different character and of varying age. 

 These range from the granite of Saganaga Lake in sees. 23 and 24, T. 65 N., 

 R. 4 W., through the Ely greenstone to the west of this area, and then up 

 to the Ogishke conglomerate and the Knife Lake slates still farther west. 

 The Duluth gabbro lies against and upon the southern edge of the Gunflint 

 formation. 



The Gunflint formation of the Animikie series is found in relationship 

 with the Ely greenstone of Archean age, at one especially well-exposed 

 place in the north side of the cut of the Duluth, Port Arthur and Western 

 Railroad, where it cuts the east end of the high cliff of greenstone on the 

 north shore of Gunflint Lake. Here the iron formation is well banded, 

 and rests, with a very slight dip to the south, on the crinkled green schists 

 derived from the Ely greenstone. At one place about 1 foot of con- 

 glomerate was found at the base of this formation. This conglomerate con- 

 sists of green schist and quartz pebbles, and above this comes a layer of 

 banded white chert about a foot thick in places, and somewhat brecciated. 

 The iron formation proper does not actually appear at the particular 



« Geol. and Nat. Hist. Survey of Minnesota, Bull. No. 10, 1894, pp. 199, 358. 



