438 THE VERMILION IRON-BEARING DISTRICT. 



unite the events preceding the time of the Ely greenstone with that of the 

 greenstone itself the time thus represented would be very long indeed. 

 This time is only a part of the Archean. So far as the Vermilion district 

 is concerned, and, indeed, so far as the entire Lake Superior region is 

 concerned, we get no farther back than that period of the primeval ocean 

 in which the Ely greenstone was formed. If there were land areas in this 

 region or elsewhere in the world at an earlier time we have no evidence of it. 



After regional volcanic activity continued for an unknown time, it 

 died out, as volcanic activity has elsewhere. Probably this process was a 

 very slow one, although we have no direct evidence upon this point. 



Following the Ely greenstone there were orogenic movements and 

 erosion. Correlative with these was the deposition of sediments in the 

 Vermilion district. The mechanical sediments formed at this time are 

 insignificant in amount.. The depth of water over the Vermilion district was 

 sufficiently great to make the mechanical sediments entirely subordinate. 

 Here, under quiescent conditions, the iron-bearing carbonate of the Soudan 

 formation was laid down. For much of the district this formation rests 

 directly upon the Ely greenstone, with no intervening mechanical material. 

 The iron-bearing carbonates are chemical or organic sediments, or were 

 more probably formed by a combination of chemical and organic agents. 



That life was present in the sea at the time of the deposition of the 

 Soudan formation and furnished organic material to reduce the limonite 

 and carbonate to protoxide is indicated by the graphitic material now 

 associated with these rocks. The iron for the carbonates may have been 

 partly absorbed from the Ely greenstone underlying the formation, but 

 probably was more largely abstracted from the areas of Ely greenstone 

 raised above the water outside the district at present considered. The 

 underground and surface streams would there dissolve the iron salts. They 

 were brought to the Vermilion sea, and there were probably precipitated 

 as limonite and transformed to iron carbonate by processes previously 

 explained. 



Following the deposition of the Ely greenstone there was a second 

 great outbreak of igneous activity. At this time various igneous rocks 

 were intruded within the Ely greenstone and the Soudan formation. At 

 this time, or earlier, were the great intrusions of granite represented by the 



