228 THE VERMILION IliON-BEARING DISTRICT. 



be a structural feature which has at some point or points exits for the 

 inflowing descending currents of water. Since the accumulation of the ore 

 depends on the circulating waters, of which some parts bring in and other 

 parts cause the deposition of the iron, and since any given amount of water 

 must carry an exceedingly small percentage of iron in solution, it is evident 

 that the free circulation of large quantities of water must be an extremely 

 important factor in the production of an ore deposit. The second great 

 factor is time — a long period, in which comjilete replacement may take 

 place, being more favorable than a shorter one. Of course it is here pre- 

 supposed that there exist the other conditions necessar)" to the acciunula- 

 tion of the ore, which are the presence of ii-on-bearing mateiial as the 

 source of the iron and the structural features for its accumulation. 



A full discussion of the chemical reactions which result in the deposi- 

 tion of the ore has already been giA^en by Irving and Van Hise in the mono- 

 graph on the Penokee series," and the reader is referred to this for the details. 



The following is a summarized statement of Van Hise's views of the 

 general chemical process of concentration as the result of which this and 

 similar ore deposits have been produced: 



The next question to be considered is the chemical process of concentration of 

 the ores. For places where waters from different sources are converged, this proc- 

 ess has been fully given in Monographs XIX and XXVIII of the United States 

 Geological Survey. In this paper the discussion will be only summarized. A part 

 of the iron oxide of the ore Avas deposited in its present condition as an original 

 sediment containing silica and other impurities. However, the nature of the sediment 

 ma}^ have been changed — that is to say, it may have been deposited in part as iron 

 •carbonate, or in small part as iron sulphide or iron silicate, and later transformed to 

 iron oxide in situ. The lean material originally deposited where the ore bodies now 

 are has been enriched by secondary deposition of iron oxide. Briefly, the process of 

 enrichment is believed to have been as follows: 



The source of the iron for the enrichment of the ores is believed to have been 

 mainly iron carbonate. Meteoric waters are charged with oxygen. As thej^ enter 

 the soil the}^ would be dispersed through innumerable minute openings. The waters 

 which early in their journey come into contact with iron carbonate would have their 

 oxygen abstracted. Such waters would be likely to be those following circuitous 

 routes. The deoxidation of the waters bj^ the iron carbonate would produce ferrugi- 

 nous slates and ferruginous cherts. In this alteration the carbon dioxide would be 



« The Penokee iron-bearing series of Michigan and Wisconsin, by E. D. Irving and C. R. Van 

 Hise: Mon. U. S. Geol. Survey Vol. XIX, 1892, pp. 283-284. 



