11 94 A TREATISE ON METAMORPHISM. 



shown are produced by descending- waters alone, those of the Lake 

 Superior region are unquestionably of the greatest importance. 



In another place I have fully discussed the segregation of these 

 iron-ore deposits." Without exception the great iron-ore deposits of the 

 Lake Superior region are found resting upon impervious formations. In 

 the great majority of cases these impervious formations are in pitching 

 troughs. (See PL XIII.) Thus the trunk channels of circulation are 

 formed. For the most part these troughs are somewhat close and have 

 steep pitches, but in the Mesabi district the troughs are less definite in form 

 and have a very shallow pitch. The impervious basement for an iron ore 

 deposit may be an impervious sediment such as a slate, may be an igneous 

 rock, or a combination of the two. The essential thing is that the basement 

 shall be relatively impervious to water as compared with the iron-bearing 

 formation in which the ores occur. The source from which the iron oxide 

 is obtained is the original rock of the iron-bearing formation, either siderite 

 or hydrous ferrous silicate, greenalite. As the water starts on its down- 

 ward journey the oxygen it contains is exhausted in the oxidation of the 

 iron carbonate or ferrous silicate, thus producing the ferruginous slates and 

 cherts, at the same time liberating the carbon dioxide of the carbonates. 

 The waters from which the oxygen is exhausted and which are enriched in 

 carbon dioxide are now able to take the ferrous carbonate and hydrated 

 ferrous silicate into solution. Where impervious basements exist in pitch- 

 ing troughs, the downward-moving waters are deflected toward these, and 

 thus are converged into such troughs. Other waters more directly from 

 the surface which have not come into contact with iron carbonate and iron 

 silicate, and therefore bear oxygen, are also converged into the troughs. 

 The mingling of the two classes of waters in the troughs results in the 

 precipitation of the hydrated hematite and limonite, the reaction in the 

 case of the iron carbonate being: 



2FeC0 3 +nH,0+0=Fe 2 3 - nH 2 0+2C0 2 



At the same time the abundant waters converged in the trunk channels 

 dissolve the silica, and thus the ores are depleted in that compound. So 

 far as there was iron carbonate or iron silicate in the pitching- troughs the 

 ferrous oxide is largely oxidized in situ. ThuSj. the ore deposits are the 



« Van Hise, C. R., The iron-ore deposits of the Lake Superior region: Twenty-first Ann. Kept. 

 U. S. Geol. Survey, pt. 3, 1901, pp. 305-434. 



