304 THE JOURNAL OF GEOLOGY. 



The iron deposits of the Mesabi Range in Minnesota, which 

 have lately been described by H. V. Winchell ' are supposed to 

 have had a somewhat similar origin to that given for the Michi- 

 gan and Wisconsin ores by Van Hise. Winchell believes that 

 they are due to the concentration by surface agencies of iron 

 disseminated as oxides in a highly siliceous rock, and that in this 

 concentration the silica has been replaced by iron. 



The red hematites of the Clinton horizon of the Upper Silu- 

 rian in the Appalachian region have been at least partly formed 

 by superficial concentration which extends to only limited depths. 



The iron deposits in other geologic horizons of the Appala- 

 chian valley, especially in the Cambrian, Lower Silurian and 

 Carboniferous rocks, are also often much changed by the action 

 of surface influences. Many of the deposits in the Cambrian 

 and Lower Silurian can be clearly shown to be due to a super- 

 ficial replacement of limestone, or even of more siliceous rocks 

 like shales, by iron dissolved from ferruginous rocks in the neigh- 

 borhood. In such cases, the iron in the original rock has been 

 dissolved and carried off in carbonated surface water, and re-pre- 

 cipitated in the other rocks, all these stages being directly due to 

 surface influences. Many of the carbonate iron ores of the Car- 

 boniferous rocks are rendered not only of higher grade, but also 

 more easy to treat, by the oxidation of the carbonate to the 

 sesquioxide and the removal of the carbonic acid. Moreover, 

 these carbonate ores often occur as nodules, " kidney ores," in 

 shale, and, on the surface, this shale has been softened by atmos- 

 pheric conditions, thus facilitating mining; while away from the 

 surface, the shale becomes harder and makes mining more expen- 

 sive. 



Surface influences on carbonate of iron have been made use 

 of artificially in Styria, where a very hard spathic iron ore has 

 been mined and spread out on a hill side for from 20 to 25 years. 

 By this process the ore was oxidized and made more porous, and 

 thus became very much more cheaply treated.^ 



'Minnesota Geological Survey, Twentieth Annual Report, pp. 136-148. 

 ^^ Letter from Mr. Charles E. Smith, Philadelphia, Pennsylvania. 



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