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THE JOURNAL OF GEOLOGY.

The iron deposits of the Mesabi Range in Minnesota, which have lately been described by H. V. Winchell[1] are supposed to have had a somewhat similar origin to that given for the Michigan 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agencies of iron disseminated as oxides in a highly siliceous rock, and that in this concentration the silica has been replaced by iron.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 Silurian 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 Appalachian 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 superficial replacement of limestone, or even of more siliceous rocks like shales, by iron dissolved from ferruginous rocks in the neighborhood. In such cases the iron in the original rock has been dissolved and carried off in carbonated surface water, and re-precipitated in the other rocks, all these stages being directly due to surface influences. Many of the carbonate iron ores of the Carboniferous 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 atmospheric conditions, thus facilitating mining; while away from the surface, the shale becomes harder and makes mining more expensive.

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.[2]

  1. ↑ Minnesota Geological Survey, Twentieth Annual Report, pp. 136-148.
  2. ↑ Letter from Mr. Charles E. Smith, Philadelphia, Pennsylvania.
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