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SUPERFICIAL ALTERATION OF ORE DEPOSITS.
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At the celebrated Iron Mountain in Missouri, a large part of the ore came from conglomerates composed largely of fragments of iron ore, which had been weathered out of the pre-Cambrian rocks that had originally contained them. These conglomerates lie at the base of the Cambrian strata which overlie the pre-Cambrian rocks, and even in the latter rocks, where exposed, the original ore has been made much more easy to work by the decay of the enclosing material and its conversion to clay.

In the iron region of eastern Texas, the limonite ores are often a result of the solution of iron from the superficial oxidation of iron pyrites, iron carbonate and glauconite. Sometimes the sequel of this process is the downward passage of the solution to an underlying laminated clay, and the gradual replacement of this bed, forming a hard limonite,[1] which still preserves the laminated structure of the clay.

In Mexico certain hematite deposits described by R. T. Hill[2] as occurring in Lower Cretaceous limestone at or near the contact with intrusive masses of diorite, and sometimes even in the diorite itself, may, as Hill suggests, be the result of superficial concentration from the limestone.

Very large deposits of hematite also occur in Grant county, New Mexico, at the contact of limestone and an eruptive. The origin of this ore is as yet somewhat obscure, but is probably due to a concentration after the original deposition of the iron.

The iron deposits in the lakes of Sweden and Norway are most striking instances of a concentration of iron ore due to surface influences and going on at the present time. The iron is derived from the oxidation of the neighboring rocks, carried by carbonated surface waters to the lakes, and there, by further oxidation and hydration, precipitated as hydrous sesquioxide (limonite). The iron ore is dredged up and used, but the processes of nature gradually replace it, and, in the course of years, the lakes again accumulate a considerable thickness of ore.

  1. ↑ R. A. F. Penrose, Jr., Geological Survey of Texas, First Annual Report, 1890, pp. 72-76, 79-81; also Bulletin Geological Society America, 1892, pp. 47-50.
  2. ↑ Amer. Jour. Sci., Vol. XLV., 1893, pp. 111-120.
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