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THE JOURNAL OF GEOLOGY.

Many other similar cases of superficial enrichment in iron deposits might be mentioned, but the above are enough to illustrate the point in question, and it will be seen that, of the regions which are the active producers of iron ore in this country, almost all, if not all, owe the existence, or at least the availability of their large bodies of ore, to superficial concentration.

Alteration in manganese deposits.—Manganese deposits are affected by superficial influences in much the same way as iron deposits. Many of the manganese deposits in the Cambrian and Lower Silurian rocks of the Appalachian Valley were concentrated in a manner somewhat similar, though not always so, to the iron deposits in the same regions.[1]

In the Batesville manganese region of Arkansas, the ore originally occurred in irregular masses in Silurian limestone, but surface decay has leached the carbonate of lime out of the limestone, leaving a red siliceous clay, which represents the less soluble part of the original rock. This clay now lies in hollows on the surface of the limestone and contains the masses of ore once disseminated through that rock. The removal of the carbonate of lime has concentrated the ore masses in the clay, and has also rendered them more easily mined; in fact, the only manganese ore that can now be profitably mined in this region is that in the residual clay.[2]

The frequent occurrence of deposits of bog manganese ore in the areas of crystalline rocks, generally represents a concentration of manganese resulting from the oxidation of disseminated carbonate and silicate of manganese in the country rock. This oxidation product is taken into solution in surface waters, and transported until subjected to such conditions that it is oxidized and precipitated as a hydrous oxide.

Alteration in copper deposits.—In many copper deposits superficial alteration has produced very remarkable chemical and economic results, and this is especially well seen in the copper

  1. ↑ R. A. F. Penrose, Jr., Journal of Geology, No. 4, Vol. I., 1893, pp. 356-370.
  2. ↑ R. A. F. Penrose, Jr., Manganese: Its Uses, Ores, and Deposits; Arkansas Geological Survey, 1890, Vol. I., pp. 166-209.
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