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SUPERFICIAL ALTERATION OF ORE DEPOSITS.
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  • bility of the compounds formed by the action of the materials in the surface waters on the constituents of the ores.
  • (3) The physical effect of superficial alteration is generally to make the deposit more open an porous, to cause it to shrink, and, in some cases, to convert it to a loose material of the consistency of sand and clay. In some cases, however, especially where hydration is active, and expansion may be caused.
  • (4) Superficial alteration extends downwards as far as surface influences are able to act, though generally alteration is not complete down to the possible limit. The depth of alteration depends on the topography of the region, the nature of the rocks, and on the climate. In glaciated regions, the glacial action has swept away the products of alteration, and sufficient time has not yet elapsed since then for alteration to have gone on to any great extent, but in many other regions the products of alteration have accumulated to considerable depths. The depth of alteration, under different conditions, varies from a fraction of a foot to 1,500 feet, or possibly more.
  • (5) Superficial alteration is well illustrated in iron, manganese, copper, lead, zinc, silver, gold, tin, and many other deposits. For special descriptions see text.
  • (6) The accumulation of soluble saline materials, like salt, on the surface has a very important effect in converting certain materials in underlying ore deposits to chlorides, etc.

R. A. F. Penrose, Jr.

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