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larger percentage of the gold production of the world. Hence, it will be seen, that in gold deposits, surface alteration not only plays an important part in freeing the gold from the iron pyrites, but also in forming placer deposits. Detrital deposits similar to gold placers and carrying various other materials are not at all uncommon, as in the cases of the platinum group of metals, cassiterite, diamonds and many other gems, chromite and magnetite sands, and, in fact, even with some of the more common ores, as with the iron conglomerate at Iron Mountain, Missouri.

Alteration in tin deposits.—In tin deposits, the typical mode of occurrence of the metal is in veins, dikes, or country rocks, in the form of the oxide known as cassiterite. Cassiterite is not easily affected chemically by surface influences, so that it is not much changed by superficial alteration, but for this very reason, its concentration is most markedly affected by surface alteration, for in the erosion of tin-bearing deposits the masses of cassiterite are broken up and carried off mechanically by surface waters, to be deposited somewhere else in the form of gravel beds, instead of being dissolved and possibly disseminated. In this transition, the fragments of cassiterite are largely separated from the accompanying materials by reason of their greater specific gravity, and hence, gravel deposits rich in cassiterite frequently occur. These represent the stream tin of the miner, and have been formed in much the same manner as have the placer gold deposits. Some chemical action, however, has gone on in the tin ore itself, but this seems to have been simply a process of solution and redeposition, as is seen in the pseudomorphs of cassiterite after other minerals and in the impregnations of animal remains in Cornwall, such as antlers, with oxide of tin.^[1]

Alteration in antimony deposits.—In many antimony deposits, alteration similar to that described in some of the deposits already mentioned frequently occurs. The metal occurs most commonly as the sulphide known as stibnite. By alteration, however, this passes into the oxides valentinite, senarmontite, cervantite, stibiconite, etc., or into the combined sulphide and oxide known