The Superficial Alteration of Ore Deposits

Scope of the subject.-The modern idea of ore deposits teaches that formations of this kind represent a process of concentration of mineral matter, either by chemical or physical means; in other words, that they are unusual localizations of certain minerals which are often found disseminated in smaller quantities in many common rocks, and that they differ from the same minerals situated in other conditions, only in their degree of concentration. These concentrations may take place at different times in the history of the rocks in which the deposits occur. If they occur in sedimentary rocks, they may sometimes be formed during the deposition of the rocks with which they are associated, as in the cases of placer gold, stream tin, and sometimes of other ores;