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tion, and the reducing action follows as the effect of one of the partially oxidized compounds on the other, as in the case of copper just mentioned. In deposits, such as gypsum, a reduction, due sometimes to superficial influences, is seen in the occasional formation of sulphur from gypsum.
An important chemical effect of surface influences is the removal in solution of certain ingredients of the ore deposit which are soluble in surface waters; as the removal of the calcite gangue of many silver and other deposits; the oxidation and removal of the sulphur in various silver, lead, zinc, copper, and other deposits; the oxidation and removal of both the iron and sulphur of iron pyrites in auriferous quartz veins; the removal of silica from certain iron deposits, such as those in the Lake Superior region, etc. Probably many phosphate deposits are formed by the superficial leaching of carbonate of lime from calcareous beds, and the corresponding concentration of phosphate of lime once finely disseminated in the same beds.
Another chemical effect of superficial alteration is seen in the occasional formation of mineral deposits of importance by certain materials carried from outside sources and deposited in a rock of otherwise no commercial value. Thus certain phosphate deposits of the South Pacific Ocean, the West Indies and possibly of Florida, are formed by the leaching of soluble phosphates from guano, their transportation down into underlying limestone or coral reefs, and the precipitation of the phosphoric acid as tribasic phosphate of lime, which, being almost insoluble, arrests further escape of the phosphatic materials.
Again, another chemical effect is seen in the incrustations, and even extensive beds, of saline materials, like borax, nitre and the various alkaline salts of the western arid regions, formed by precipitation from water rising by capillary action through the soil, becoming evaporated on the surface and depositing the saline materials which they have dissolved from below. Many saline deposits are formed by the simple evaporation of surface waters, such as lakes, seas, etc., but certain deposits undergo only an initial concentration in this way, and are laid down with