292 THE JOURNAL OF GEOLOGY. 



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 reduc- 

 tion, due sometimes to superficial influences, is seen in the occa- 

 sional 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 cal- 

 cite 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 cal- 

 careous 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 cer- 

 tain 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 possi- 

 bly 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 



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