SUPERFICIAL ALTERATION OF ORE DEPOSITS. 293 



clay, sand, and gravel, while further concentration is due to this 

 capillary action. In the case of nitre, indeed, the saline material 

 is very often, if not generally, formed in soils or guano beds and 

 undergoes its first concentration by this capillary action. 



In the various chemical changes mentioned above, the class 

 of salts that remains, whether oxides, carbonates, haloid 

 compounds, etc., varies with the nature of the bases affected. 

 Thus, iron sulphides and copper sulphides are both oxidized 

 and form sulphates. But here the similarity of their behavior 

 ends, for the iron sulphate probably passes then into a basic sul- 

 phate and then into a hydrous sesquioxide, while the copper sul- 

 phate takes up carbonic dioxide and water and forms basic car- 

 bonates. The iron sulphate might, under certain conditions, 

 form a carbonate in a similar manner, but this compound would 

 be very unstable under the conditions existing in the alteration 

 of sulphide deposits and would quickly go into the form of the 

 hydrous sesquioxide, while the carbonate of copper is stable 

 under existing conditions and remains. 



In the same way, if silver sulphide and iron sulphide are both 

 oxidized and then affected by waters carrying common salt or 

 other chlorides in solution, the silver is converted to chloride, 

 which is insoluble and remains ; while the chlorides of iron are 

 much' less liable to be formed, as they are soluble, and some of 

 them unstable, compounds, and even if they were formed, they 

 would be leached out or oxidized. Hence, though chloride of 

 silver is a common product of alteration in silver deposits, chlor- 

 ide of iron is never found, at least to any extent, as a product of 

 alteration of iron deposits. 



Again, it is frequently found that unaltered auriferous iron 

 pyrites contains a certain amount of silver, while the altered part 

 often carries almost none. In such cases, the gold has remained 

 stable during the alteration, while the silver, in the absence of a 

 chloride or other reagents to convert it to an insoluble compound, 

 has been dissolved and carried away in solution by the acid 

 materials generated during alteration. 



Hence, the materials in surface waters affect different bases 



