3l6 THE JOURNAL OF GEOLOGY. 



in many cases it probably also occurred after they were desic- 

 cated.' 



SUMMARY. 



It will be seen from the above discussion that: 

 (i) After the deposition of ore deposits and their subsequent 

 exposure to surface influences, such as air, water and the mate- 

 rials contained in it, changes of temperature, etc., chemical and 

 physical alterations occur which cause a total change in the min- 

 eralogical condition, and generally in the economic value, of the 

 ore deposit. 



(2) The process of this alteration is primarily one of oxida- 

 tion and generally of hydration, and both of these actions may 

 go on alone, but generally both have their effect on the same 

 material. The other materials in solution in surface waters also 

 react on the substances in the ore deposit, either before or after 

 the oxidation of the latter, though generally after at least partial 

 oxidation, and form various compounds different from those 

 originally in the deposit. The difference, however, with few 

 exceptions, is not in the metal or other base which forms the 

 important feature of the deposit, but in the acidic portion or 

 material representing this portion of the mineral. Thus, sulphide 

 of copper may be altered to carbonate of copper, but the base 

 remains the same. The action of surface influences is in rare 

 cases one of reduction, -which, however, often follows a previous 

 oxidation. The process of alteration also frequently causes a 

 leaching of certain ingredients of the ore deposit, either with or 

 without previous oxidation, as in the removal of iron pyrites, 

 calcite, etc. It also sometimes renders a hitherto worthless 

 material valuable by the introduction of a valuable constituent, 

 as in the replacement of carbonate of lime by phosphate of lime. 

 It also causes the concentration, by capillary action in soils, of 

 certain deposits like nitre, etc. The compounds formed with 

 different ore deposits vary with the ores affected and the sta- 



' Chlorides of other materials than silver and copper may also have been formed 

 by a similar process, but the solubility of many metallic chlorides would prevent their 

 being accumulated in any but very dry regions. 



