SUPERFICIAL ALTERATION OF ORE DEPOSITS. 297 



The process of hydration, when the materials affected do not 

 require oxidation before they can become hydrated, may extend 

 down indefinitely below the limit of oxidation ; but when oxida- 

 tion is necessary before hydration is possible, the latter process 

 of course can extend no deeper than oxidation. Thus the silicate 

 of alumina in feldspar may become hydrated and form kaolin 

 without the intervention of oxygen. This is brought about by 

 the action of carbonic acid and water, which react on the feldspar 

 and form alkaline carbonates, kaolin and free or hydrous silica. 

 Theoretically, therefore, kaolinization ought to go on to any 

 depth that can be reached by water and its almost universal 

 accompaniment, carbonic acid. In this case, however, the base 

 in question is already in its peroxide condition (AI3 O3), but 

 when a base is not in this condition, it frequently requires oxida- 

 tion previous to hydration. Thus sulphide of iron does not 

 become hydrated until it is peroxidized, and this mineral, there- 

 fore, requires oxidation previous to hydration.^ 



The various materials other than oxygen in surface waters 

 have a very important effect on the mineral matter with which 

 they come in contact, and their action sometimes takes place 

 before that of oxidation, though it often requires at least a par- 

 tial previous oxidation. The effect is both to form new chemical 

 compounds with the materials involved, and to dissolve and 

 bodily remove certain materials. As with oxygen, however, so 

 with these other agents of alteration, they are more active above 

 the drainage level of the country than below it, and an additional 

 reason for this is that many of the materials affected require a 

 primary oxidation before they enter into other chemical combi- 

 nations. Thus sulphide of lead is oxidized to sulphate of lead 

 before it can take up carbonic acid and form carbonate of lead ; 

 while on the other hand, carbonate of lime can be converted to 

 sulphate of lime (gypsum) by the action of sulphuric acid or 

 certain sulphates without any change in the degree of oxidation 

 of the lime. 



' For a full discussion of this subject see H. Rose, Ueber den Einfluss des Wassers 

 bei chemischen Zersetzungen, Pogg. Ann. der Physik und Chemic, 82 et seq. 



