LEAD AND ZINC ORES. 1149 



occurrence of many detached fallen crystals and masses of galena in the 

 openings above the level of ground water, and also at the bottoms of the 

 wider openings and caves a short distance below it. Indeed, a considerable 

 portion of the lead which has been mined was taken above or within a short 

 distance below the level of ground water. This strongly corroborates the 

 idea that concentration resulted from the solution of the other sulphides 

 which held the galena to the walls and from the erosion of the limestone, 

 thus permitting the material to drop to lower positions in the crevices. 



While the concentration of the galena was partly as above stated, it 

 may have been caused in part by chemical reactions between the various 

 compounds. In the belt of weathering part of the galena, as already noted, 

 is being oxidized, as is shown by the incrustations and superimposed crystals 

 of cerussite and anglesite. During the formation of the carbonates and 

 sulphates a certain amount of these salts is taken into solution and carried 

 downward. These carbonates and sulphates react upon the other sulphides 

 present and reprecipitate the lead as galena. These reactions may take 

 place to some extent above the level of ground water, but are especially 

 likely to occur below it. As a result of the downward migration of the 

 belt of weathering, there is in the downward- moving waters a continual 

 supply of the sulphates and carbonates of lead. The chief reaction is that 

 between the lead salts and the dominant iron sulphide. Supposing the iron 

 were in the form of FeS, the reactions may be written as follows: 



PbS0 4 +FeS=PbS+FeS0 4 . 

 PbC0 3 +FeS=PbS+FeC0 3 . 



If the iron be supposed to be in the form of FeS 2 , as is most likely, and 

 oxygen were present, the reactions may be — 



PbS0 4 +FeS 2 +0 2 =PbS+FeS0 4 +S0 2 . 

 PbC0 3 +FeS 2 +0 2 =PbS+FeC0 3 +S0 2 . 



Stokes has also shown that the precipitation of lead as a sulphide from 

 its oxidized solutions may be accomplished by the bisulphide of iron 

 without the presence of oxygen. The reaction by which this takes place is 

 as follows, with lead sulphate: 



7PbS0 4 +4FeS 2 +4H 2 0=7PbS+4FeS0 4 +4H 2 S0 4 . 



