1150 A TREATISE ON METAMOEPHISM. 



A similar reaction takes place with lead carbonate and bisulphide of iron 

 where sodium carbonate is present. Stokes writes the reaction as follows: 



14PbC0 3 + SFeS 2 +Na,C0 3 =14PbS+4Fe 2 3 +Na 2 S 2 3 +15C0 2 . 



Subsequent experiment has shown that calcium carbonate may be substituted 

 for the sodium carbonate. The last two reactions were performed at tem- 

 peratures of 100° and in the absence of oxygen. Since in the upper part 

 of ore deposits such temperatures are seldom found, and there is no reason 

 whatever to suppose that they existed during the development of the lead 

 and zinc ores of the Mississippi Valley, and as oxygen is likely to be present, 

 it is probable that the precipitation of the sulphide of lead by the bisulphide 

 of iron in the upper part of veins was by the reactions written with oxygen 

 rather than by the reactions without oxygen. 



However, it has been stated that zinc sulphide is present with the 

 original sulphides, and this may also react upon the lead salts, according to 

 the followiug equations: 



PbC0 3 +ZnS=PbS+ZnC0 3 . 

 PbS0 4 + ZnS =PbS + ZnSOi- 



In the case of the former reaction, smithsonite would be formed. In 

 this connection it is notable that frequently smithsonite is associated with 

 the galena for some distance below the level of ground water. While a 

 part of the smithsonite below ground water is of this origin, doubtless the 

 larger portion of it is differently explained. (See p. 1147.) 



To the foregoing reactions, partly explaining the concentration of 

 galena, objection may be made on account of the slight solubility of lead 

 carbonate and lead sulphate. It is true that these substances are very 

 sparingly soluble in pure water ; however, they are sufficiently soluble in 

 waters bearing carbon dioxide to account for the phenomenon. But some 

 of the lead may have been carried downward as a chloride. Independently 

 of chemical theory, we know that much of the galena has been changed to 

 some soluble form upon" an extensive scale. As evidence for this inference, 

 galena crystals above the level of ground water are much corroded, and the 

 amount of cerussite and anglesite associated with them is so small as not to 

 account for the corrosion, therefore the lead has been transformed to a 

 soluble salt which has been transported below in important amounts. In 

 the deposits of southwestern Missouri, and in those of Wisconsin adjacent to 



