PRECIPITATION OF SULPHIDES. 1113 



illustrated by the artesian waters of Wisconsin," which are contained in a 

 series comprising limestones and shales that are rich in organic matter. The 

 influence of organic matter in producing hydrogen sulphide is further 

 strongly suggested by the conditions at Sulphur Bank, where hydrogen sul- 

 phide is especially abundant in the solutions and the circulation of the ground 

 water is through sedimentary rocks bearing abundant organic material. 



Another source of hydrogen sulphide is the action of the dilute strong 

 acids upon the sulphides. For instance, the reaction of sulphuric acid 

 upon many of the base sulphides liberates hydrosulphuric acid. 



Even the weak acid, carbonic, where abundant, produces hydrogen 

 sulphide by reaction upon the alkaline sulphides, such as sodium sulphide. 

 As already noted, Stokes has shown that the carbonates of the alkalies, by 

 reaction upon the sulphides of the base metals, such as pyrite, form alka- 

 line sulphides. If the sodium sulphide thus produced comes into contact 

 with streams bearing carbonic acid, hydrogen sulphide will be produced. 

 When it is remembered that in the solutions sodium carbonate is an abun- 

 dant salt, that pyrite is the most plentiful sulphide, and that carbonic acid 

 is the most abundant acid, it will be realized that the conditions must 

 frequently arise which result in the production of hydrogen sulphide on a 

 considerable scale. 



Another way in which hydrogen sulphide can be formed is by the 

 partial oxidation of the sulphides, especially the abundant iron bisulphide 

 and pyrrhotite. On page 214 it was seen that the following reactions may 

 take place: 



3FeS 2 +4H 2 0+40=Fe 3 4 +4H 2 S+2S0 2 

 3Fe n S 12 +36H 2 0+80=llFe 3 4 +36H 2 S 



The frequent association of magnetite with iron sulphide where the con- 

 ditions have been favorable for partial oxidation suggests that this reaction 

 may have taken place upon a considerable scale. 



The hydrogen sulphide from any of the above sources may join the 

 circulating waters bearing oxidized salts in solution and precipitate copper, 

 zinc, lead, silver, and other metals as sulphides in the veins. 



In this connection it is very interesting to note that Doelter, by treating 

 metallic salts of the metals by hydrogen sulphide in somewhat dilute solu- 



"Chamberlin, T. C, The ore deposits of southwestern Wisconsin: Geol. of Wisconsin, vol. 

 4, 1882, p. 547. 



