1110 A TREATISE ON METAMORPHISM. 



Very frequently the mingling of alkaline solutions of sulphides with 

 acid solutions is at the points where descending and ascending waters meet. 

 This is very well illustrated by the conditions at Steamboat Springs and 

 Sulphur Bank, described by Le Conte," where the surface descending 

 waters are strong-ly acid, largely resulting from the oxidation of sulphides 

 to sulphates, whereas the waters rising- from the deep sources are strongly 

 alkaline and bear snip, ides in solution. 



Precipitation of sulphides . ansported as oxidized salts. Wherever below the level of 



ground water in the belt of cementation the sulphates and sulphites come 

 in contact with buried organic material, or with solutions carrying organic 

 compounds, the sulphates and sulphites may be reduced to sulphides. By 

 such reduction ores may be directly produced. Where the salts of the 

 metals are transported as sulphates the organic matter has merely to take 

 away the oxygen of the sulphate in order to transform it to a sulphide. 

 For instance, if the compound be lead sulphate (PbS0 4 ) the abstraction 

 of the four atoms of oxygen produces galena (PbS). But in order that 

 such compound be formed it is not necessary to suppose that the metals 

 precipitated are transported as sulphates. It is only necessary to suppose 

 that oxidized compounds of the metals and sulphates, with a reducing- 

 compound, are present. To illustrate, if silver be transported as a chloride 

 with sodium sulphate in the solutions and oxygen be taken away from 

 the sodium sulphate, this would be transformed to sodium sulpiride, 

 which would immediately precipitate the silver as silver sulphide. Again, 

 supposing- copper to be traveling mainly as carbonate, sodium sulphate 

 to be present, and the reducing agent to be carbon, the end result is repre- 

 sented by the following reaction : 



CuC0 3 +Na 2 S0 4 +2C=CuS+Na, 2 C0 3 +2C0 2 



Of course the actual process would be as in the previous case. The 

 compound containing the sulphur would be reduced to sulphide. This 

 would react upon any of the compounds present which could produce 

 insoluble sulphides. 



Many ore deposits give evidence that the reducing action has been 

 caused by organic material. Such cases are illustrated by disseminated 

 sulphides occurring through carbonaceous material, as the graphite schist 



« Le Conte, Joseph, Genesis of metalliferous veins: Am. Jour. Sci., 3d ser., vol. 26, 1883, p. 9. 



