1176 A TREATISE ON META^JORPHISM. 



tend to sink downward, while those from below are warm and less dense 

 and tend to rise ; thus the waters are mingled. But even if the water were 

 supposed to be stagnant at the neutral belt, it is probable that by diffusion 

 the materials contributed by the descending and ascending waters would 

 be mingled. 



Ascending and descending solutions are sure to have widely different 

 compositions, and an accelerated precipitation of metalliferous ores is a 

 certain result of their mixture. As an illustrative case in which precipita- 

 tion is likely to occur, we may recall that ascending waters contain prac- 

 tically no free oxygen, frequently contain hydrogen sulphide, and are 

 often somewhat alkaline, while descending waters are usually rich in 

 oxyg-en and frequently contain acids, as at Sulphur Bank, described by 

 Le Conte." The mingling of such waters as these is almost sure to result 

 in precipitation of some kind. As illustrating the effect of the mingling of 

 descending and ascending solutions we may suppose that the sulphides 

 of any of the metals are rising in solutions of sodium sulphide, and that 

 the descending waters are carrying sulphates of the metals and sulphuric 

 acid. The sulphuric acid would destroy the sodium sulphide according to 

 the following reaction: 



N^S+HjSO^N^SOi+HiiS 



The sodium sulphide being destroyed, the sulphides traveling as such in 

 the ascending solutions would be thrown down. This reaction also pro- 

 duces hydrogen sulphide, which, formed in this way or originally present, 

 would throw down metals from the descending- oxidized salts. For 

 instance, sulphate of copper would be thrown down as a sulphide by 

 hydrog-en sulphide. Another illustrative case is the mingling of descending- 

 waters bearing oxygen with ascending waters bearing iron carbonate. The 

 result is to throw down the iron as ferric oxide, according to the following 

 reaction : 



2 FeC0 3 +0+nH 2 0=Fe,0 3 . nH 2 0+2C0 2 



Le Conte also suggests that by the mingling of the waters from 

 below with those from above the temperature of the ascending column is 

 rapidly lessened, and this also may result in precipitation. The dilution 

 may work in the same or in the reverse direction. 



^Compare Le Conte, Am. Jour. Sci., 3d series, vol. 24, 1882, p. 33, and vol. 26, 1883, p. 9. 



