PRINCIPLES CONTROLLING ORE DEPOSITS 763 



rom above are cool and dense, while those from below are warm 

 and less dense. In the neutral zone of circulation the waters 

 from above would thus tend to sink downward, while waters from 

 below would tend to rise, and thus the waters would be mingled. 

 Still further, even if the water were supposed to be stagnant at 

 the neutral belt, it is probable that by diffusion the materials con- 

 tributed by the descending waters would be mingled with the 

 materials contributed by the ascending waters. 



Ascending and descending solutions are sure to have widely 

 different compositions, and precipitation of metalliferous ores 

 is a certain result. As a specific case in which precipitation 

 is likely to occur, we may recall that waters ascending from 

 below contain practically no free oxygen and are often some- 

 what alkaline, while waters descending from above are usually 

 rich in oxygen and frequently contain acids, as at Sulphur 

 Bank, described by Le Conte. 1 The mingling of such waters 

 as these is almost sure to result in precipitation of some kind. 

 Le Conte further suggests 2 by the mingling of the waters from 

 below with those from above that the temperature of the 

 ascending column will be rapidly lessened, and this also may 

 result in precipitation, but the dilution would work in the reverse 

 direction. 



The metals precipitated by the mingling of the waters may 

 be contributed by the descending waters, by the ascending waters, 

 or partly by each. In so far as more than an average amount of 

 metallic material is precipitated from the ascending waters, this 

 would result in the relatively greater richness of the upper part 

 of veins independently of the material carried down from above. 



In all the cases considered the precipitation and enrichment 

 of the upper parts of deposits follow from the reactions of 

 downward moving waters. Their effect may be to precipitate the 

 metals of the ascending water to some extent and thus assist 

 in the first concentration. But the results of these processes 



J On the Genesis of Metalliferous Veins, by Joseph Le Conte : Am. Jour. Sci., 

 3d ser., Vol XXVI, 1883, p. 9. 

 2 Le Conte, op. cit., p. 12. 



