790 



SCIENOK 



[N. S. Vol. XIV. No. 360. 



the waters would be mingled. Still further, 

 evea if the water were supposed to be 

 stagnant at the neutral belt, it is probable 

 that by diffusion the materials contributed 

 by the descending waters wouhl be mingled 

 with the materials contributed by the as- 

 cending 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 whicii 

 precipitation is likely to occur, we may 

 recall that waters ascending from below 

 contain practically no free oxygH,n and are 

 often somewhat alkaline, while waters de- 

 scending from above are usually rich in 

 oxygen 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. Le Conte further suggests f by 

 the mingling of the waters from below with 

 those from above that the tempera^.ure of 

 the ascending column will be rapidly 

 lessened, and this also may result in pre- 

 cipitation. 



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 rich- 

 ness of the upper part of veins independ- 

 ently of the material carried down from 

 above. 



In all the cases considered the precipita- 

 tion and enrichment of the upper parts of 

 deposits follow from the reactions of down- 

 ward-moving waters. Their effect may be 

 to precipitate the metals of the ascending 

 water to some extent, and thus assist in the 



* ' On the Genesis of Metalliferous Veins, ' by Joseph 

 Le Conte, Am. Jour. Sci., 3d ser., Vol. XXVI., 1883, 

 p. 9. 



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



first concentration. But the results of these 

 processes cannot be discriminated from the 

 concentration resulting from an actual 

 downward transportation of the material of 

 an earlier concentration. In concluding 

 this part of the subject, it is held that the 

 downward transportation of metals already in 

 lodes is the most important of the causes explain- 

 ing the character of the upper portions of ore 

 deposits; and that their peculiar characters are 

 certainly due to the effect of descending waters. 



The concentrations by ascending and de- 

 scending waters have been considered as if 

 they were mainly successive. In some in- 

 stances this may be the case ; but it is much 

 more probable that ascending and descend- 

 ing waters are ordinarily at work upon the 

 same fissure at the same time, and that 

 their products are, to a certain extent, si- 

 multaneously deposited. For instance, un- 

 der the conditions represented by this chart 

 (Fig. 6) a first concentration by ascending 

 waters is taking place in the lower part of 

 the fissure, and a reconcentration by de- 

 scending waters is taking place in the upper 

 part of the fissure. Between the two there 

 is a belt in which both ascending and de- 

 scending waters are at work. The rich 

 upper part of an ore deposit which is worked 

 in an individual case may now be in the 

 place where ascending waters alone were 

 first acting, where later, as a consequence 

 of denudation, both ascending and descend- 

 ing waters were at work, and still later, 

 where descending waters alone are at work. 

 The more accurate statement concerning 

 ore deposits produced by ascending and 

 descending waters is, therefore, that as- 

 cending waters are likely to be the potent 

 factor in an early stage of the process, that 

 both may work together at an intermediate 

 stage, and that descending waters are likely 

 to be the potent factor in the closing stage 

 of the process. 



Also, for the sake of simplicity in the 

 consideration of the concentrations I have 



