788 



SCIENCE. 



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



it still seems to me that the majority, per- 

 haps the great majority, of very rich de- 

 posits are below slopes and crests, and 

 not below the valleys. I believe the richer 

 deposits are below the slopes, because at 

 these places a second concentration is pos- 

 sible and probable. 



Returning now to this chart (Fig, 6), we 

 shall direct our attention to the fissure on 

 the slope. This fissure once extended up 

 through the overlying rocks which have 

 been removed by denudation. What has 

 become of the ore in the part of the fissure 

 which has been worn away ? If, for instance, 

 it carried five per cent, of copper, what 

 has become of it ? A part of it would have 

 been scattered far and wide through erosive 

 action ; but a part of it would have been 

 taken into solution and redeposited in the 

 same vein deeper down. In the belt of 

 weathering oxidized salts, such as sulphates, 

 would form ; the descending waters would 

 carry these products downward ; and it is 

 my belief that they would react upon the 

 solid, lean sulphides below with the result 

 of precipitating the metals from the de- 

 scending solutions. N"ow this has been 

 held to be a mere unverified assumption 

 by some geologists, but it seems to me that 

 they have not fully considered the certain 

 efi"ects of the chemical laws concerned. 

 We know if in a laboratory a solution of 

 copper sulphate or other copper salt be 

 placed in contact with iron sulphide, that 

 copper will be thrown down as copper sul- 

 phide. If the copper solution be placed in 

 contact with a lean copper-iron sulphide, 

 a sulphide richer in copper will be pro- 

 duced. And if these reactions occur in 

 the chemical laboratory, will they not as 

 certainly occur in the laboratory of nature, 

 although perhaps more slowly ? 



At this point it is to be recalled that in 

 many ore deposits above the level of ground- 

 water oxides and carbonates occur, while 

 below the level of ground- water are sul- 



phides. Moreover, at high levels these sul- 

 phides are rich in valuable metals, and 

 usually become poorer in these metals and 

 richer in iron sulphide at the lower levels. 

 You will remember at Butte, Mont., at and 

 for a distance below the level of the ground- 

 water, are rich copper sulphurets which 

 grade at depth into leaner copper sulphides 

 containing correspondingly large amounts 

 of iron sulphide. You will remember the 

 same is true of the entire Appalachian 

 region. You will remember that frequently 

 above the level of the ground-water gold 

 lodes are exceedingly rich. What is the 

 explanation of these similar facts? What 

 is the explanation of the exceptional or 

 even extraordinary richness of the deposits 

 at and near the level of ground- water, and 

 of the low grade of galena, blende or pyrites 

 deep below the level of ground-water? In 

 my opinion the only plausible explanation 

 is that the rich parts of the deposits have 

 received two concentrations, the first by as- 

 cending waters and the second by descend- 

 ing waters. The metals of the rich portions 

 of the deposits were largely contributed by 

 the parts of the deposit above, or once 

 above, the rich parts. In some cases por- 

 tions of the depleted veins remain, as at 

 Butte ; but frequently the depleted parts of 

 the veins have been removed by erosion. 

 The remote source of the material was, 

 therefore, the metals deposited by the first 

 concentration. But let us follow the matter 

 still further. In the majority of cases, as 

 denudation continued, the parts of the ore 

 deposits produced by the second concen- 

 tration rise into the belt of weathering. 

 They may there be partly or wholly trans- 

 formed into rich oxidized products, or they 

 may be depleted to extend the rich deposits 

 below. In the concentration by descend- 

 ing waters the chief chemical reactions are 

 believed to be between the oxides or salts 

 of the valuable metals and the sulphide of 

 iron; although, of course, similar reactions 



