ILLUSTRATIONS OF SECONDAEY ENRICHMENT. 1189 



there must have concentrated in the shallow deposits at the surface an 

 amount of copper which was originally distributed through a vertical 

 distance of hundreds of meters. At Butte, Mont, the production of the 

 rich sulphide deposits characteristic of that district must also have required 

 enormous denudation. In this case where the belt of weathering is depleted 

 in copper, it may be supposed that during the secondary concentration the 

 major part of the copper originally distributed through a great vertical 

 distance was carried downward and reprecipitated ; and that the losses in 

 consequence of denudation were not great. But in other cases, where 

 geological studies show that great denudation must have taken place, the 

 amount of enriched material is comparatively small. For example, in the 

 Sierra Nevada gold belt, the enriched product has a thickness of not more 

 than 30 meters and is not more than five times as rich upon the average as 

 the sulphides of the first concentration below. It would therefore follow 

 that the gold distributed through 150 meters of the first concentration 

 would be sufficient to account for all of the enriched product, but Lindgren 

 places the extent of the denudation in this district as about 3,000 meters. 

 Hence in this case but a small part of the gold has been taken into solution 

 by descending waters and transported below and reprecipitated. It 

 therefore follows that the larger part of the gold produced by the original 

 concentration has been carried away from the fissures by the processes of 

 denudation, and correlative with these great losses to the veins are the 

 placer deposits of California. While the material was mainly carried away 

 from the veins, it was in g-reat measirre segregated in the beds of streams, 

 past and present, from which deposits it is being extracted. 



These instances of the relation of depth of denudation and amount of 

 enriched deposits, where the evidence seems to be particularly clear, are 

 mentioned with a view of suggesting an interesting- line of stud)- with 

 reference to ore deposits rather than with an idea of giving exact or 

 comprehensive data upon the subject. 



GENERAL STATEMENTS. 



It is believed to be a very general rule, if a long enough scale be used, 

 that ore deposits diminish in richness with depth. But it is well known 

 that above the level of ground water the valuable materials may be almost 

 wholly dissolved and deposited at or below the level of ground water 



