98 F. R. VAN HORN PROUSTlTE AND ARGENTITE FROM COLORADO 



last to be precipitated was the mixture of proustite and quartz or argen- 

 tite and quartz. From the solubility of these various minerals it would 

 seem probable that here, also, the ores were deposited from different solu- 

 tions at different times, which was the conclusion drawn from the assays 

 given above, all of which were made before the discovery of the rich silver 

 minerals. 



It seems plausible that there were two silver solutions, one first fur- 

 nishing the argentiferous galena, while the later one deposited the prous- 

 tite and argentite along with quartz. Although apparently of later origin, 

 in no case were the silver minerals found penetrating through the siderite 

 into the galena and sphalerite, ' which would indicate that there was no 

 later fracturing or opening of the fissure while the filling of the same was 

 taking place. The walls of the vein seem everywhere sharply defined, 

 and no replacement of the country rock was noticed. It might be men- 

 tioned that the officials of the company working the mine have a decided 

 opinion that more sphalerite has been encountered in the lower levels than 

 above. Likewise no proustite and argentite were ever found in the older 

 workings above. This indicates an enrichment of the vein below, which 

 might be due to solution of upper deposits and precipitation of the same i 



in the lower levels. There was no evidence observed which would warrant 

 this assumption, however, and it seems more probable that the change of 

 ores has been caused by other conditions, such, possibly, as variation in 

 pressure, temperature, or chemical composition. 



Conclusion 



The chief incentive in writing this article was to record the new occur- 

 rence of proustite and argentite in this particular locality. The writer 

 has also never seen or heard of such massive specimens in such large 

 amounts from any other region. On account of these facts, this paper 

 has been deemed worthy of presentation before this Society, and specimens 

 of both minerals in vein sections are submitted for inspection, hoping that 

 they may prove of interest. 



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