COMPOSITION OF OEES. 545 



In discussing the above analyses the combinations of different ele- 

 ments in the ores and the indications afforded. as to the state in which they 

 existed prior to alteration will first be considered, next the possible proc- 

 esses, and finally the agencies, by which this alteration may have been 

 effected. 



Gold exists in these ores only in traces; it has never, so far as known, 

 formed any considerable value in the limestone ores, except in the Florence 

 mine, where it has been observed in the native state. It is in this form 

 without doubt wherever it occurs. It is generally supposed to be most 

 .commonly associated with pyrite in sulphuret deposits, and the assays of 

 the Pyritiferous Porphyry given below (Table IV, Appendix B) confirm 

 this hypothesis. They show, however, that it may be associated with 

 galena, and it is said that from the Printer Boy mine a specimen of galena 

 was obtained in which two crystals of this mineral were connected by a 

 thread of gold. Most of the small amount of gold that is produced by 

 Leadville mines comes from deposits in porphyry or sandstone, which 

 sometimes carry a little copper also. The greater part of the gold in 

 Leadville bullion conies from ore shipped to the smelting works from 

 deposits in the Archean rocks of neighboring districts. 



Silver exists in the oxidized ores invariably in the form of chloride, as 

 far as can be judged by actual observation. With a strong lens, minute 

 crystals and flakes of chloride can be detected on the crystals of cerussite, 

 or coating cleavage surfaces and cracks in the various vein materials, even 

 in comparatively poor ores. The above analyses confirm this observation. 

 The first three show more than enough chlorine for combination with the 

 silver they contain. In the mixture Hie amount of chlorine is 0.01 less than 

 is required by the amount of silver; if it could be assumed that the de- 

 termination of chlorine was absolutely accurate, a small portion of the 

 silver might be assumed to be combined with the sulphur, antimony, and 

 arsenic. Since there is a reasonable doubt on this point, the weight of evi- 

 dence is still in favor of its probable combination with chlorine. 



The association of minerals in the Leadville ore deposits generally 

 furnishes a priori grounds for the assumption that in the original deposit 

 silver was in the form either of simple sulphide or of some of the antimo- 



7.ION XII 35 



