558 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



its outward appearance would indicate. Both contain a little antimony, 

 and the former a little PbO, which were not determined quantitatively. 

 Both contain also a small percentage of silver, but not enough to constitute 

 pay ore. 



No. 3, though classed as an iron ore by the miner, is more properly an 

 ore of manganese. 



Nos. 4, 5, and 6 are given to illustrate the different stages in the alter- 

 ation of pyrite, which would lead to the above vein materials. The speci- 

 men was obtained at some distance from Leadville, it not being possible to 

 find any pyrite in the mines themselves; it occurs, however, at the Blue 

 Limestone horizon and in analogous conditions to the Leadville ores, except 

 that no rich silver-lead ores had yet been found at the locality. No. 4 is 

 the comparatively unaltered pyrite nucleus; No. 5, the inner zone of alter- 

 ation; and No. 6, the lighter- colored outer zone. Only traces of sulphuric 

 acid are found in either. The percentage of metallic iron has slightly in- 

 creased from No. 4 to No. 5, as has that of insoluble matter; sulphur alone 

 has, therefore, been removed, probably as sulphate of lime. From No. 5 

 to No. 6 iron has disappeared rapidly and been replaced by insoluble mate- 

 rial; it has presumably been carried away largely as hydrated oxide, which 

 is probably commencing to replace the carbonates of lime and magnesia in 

 the adjoining rock. The rapid increase in insoluble matter could not, it 

 would seem, be entirely original matter, but must in part have been brought 

 in by the decomposing waters. The conditions here may be contrasted 

 with those which probably existed where the basic ferric sulphates above 

 described were formed. In the latter case the decomposition probably took 

 place within the mass of a large body of metallic sulphides; here it was a 

 small amount of sulphide in direct contact with the country rock, and free 

 oxygen would have been present in relatively greater proportion, so that 

 the sulphide would have been more readily decomposed by the oxidation of 

 the iron. 



. Nos. 7 and 8 represent the early action of Welters coming from such a 

 decomposition upon the country rock, in this case a fragment of Blue Lime- 

 stone found on the south slope of Iron Hill, near the fault. No. 7 is the 

 lighter portion, No. 8 the darker, in which the replacement has apparently 



