552 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



in volume from sulphide to carbonate is 28.13 per cent. Such changes of 

 weight and volume might account for the prevailing sandy condition of the 

 carbonate ores. It may be assumed that they were once comparatively solid 

 masses of galena, and during these transformations occupied, first a larger, 

 then a smaller space, thus leaving interstices between the minute crystals 

 of cerussite of which the sand carbonate consists. In the presence of phos- 

 phoric acid and alkaline chlorides, sulphate of lead may be transformed into 

 pyromorphite. Sulphate of zinc is readily transformed into carbonate, and, 

 as a further stage of alteration, finally becomes a silicate. 



Sulphate of protoxide of iron may become carbonate in the presence 

 of earthy carbonates. Carbonate of protoxide by oxidation and hydration 

 becomes hydrated sesquioxide or limonite, which on loss of its water 

 becomes hematite; the latter in contact with organic matter may be reduced 

 to the protoxide or magnetite. 



The sulphate of the protoxide may by oxidation change directly to 

 sulphate of sesquioxide or basic ferric sulphate, which in turn becomes 

 limonite and hematite. 



The fact that carbonate of iron is so rarely found in the Leadville 

 deposits would suggest that their limonite might have been formed in the 

 latter way, and that the basic ferric sulphates, of which analyses are given 

 above, may have been formed directly from the sulphide. 



Agents of alteration. — The alteration having taken place through the 

 agency of waters coming from the surface, it remains to consider whence 

 they may have derived the substances which would have facilitated the 

 alteration of the sulphide. Surface waters in general are said to carry a 

 certain amount of atmospheric oxygen, of organic matter, of chloride of 

 sodium, and of phosphoric acid. An analysis of surface water at Leadville, 

 taken from the reservoir in Big Evans gulch, was found by Mr. Hillebrand 

 to contain, in a million parts, ICO— 1. 12, Na20rzl.92, SO3 = 7.20, and 

 Clzrl.14.1 



Of the rocks through which they may have passed, out of eight erup- 

 tive rocks analyzed, six were ioxmd to contain phosphoric acid and five 



' The Delaware River at Trenton contains 1.20 anil two glacier streams in the high Alps an 

 average of 0.4 CI in 1,000,000 parts. 



