LEAD AND SILVER. 183 



in starting ore deposition, and the solutions favored the under side 

 of the sheets, along their contacts with the blue Carboniferous 

 limestone. 



2.08.06. The early "history of Leadville will be subsequently 

 referred to in speaking of auriferous gravels. The lead-silver 

 ores first became prominent in 1877, although discovered in 1874, 

 and by 1880 the development was enormous. The region grew at 

 once to be the largest single producer of these ores, and has re- 

 mained such ever since. The mines are situated east of the city 

 on the three low hills, Fryer, Carbonate, and Iron, but recently a 

 deep shaft in the city itself has found the extension of the ore 

 chutes and opened up great future supplies. The ores have chiefly 

 come in the past from the upper oxidized portions of the deposits. 

 Of late years, however, the older and deeper workings have been 

 showing the unchanged sulphurets. The ores are chiefly earthy 

 carbonate of lead, with chloride of silver, in a clayey or siliceous 

 mass of hydrated oxides of iron and manganese. In the Robert 

 E. Lee mine silver chloride occurred without lead. Some zinc is 

 also found, and a long list of rare minerals. Where the ore is in 

 a hard, siliceous, limonite gangue it is called hard carbonate, but 

 where it is sandy and incoherent it forms a soft carbonate, or 

 sand carbonate. All the mines produce small amounts of gold, 

 which in one case (the Printer Boy) has been of more importance 

 than the silver. A few ore bodies are found at other horizons 

 than the Carboniferous. They also run in instances as much as 

 100 feet from the contact, and may likewise be found in the por- 

 phyry, doubtless replacing included limestone. They were all 

 deposited as sulphides, and, according to Emmons, when the rocks 

 were at least 10,000 feet below the surface. 



2.08.07. In the valuable monograph on the region, which is 

 now a classic on the subject and which is cited below, Emmons 

 endeavors to prove the following points : 



I. That the ores were deposited from aqueous solution. 



II. That they were originally deposited mainly in the form of 

 sulphides. 



III. That the process of deposition involved a metasomatic 

 interchange with the material of the rock in which they were de- 

 posited. 



IV. That the mineral solutions or ore currents were concen- 

 trated along natural water channels, and followed, by preference, 

 the bedding planes at a certain geological horizon, but that they 



