MINES OUTSIDE THE LEADVILLE DISTRICT. 



537 



TEN-MILE DISTRICT. 



The ore deposits of this district, which lies north of the western half 

 of the Mosquito map, will be made the subject of a separate monograph, 

 and only a brief mention of their prominent characteristics need be made. 



The larger and more prominent ore bodies of this district resemble 

 the Leadville deposits in that they occur as a replacement of limestone 

 strata, and generally along the upper surface of these strata. In this re- 

 spect they are contact deposits, but the immediately overlying rock is gen- 

 erally a sandstone instead of a porphyry sheet. The development of erup- 

 tive rocks is even more remarkable than at Leadville, and they occur gen- 

 erally as intrusive sheets nearly parallel with the stratification, sometimes 

 also in the form of transverse sheets and dikes. Thus, although the ore 

 body may not be found in actual contact with an eruptive rock, it is never 

 very far removed from one. 



The ore bodies themselves are less completely oxidized than those of 

 Leadville, probably because of the greater elevation above sea-level, in 

 consequence of which surface waters are imprisoned by frost during a 

 greater proportion of the year. The unoxidized portion of these larger 

 ore bodies consists entirely of sulphides, mainly pyrite, zinc blende, and 

 galena, while the oxidized portion, near the surface, consists of iron-stained, 

 clayey vein material, carrying carbonate of lead and chloride of silver, 

 scarcely to be distinguished from the average Leadville ore, though it is 

 generally less rich in silver. The manner of occurrence of ore bodies 



