376 GEOLOGY AND MmiNG INDCTSTRY OF LEADVILLE. 



extending to varying depths from the surface, and even occupying at times 

 the entire thickness of the Blue Limestone formation. This ma}^ be re- 

 garded as the typical form of the Leadville deposits; there are, however, 

 variations from it, and also in the character of the inclosing rock, which do 

 not necessarily involve any difference in origin or mode of formation. As 

 variations in form, the ore sometimes occurs in irregularly-shaped bodies, 

 or in transverse sheets not always directly connected with the upper or 

 contact surface of the ore-bearing bed or rock; it also occurs at or near the 

 contact of sheets of Gray or other porphyries with the Blue Limestone, and 

 less frequently in sedimentary beds, both calcareous and silicious, and in 

 porphyry bodies, sometimes on or near contact surfaces, sometimes along 

 joint or fault planes. 



Composition. — The prevailing and bj^ far the most important ore, from 

 an economical point of view, is argentiferous galena, with its secondary 

 products, cerussite or carbonate of lead and cerargj-rite or chloride of silver. 



Lead is also found as anglesite or sulphate, as pyromorphite or chloro- 

 phosphate, and occasionally as oxide in the form of litharge or more rarely 

 of minium. 



Silver frequently occurs as chloro-bromide, less frequently as chloro- 

 iodide, and very rarely in the native state. Chemical investigation has 

 failed to detect sufficient regularity in the proportions of chlorine, bromine, 

 and iodine, combined with the silver, to justify the determination of distinct 

 mineral species. 



A frequent alteration product of mixed pyrite and galena, which occurs 

 in considerable quantity, associated with the ore bodies, is generally called 

 "basic ferric sulphate." It is an ocherous-looking substance of somewhat 

 uniform outward appearance, but of varying composition, being mainly a 

 mixture of jarosite, or yellow vitriol, and hydrated basic ferric sulphate, 

 with more or less anglesite and pyromorphite. 



Gold occurs in the native state, generally in extremely small flakes or 

 leaflets. It is also said to have been found in the filiform state in galena. 



As accessory minerals are: 



Zinc blende and silicate of zinc or calamine. 



Arsenic, probably as sulphide, and as arseniate of iron. 



