540 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



which rendered it difficult to arrive definitely at the original form in which 

 they were laid down. When the mine workings shall have been extended 

 to such depths that the mineral is found practically sheltered from *the 

 secondary action of surface waters the study will be rendered more easy 

 and the results arrived at will be more certain. 



In this discussion it will not always be possible to follow the exact 

 order given in Chapter I, which was adopted rather for the purpose of 

 presenting a concise and readily comprehensible statement than as the 

 logical sequence of the processes actually involved in the ore deposition. 



MANNER OF OCCURRENCE. 



Perhaps the most suggestive fact in the manner of occurrence of the 

 Leadville ores is their predominance in the beds of the Lower Carbonifer- 

 ous or Blue Limestone, and one is naturally led to seek a cause for this 

 preference. It being admitted that the ores were deposited from aqueous 

 solutions, it is readily apparent that the more soluble limestone beds would 

 be more easily acted upon by these solutions than the other sedimentary 

 beds of the region, which consist mainly of sandstones and argillaceous 

 shales, and are much less susceptible to the action of percolating waters. 

 This, however, does not explain why the Blue Limestone should have been 

 chosen rather than any of the other calcareous beds of the region. 



There was a theory at one time prevalent among mining men in the 

 West that the great silver deposits in limestone were peculiar to a definite 

 geological horizon, a conception that perhaps had its origin in the tendency 

 of some geologists to generalize from the coincidence that certain classes 

 of deposits have been found in different parts of the world at the same 

 geological horizon. Indeed, some have gone so far as to base their deter- 

 mination of the horizon of certain beds, when other evidence was wanting, 

 on the occurrence in them of this class of deposits. The great silver 

 deposits in limestone of the western United States are, in point of fact, 

 found throughout the whole range of the Paleozoic system, and the hori- 

 zons of no two districts have as yet been proved to be absolutely identical. 

 As a case in point, the vast deposits in the Ten-Mile district, only 16 miles 

 from Leadville, occur in the Upper Carboniferous Limestones, and not in 



