378 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



These associations have already been remarked in other mining dis- 

 tricts. 



Secondary alteration. Here, as elsewhere, the ores found near the surface 

 are mostly oxidized or chloridized ores, and those farther removed from it, 

 or comparatively unexposed to the direct action of surface waters, are 

 mostly sulphides. It may be observed, moreover, that the zone of sec- 

 ondary deposition, or that in which oxidized ores predominate over sul- 

 phides, varies in the depth to which it extends with the relative altitude of 

 the deposit; or that in higher altitudes, where surface waters are imprisoned 

 by frost during a larger portion of the year, the proportion of secondary 

 products is less. 



There is a contrast in this respect, however, between the deposits of 

 Leadville and those of the more arid regions of the Great Basin. In the 

 latter the surface zone, or zone of oxidation, is generally more sharply 

 defined and extends down to what is known as the water level. This con- 

 trast is more apparent than real, for the zone of oxidation is there dry, be- 

 cause of the limited atmospheric precipitation, and in Leadville generally 

 wet, partly because of the relatively great precipitation and partly because 

 of the peculiar geological position of the deposits, which renders them more 

 accessible to surface waters. The alteration of the ore deposits is produced, 

 not by the water alone, but by the atmospheric agents which it brings from 

 the surface with it; whereas in the case of deposits below the water level 

 the water which reaches them, not coming directly from the surface, but 

 through a relatively long underground passage, has during that passage 

 been deprived of these active agents of oxidation or neutralized. 



Mode of formation. From the present investigation it has been assumed, 

 with regard to the mode of formation of these deposits: 



I. That they were deposited from aqueous solutions. 



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

 III. That the process of deposition teas a metasomatic interchange with the material 

 of the rock in ichich they icere deposited. That is, that the material of which they were 

 composed was not a deposit in a pre-existing cavity iu the rock, but that the solu- 

 tions which carried them gradually dissolved out the original rock material and left 

 the ore or vein material in its place. 



IV. That the mineral solutions or ore currents concentrated along natural icater 

 channels and followed by preference the bedding planes at a certain neohf/ical horizon, but 

 that the;/ also penetrated the adjoining rocks through croxx joint* and clcaragu planex. 



