BRIEF OUTLINE OF RESULTS. 



GEOLOGY. 



The Mosquito Range, the study of whose geological structure formed a necessary basis for that of 

 the ore deposits of the Leadville region, is the western boundary of the South Park, and has thus been 

 considered from a topographical standpoint to form part of the Park Range. Geology shows, however, 

 that in Paleozoic times the boundaries of the depressions now known as the Parks were formed by the 

 Archean land masses of the Colorado Range on the east and of the Sawatch and its continuation to the 

 north, the Park Range on the west, and that the uplift of the Mosquito Range did not occur until the 

 close of the Cretaceous. 



Prior to this uplift the various porphyry bodies, which now form a prominent feature among the 

 rock formations of the region, were intruded into the sedimentary beds deposited during Paleozoic and 

 Mesozoic times, spreading out between the beds and sometimes crossing them, but being most uniformly 

 distributed at the top of the Lower Carboniferous or Blue Limestone. It was in this limestone that the 

 greater part of the ores were deposited, and the original deposition must have taken place after the 

 intrusion of the porphyry aud before the uplift of the range. 



In the uplift of the range both eruptive sheets and sedimentary beds, with the included ore 

 deposits, were plicated and faulted, aud by subsequent erosion an immense thickness of rocks has been 

 carried away, laying bare the very lowest rocks in the conformable series; the outcrops are, however, 

 frequently buried beneath what is locally called " wash," a detrital formation of glacial origin. In the 

 Leadville region, owing to the reduplication caused by faulting, a series of outcrops of easterly dipping 

 beds of the Blue Limestone are exposed beneath the wash, of which all are metalliferous and a consid- 

 erable proportion carry pay ore. 



ORE DEPOSITS. 



The principal ore deposits of Leadville occur, as above indicated, in the Blue Limestone and at or 

 near its contact with the overlying bodies of porphyry. The ores consist mainly of carbonate of lead, 

 chloride of silver, and argentiferous galena, in a gangue of silica and clay, with oxides of irou and 

 manganese and some barite. These materials are mainly of secondary origin, nnd result from the altera- 

 tion by surface waters of metallic sulphides. 



The study of these deposits has shown : 1, that they were originally deposited as sulphides, and 

 probably as a mixture, in varying proportions, of galena, pyrite, and blende; 2, that they were de- 

 posited from aqueous solutions ; 3, that the process of deposition was a metasomatic iuterchaugi 1 be- 

 tween the materials brought in by the solutions and those forming the country rocks, consequently 

 that they do not fill pre-existing cavities; 4, that the ore currents from which they were deposited 

 did' not come directly from below, but were more probably descending currents ; and 5, that these 

 currents probably derived the material of which the ore deposits are formed mainly from the por- 

 phyry bodies which occur at horizons above the Blue Limestone. 



PRACTICAL CONSIDERATIONS. 



Inasmuch as the ore currents did not come directly from below, it is not advisable to search for 

 ore below the Blue Limestone horizon. This horizon, however, should be thoroughly prospected, and 

 the maps and sections show its probable position in the as yet unexplored areas ; the explorations, 

 moreover, should not be confined to the upper surface of this limestone, but carried into its mass 

 wherever there are indications of ore, and especially along the contact of transverse bodies of Gray 

 Porphyry. The probabilities are that very considerable bodies of ore remain as yet undiscovered, and 

 the most promising areas for prospecting are indicated. It is also probable that as the distance from 

 the surface increases the ores will be found less altered, and that they will therefore be lesseasily 

 reduced by the smelting processes now employed. 



The petrography of the district is treated by Mr. Whitman Cross in Appendix A. The results of 

 chemical investigation aud the methods of research are given in Appendix B by Mr. W. P. Hillebrand, 

 and in Appendix C Mr. Guynrd has given a memoir on lead smelting as conducted at Leadville, show- 

 ing the character of the plant, the composition of ores, fluxes, aud furnace products, aud discussing the 

 reactions which take place in the blast furnaces. 



XXIX 



