420 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



ores, except in certain limited areas, do not contain an appreciable 

 amount of this metal. . 



The geological structure of this district is extremely complicated. 

 Its principal mines are situated on one of the western spurs of the 

 Mosquito range, which range, as has already been explained, is a por- 

 tion of the Sawatch massive, uplifted into its present position by fault- 

 ing. This faulting was not, however, a simple uplifting of the strata 

 wit hont plication or compression, as has been described as the charac- 

 teristic Structure of the Plateau region. It was, on the contrary, pri- 

 marily a plication produced by powerful compression against the 

 unyielding Archean massive (Horst, Butoir) of the Sawatch. Faulting 

 has been the final result, where the limit of plasticity of the involved 

 rock masses had been reached. The relative plasticity of different por- 

 tions of these rock masses appears to have borne an inverse relation to 

 the amount of intrusive beds and laccolith- masses, which already formed 

 an integral part of the sedimentary beds involved before the inception 

 of the plication. Where these were only in thin sheets, as on the east 

 em thinks of this range, sharp anticlinal folds, generally much Bteeper 

 on the western side, were formed before actual fracturing and displace- 

 ment took place. The faulting in such cases occurred on the steeper 

 side of the fold, where the tension was greatest. With a relatively 

 greater proportion of eruptive rocks to sedimentary strata, plication 

 was less marked and faulting more frequent. In the immediate vicinity 

 of Leadville, where the eruptive sheets greatly exceed in volume the 

 sedimentary beds above the Archean, with which they are practically 

 interstratitied, faulting lias been a much larger factor in the uplift than 

 plication, and the spur in which the principal ore deposits occur con- 

 sists of a number of blocks, each faulted up to the eastward above the 

 other, and traceable in the present topography as shoulders or hills. 

 These faulted blocks are not, however, as might appear at a first glance, 

 simple uplifted monoclinals, but form part of a system of synclines and 

 anticlines, whose axes bear a definite, though sometimes rather obscure, 

 relation to the planes of the various faults. 



The prominent phases in the geological history of the region, as far 

 as they have yet been deciphered, are — 



1. Successive intrusions of porphyry, the earliest of which (White 

 porphyry) was generally parallel to the stratification, spreading out in 

 places into laccolitic bodies. The later intrusions (Gray porphyry) 

 were more frequently tranverse to the bedding, and formed thinner 

 and more dike like bodies, probably following to some extent planes of 

 fracture produced by the shattering attendant upon the earlier intru- 

 sions. 



2. During the second phase occurred the orignal ore deposition, 

 which was a concentration of the metals disseminated through the rock 



