284 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



been derived from a limestone bed relatively free from quartz, to a less com- 

 pletely altered coiidition, where it might have been seen whether there had 

 been a previous formation of amphibole. In the dark-green rock, however? 

 there seems little doubt that the serpentines are derived from silicates. 



With regard to the formation of amphibole and pyroxene, their distri- 

 bution seems wider and more even, and the question presents itself whether 

 they have been formed in situ by a slow process of metamorphism pre- 

 ceding the appearance of the eruptive rocks, or after this period and imme- 

 diately preceding that of the serpentine, or, again, whether they are simply 

 derived from the Archean rocks mechanically. In favor of the last sup- 

 position is the fact, observed by Mi-. Cross in one specimen, that the amphi- 

 bole penetrates the quartz grains and is sometimes entirely encircled in 

 them, and that these latter contain fluid inclusions with moving bubble. 



STRUCTURAL FEATURES. 



The most striking features in the geological structure of this region 

 are the forms of the folds and the close relation between them and the 

 great faults which traverse it from north to south. 



Folds and faults. — The typical form of the former is what has been called 

 the S-fold, in which the anticline has a steep and almost vertical face to 

 the Avest, or towards the original land mass of the Sawatch, and a gentle 

 slope to the east, while in the adjoining syncline the conditions are re- 

 versed, and the gently rising slope is to the west. This is the most natural 

 form of fold which would result from the supposed cause of uplift of the 

 range, namel}', a horizontal thrust of the beds against the Archean mass of 

 the Sawatch. In a fold produced in this way the line of greatest tension, 

 and where the tendency to fracturing and displacement would be greatest, 

 is, as shown by Daubrde's well-known experiments,^ along this steep side 

 of the fold, and in point of fact it was found that along this line occur the 

 great strike-faults of the range. 



By reference to the sheets of sections (Atlas Sheets VIII and IX) it will 

 be seen that the great Mosquito fault, which extends for an unknown dis- 

 tance beyond the northern limits of the map, and its two southern branches, 

 the. London and Weston faults, fulfill in the main these theoretical conditions. 



'A. Daubr^e: Geologic Exp6rimentale, p. 321. Paris, 1879. 



