290 GEOLOGY AND MINING INDUSTRY OF LEADVILLE, 



ing abruptly along given lines by monoclinal flexures and faults to lower 

 levels, to be more frequently the result of subsidence, while the movements 

 in the adjoining mountain masses were probably more often true move- 

 ments of elevation. 



The one-sided or p -shaped fold. — In the above remarks considerable stress has 

 been laid upon the one-sided or S-fold and its frequently associated faulting, 

 because it seems to be the extreme development of the most common form 

 of plication throughout the Rocky Mountains and the region of the Great 

 Basin. In the latter region it often happens that only one side of the fold 

 protrudes above the Quaternary deposits, wliicli cover the greater portion of 

 its surface, so that the narrow mountain ridges present only a monoclinal 

 slope. For this reason the structure of what is called the Basin province 

 has been characterized as a region of faulted blocks uptilted in different 

 directions and practically without plication. 



I dissent from this reading of the geological structure, first, because 

 my own observations in the region mentioned have shown many unmistak- 

 able instances of the above-mentioned structure, in which it is true the 

 flexing is often gentle, but nevertheless a true plication, and which have led 

 me to believe by analogy that in other cases, could the structure beneath 

 the valleys be seen, the missing faulted-down members of the fold would 

 be found ; secondly, because the diversely tilted blocks which are given in 

 the sections involve what seems to me to be a geological impossibility, or at 

 least one which is not yet found possible by observation, namely, the actual 

 annihilation of considerable wedge-shaped segments of stratified beds by 

 the simple action of faulting. Even in the Uinta Range, which differs from 

 the Rocky Mountain ranges in that it is the truncation of a complete arch 

 of sedimentary strata, with no evident pre-existing elevation beneath it, the 

 anticlinal fold has the one-sided structure. The axis of this range is along 

 the northern edge of the uplift; to the south of the axis the beds descend 

 in gentle slopes; to the north they dip steeply at angles of 40° or 50°, and 

 are partly faulted along this steep side. On a line with this steep side, at 

 the eastern end of the range, is a submerged ridge of Arcliean, whose resist- 

 ance, as I have already suggested,^ probably caused the sharper and more 



'Exploration of the Fortieth Parallel, Vol. II, Descv. Gool., p. 201. Washington, 1877. 



