32 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



cessive beda varying from 50 feet to 200 feet in thickness are now found 

 intercalated between alternate strata to the number of 15 or 20 in a single 

 section — nuist necessarily have produced great irregularities in the once 

 level surface of the then existing crust: but these irregularities were largely 

 obliterated by the dynamic movements Avhich followed, and the only traces 

 still remaining are variations in the strike of the inclosing beds, which show 

 a tendency to curve around any concentration of eruptive masses. 



At some time during the long period which intervened between the 

 final deposition of the latest sediments of the Cretaceous ejiocli and the 

 succeeding deposition of Tertiary strata, and during which the waters of 

 the ocean gradually receded from the Kocky Mountain region, the pent-up 

 energy of the force of contraction of the earth's crust, which had accumu- 

 lated during ages of comparative geological trancpiillity, found expression in 

 intense and ])rolonged dynamic movements of the rocky strata forming the 

 immediate crust of the earth in this region. These dynamic movements in 

 their simplest form may be conceived as a pushing together from the east 

 and from the west of the more recent stratified rocks against the relatively 

 rioid mass of the already existing Archean land masses, and a consequent 

 folding or crumpling of the beds in the vicinity of , the shore-lines, where, 

 owing to the break in the continuity of the strata and the more irregular 

 character of the floor upon which they rested, the conditions were more 

 favorable to the crumpling movement than they would be, for instance, in 

 the open plains, where a great thickness of level and hithei-to undisturbed 

 beds offers no lines of weakness to favor a commencement of folding. It is 

 liere a (piestion only of the movement of the distinctly stratified beds, 

 because it is in these alone that the resulting flexures can be accurately 

 studied and mapped out; but it is evident that the crystalline and already 

 violently contorted beds which formed the Archean land masses must have 

 also partaken in the resulting movements, and their axial regions have been 

 lifted i.p to a great elevation, of which the present height of the culminating 

 peaks of the Rocky Mountains, formed as they are in the majority of cases 

 exclusively of Archean rocks, is only a ver}- uuich modified expression. 

 Contemporaneously with the east and west movements (the expression of 

 the major force of contraction in this region), there acted also a minor force 



