20 GEOLOGY AND MINING INDUSTRY OF LBADVILLE. 



Eastern uplift. The Colorado or Front Kange is the move extensive and 

 more important of the two Archean uplifts, and along its eastern flanks is 

 exposed, by the denudation of the overlying Tertiary formations, an almost 

 continuous fringe of upturned Paleozoic and Mesozoic beds. 



The most significant geological fact to be observed in connection with 

 these exposures of upturned beds is that the formation which is immediately 

 adjacent to the Archean varies from place to place. At one point Triassic 

 beds, sloping away at varying angles from the flanks of the mountain, rest 

 directly upon the Archean beds; at another point the lower beds of the Cre- 

 taceous; at still another, and this more rarely, the Carboniferous limestones 

 are exposed resting against the Archean, while above them, always con- 

 formable, are found the Triassic, Jurassic, and Cretaceous formations as 

 one follows the section in an ascending geological sense. At one or two 

 points only along the eastern flanks Silurian beds are exposed beneath 

 the Carboniferous. 



It has been customary with many of the early geological explorers 

 to consider the uplift of these mountain ranges to be that of a simple anti- 

 clinal fold in the sedimentary strata, which once arched over the underlying 

 nucleus of crystalline rocks; this was once considered the typical structure 

 of a mountain range. In practical field geology, however, it is found that 

 the symmetrical form resulting from this typical structure of mountain 

 range is one of the rarest occurrences, at least in the Rocky Mountain 

 region.' The one great instance of such a perfect anticlinal range is that ot 

 the Uinta Mountains, which presents exceptional features distinguishing it 

 from the majority of mountain ridges of the Rocky Mountain system ; this 

 has a peculiarly normal anticlinal structure in the first place, and in the 

 second place its trend is east and west, whereas all the other great mount- 

 ain ridges of the Cordilleran system have a direction varying between north 

 and south and northwest and southeast. 



The facts just noticed with regard to the sedimentary beds which rest 

 against the eastern flunks of the Rocky Mountains, it will be readily seen, 

 exclude the possibility of the typical anticlinal structure above mentioned. 

 If we suppose a conformable series of sedimentary beds to have been folded 

 into a long anticlinal fold and the crest of this fold subsequently planed 



