40 GEOLOGY AND MINING INDUSTEY OF LEADVILLE. 



General erosion. — It is HOW impossible to determine how much of the ero- 

 sion that has removed the ci-ests of these folds and denuded such large 

 masses of Archean rocks was accomplished earlier than the Glacial period, 

 bnt it is evident that the carving and shaping out of the valleys which score 

 the flanks of the range has been mainly accomplished since that time. It 

 is, moreover, not absolutely certain that this area was entirely covered by 

 later beds than the Triassic, since the only proofs that Jurassic and Cre- 

 taceous strata also extended over it conformably are founded on the fact that 

 no unconformability between Jura and Trias lias yet been observed here. 

 On the other hand, no opportunity was offered for a detailed study of the 

 relations of these two formations. That the beds of the Trias formed part 

 of the conformable series and were deposited along the shores of the Sa- 

 watch island is definitely proved, although they are no longer found within 

 the area of the map, by the fact that just beyond its limits to the north and 

 east, in tlie Ten-Mile and Mount Silverheels districts, respectively, they form 

 a continuous and conformable series with the Carboniferous beds, are folded 

 and faulted witli them, and carry the same intrusive sheets of eruptive rocks. 



Arkansas Valley erosion. — The manner and date of formation of the main 

 Arkansas Valley is a matter of interesting speculation. It is evident, as 

 has already been said, that it did not exist before the dynamic movements 

 which uplifted the Mosquito Range, and yet it must have already been a 

 deep valley at the commencement of the Glacial period, since a large lake 

 was formed in it daring the first melting of the ice of that period, in whose 

 bottom at least three liundred feet of sediments were deposited. Although 

 no beds of undoubted Tertiary age have yet been recognized in it which 

 would afford a definite date to reckon from, it is probable from structural 

 evidence that a line of depression was formed by the elevation of the Mos- 

 quito Range and the accompanying faulting, which corresponded approx- 

 imately with the present general direction of the valley. A new drain- 

 age system having thus been formed, the erosive agencies which have 

 carried away so many thousand feet of rocks from the range itself gradu- 

 ally deepened and enlarged this new depression, until it has now assumed 

 those majestic proportions that make it a topographical feature of scarcely 



