KMMoNs] THE ROCKY MOUNTAINS OF COLORADO. 4()5 



Mesozoic beds dipping away from it at various angles. These beds 

 give internal evidence of having been deposited around a land mass, 

 and from their present position it is evident that theeentral Archean 

 area was never completly covered by them. They also show little or 

 no discrepancy in their angle of dip. 



The Elk mountain massive, which lies immediately to the west of the 

 Sawatch, is a region of intense disturbance. The sedimentary beds 

 arc profoundly plicated and faulted; and, fchongh the peaks areas high 

 as those of the Sawatch, and the intermediate valleys even more pro- 

 foundly eroded, only an extremely limited area of Archean is exposed. 

 Many of the higher peaks are immense masses of diorite, up to L5 miles 

 in diameter, which have been protruded through the sedimentary beds 

 daring the post-Cretaceous movement. The evidence of the various 

 transgressions, which is remarkably distinct in these mountains, is 

 found in discrepancies of strike rather than of dip. Thus, where the 

 upturned sedimentary series is crossed by the train, on the Grand river 

 above Newcastle, the strata seem perfectly conformable from Laramie 

 Cretaceous down to Cambrian. Yet the sandstone series of the Dakota 

 Cretaceous (which includes also some beds of probable late Jurassic 

 age, hence sometimes called Jura-Dakota sandstones), if followed south- 

 ward along their strike into the Elk mountains, would be found to rest 

 on successively lower beds from Paleozoic to Archean. Besides the 

 diorite masses in the more disturbed central region, numerous pictur- 

 esque mountain masses are formed by laccolites of quartz-porphyry in 

 the nearly horizontal Cretaceous strata of the southern and western 

 portion of this remarkable group of mountains. On the east, between 

 the Elk mountains and the Sawatch. is a zone of profound faulting, 

 following north and south lines. 



To the east of the Sawatch mountains, and separated from them by 

 the great longitudinal valley of the Upper Arkansas, lies the Mosquito 

 Or Park range, which once formed an integral pari of the Sawatch and 

 has been uplifted into its present position by plication and faulting, 

 which was initiated during the late Jurassic movement. The fault fol- 

 lows a north and south line just west of the main crest, and has been 

 traced for over 50 miles. The Upper Arkansas valley is thus a com- 

 paratively modern topographical feature. 



The Wide valley Of South park, on the other hand, which lies between 

 the Mosquito range and the broad Archean mass of the Front or 

 Colorado range, is underlaid by the beds of the entire Mesozoic and 

 Paleozoic groups, which descend with decreasing angle of dip from the 

 crest of the former and disappear beneath the recent beds which form 

 its present floor. They are probably cut off by a fault on its eastern 

 edge, but this does not appear farther south, on the line traversed by 

 the train along the valley of the Lower Arkansas. 



