THE BUILDING OF THE COLORADO ROCKIES 249 



ening and intense horizontal thrusting. If the active crumphng in 

 a range Hke the Alps were not confined to a very thin shell the result- 

 ing vertical bulge would be stupendous. 



Those ranges in which a much thicker shell has yielded to the 

 stresses are characterized by open, gentle folding, only moderate 

 shortening, but more pronounced vertical hfting. The Colorado 

 Rockies belong to this group. Moderate folding distributed 

 through the thicker zone accomplishes the same amount of vertical 

 bulging as greater shortening of a thinner zone. Shearing along 

 a few well-defined planes may well be a less important means of 

 adjustment beneath the thicker deformed zone than beneath the 

 thinner one. Adjustment in the thicker zone is probably due to 

 gradual accommodation by the slow dying out of folds below, with 

 perhaps more or less local, distributive shearing. Upward yielding 

 in the direction of least resistance being relatively more conspicuous 

 than horizontal movement when a thick shell is deformed, this 

 type of mountain building crowds closely on the borders of the 

 plateau-forming type of diastrophism. The western portion of 

 the Lyons-Grand River section, from State Bridge to the Grand 

 Hogback, where the disturbed zone had a thickness rising from 81 

 to 107 miles, portrays what was essentially a plateau uplift, and 

 suggests the actual merging of this mountain type into the plateau 

 type. 



An incidental feature of some significance is the bordering thrust 

 fault observed in several instances on the inland margin of a strongly 

 deformed mountain belt farthest removed from the oceanic segment 

 from which the thrusting presumably has come. 



Since the Laramide disturbance the Colorado Rocky Mountain 

 region has been the scene of much vulcanism. The products of 

 this, in places, have obscured the folded strata beneath, and thus 

 have added materially to the difficulty of making an accurate 

 cross-section of the mountainous belt. In general, Tertiary vul- 

 canism has been a prevailing phenomenon throughout the Rockies 

 from Montana to New Mexico. In the region of the present Appa- 

 lachian Mountains, on the other hand, there has been very little 

 vulcanism. There were the Upper Triassic lavas of the Newark 

 series, to be sure, but they were east of the present mountains and, 



