TEE BUILDING OF THE COLORADO ROCKIES 251 



building. This seems perhaps the more remarkable when it is 

 considered that any great disturbance of the outer portion of the 

 earth is likely to stimulate outward movement^ of Hquid magmas, 

 and in some cases through relief of pressure actually to cause 

 liquefaction and the development of magmas. 



As representatives of the thicker-shell type of mountains in 

 which vertical movements are more pronounced, and horizontal 

 thrusting and shortening less conspicuous, one might select in 

 addition to the Colorado Rockies, the Cascades on the Pacific 

 Coast,^ the great chains of Andes, particularly the Western Andes, 

 and the Abyssinian Mountains. The extravasation of vast floods 

 of lava marked the growth of these ranges. In the light of this 

 inquiry, it may be inferred that these mountains had deeper roots 

 than the more sharply folded ranges, and that this greater depth 

 of disturbance was an important factor in causing the greater 

 igneous outbursts. 



In final summation it may be remarked that within Colorado the 

 Rocky Mountain system is characterized by open, gentle folding, 

 moderate crustal shortening affecting a zone several scores of 

 miles in depth in its deeper portions, by strong uplifting, and by 

 the extrusion of much lava. Extending the range of view, it is to be 

 noted that this dominantly vertical diastrophism with its ample 

 outpourings of lava is typical of much of the western United States. 

 On the other hand, in northwesternmost Montana and throughout 

 Alberta, the ranges of the Rocky Mountain system exhibit low 

 angle thrust faulting on a grand scale, greater crustal shortening 

 which presumably has affected a thinner surface shell, and has 

 been accompanied by less volcanic activity. It thus appears that 

 a single mountain system of great length may develop both types 

 of deformation in different portions of its extent. 



' See Bailey Willis, " Physiography and Deformation of the Wenatchee-Chelan 

 District, Cascade Range," U. S. Geol. Surv., Prof. Paper ig (1903), pp. 95-97. 



