26 GEOLOGY AND MINING INDUSTRY OF LEADV1LLE. 



edges are pushed past and over each other, the movement of both folds and 

 faults showing that the force which produced them was acting from either 

 side toward the center of the original land masses. 



As contrasted with the Basin region west of the Wasatch uplift, the 

 folds of the Rocky Mountains show a greater plasticity in the sedimentary 

 strata by their relative sharpness, the anticlines and synclines in the former 

 having more gentle and equal slopes, while in the latter they often have 

 the form of an S, with one member almost bent under the other into an 

 isocline. 



Compared with the remarkably compressed folds of the Appalachians, on 

 the other hand, where the isocline may be considered the type structure, the 

 flexures of the Rocky Mountains show that the sedimentary rocks are far from 

 possessing the great plasticity and compressibility that they have intheformer. 

 The contrast between the eastern and western mountain systems, in respect 

 to the relative plasticity of their strata, is so marked that it would seem that 

 the reason therefor must be readily apparent. It is not that the beds in the 

 former are thinner ; on the contrary, the corresponding Paleozoic formations 

 are many times thicker in the Appalachians than in the Rocky Mountains. 

 It is to be* remarked, however, that in the former eruptive rocks are com 

 paratively rare, especially those of Mesozoic and Tertiary age, while in the 

 Rocky Mountains they are most abundant and in the western part of the 

 Basin region they form the greater part of the surface ; to this fact may 

 probably be ascribed, as will be shown later, the less plastic condition of 

 the earth's crust in the latter regions. 



In the character of these eruptive rocks, again, there is a marked con- 

 trast between the Rocky Mountains and the Basin region of Nevada. In 

 the latter they almost exclusively belong to the Tertiary volcanics, approach- 

 ing in character the lavas of modern volcanoes, the older and more crystal- 

 line varieties, corresponding to the Mesozoic porphyries of Europe, having 

 been rarely 'observed on the surface. In the Rocky Mountain region, on 

 the other hand, while the Tertiary eruptive rocks are often developed on a 

 very large scale, the earlier and more crystalline varieties seem to have an 

 equal and even greater importance, if not in the actual amount of surface 



