26 GEOLOGY AXD MINING INDUSTRY OF LEADYILLE. 



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

 faults showing that the force which ])roduced them was acting from either 

 side toward the center of the original land masses. 



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

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

 strata by their relative sliar[)ness, the anticlines and synclines in the former 

 having more gentle and equal slopes, while in the latter the}- 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 ty])e structure, the 

 flexures of the Kockv ^Mountains show that the sedimentary rocks are far from 

 possessing the great plasticity and compressibilitv that they have in thefonner. 

 'llie contrast between the eastern and western mountain systems, in i-espect 

 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 corresjionding Paleozoic formations 

 are many times thicker in the Ap|)alachians than in the Rocky Mountains. 

 It is to be remarked, however, that in the former eruj)tive rocks ai-e com- 

 parati\ely rare, especially those of Mesozoic and Tertiary ago, while in the 

 Rock^• 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 

 probabl}' be ascribed, as will be shown later, the less pla«tic 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 t\\e\ almost exclusively belong to the Tertiary volcanics, approach- 

 ing in character the lavas of modern volcanoes, the older and more cr\stal- 

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

 been rarely observed on the surlace. 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 



