308 ELEMENTARY GEOLOGY 
and valleys, form cordilleras. ‘The Cordilleras of the 
West, which include the Rocky Mountains, the Basin 
Ranges, the Sierra Nevada, and Coast Ranges, and 
some other minor systeins, are typical illustrations. 
As mountains are exposed to denudation, if not grow- 
ing, they are gradually worn down, being first cut into 
rough and rugged outlines, and later reduced to rounded 
Fig. 181. 
The Appalachian Mountain system and the bottom of the Atlantic near the 
eastern coast. (A reduction by photograph of a part of Ward’s model.) 
and gently sloping forms. As the wearing process con- 
tinues, the harder rocks stand up as ridges or peaks ; 
and so, to take two examples from opposite sides of © 
the continent, we have the White Mountains of New 
‘Hampshire and Pike’s Peak of Colorado, located where 
granitic masses have been better able to resist the 
effects of denudation than have the surrounding rocks. 
