364 PHYSICAL GEOLOGY 



Cycle of Erosion of Mountains. — In the process of time moun- 

 tains may be wholly reduced by erosion, and plains and plateaus be 

 formed in their place, which will have all the structural features of 

 folded mountains. Examples of such plateaus are to be found in 

 the Piedmont of Virginia, in New England, and elsewhere. 



In moist, tropical regions the luxuriant vegetation checks erosion, 

 with the result that the forms are less diversified than in other areas. 



Theories of Mountain Building 



Mountain chains are more conspicuous than plateaus because of 

 their narrow crests and great length in proportion to their width, 

 but when the heights of mountains and plateaus are compared, it 

 is found that many mountain ranges are relatively low as compared 

 with many plateaus. Portions of the Appalachian Mountains, for 

 example, are lower than portions of the Allegheny plateau only a 

 few miles away. The Tibet plateau is 15,000 to 16,000 feet high, 

 being higher than many of the great mountain chains of the world. 

 The highest of the Colorado plateaus (Aquarius) is 11,600 feet, and 

 that at Grand Canyon, Arizona, is 6000 to 8000 feet above the sea. 

 It is evident from the above that the cause of the elevation of the 

 less conspicuous but more massive plateaus is as important as that 

 of the more spectacular mountains. 



Cause of Lateral Pressure. — In the discussion of the interior 

 of the earth it was pointed out that the earth is composed of a hot 

 but solid core with a cool crust. The answer to the question, "What 

 produces lateral pressure ? " will be found to depend, to some degree, 

 upon this relation. 



The explanation often given for the crumpling of the earth's crust is that, as the 

 interior heat is lost very slowly by conduction, the crust wrinkles to accommodate itself 

 to the smaller interior. The comparison usually made is that of an apple which has 

 been left in a warm, dry room. Under these conditions the interior of the fruit loses 

 water by evaporation, while the dense skin shrinks but little and is wrinkled on the 

 contracted interior. The efficacy of this cause has been proved impossible on the 

 ground that the shrinkage of the interior of the earth has not been sufficient to produce 

 the lateral compression seen in the great folded tracts of the earth's surface, and in 

 proof of this contention it is pointed out that during a single era of the earth's history 

 (Paleozoic, p. 477) the folding of the earth's crust resulted in a shortening of between 

 100 and 200 miles. Since a lateral shortening of six miles of the crust is produced by 

 one mile of radial shortening, it follows that a minimum estimate would require a radial 

 shortening of 16 miles, and a maximum, one of 32 miles. 



