THE EARTH S CONTOUR AND SURFACE SUBDIVISIONS. 



27 



In the Sierra Nevada, the western (or gentler) slope is between 100 and 

 250 feet to the mile, and the eastern, for a larger part of its length, 1000 

 feet. In the Andes the eastern slope is abont 60 feet in a mile, and the 

 western 100 to 150 feet; the passes are at heights from 12,500 to 16,160 

 feet, and the highest peak — Sorata in Bolivia — 25,290 feet. The slope is 

 much more rapid than in the Rocky Mountains. But there is the same kind 

 of mountain mass variously diversified with ridges and plateaus. The exist- 

 ence of the great mountain mass and its plateaus is directly connected with 

 the existence of the main ridges. But it will be shown in another place that 

 the ridges may have existed long before the mass had its present elevation 

 above the sea. 



In the Appalachians the mountain mass is very much smaller, and the 

 component ridges are relatively more distinct and numerous ; and still the 

 general features are on the same principle. The greatest height — Mount 

 Mitchell or Black Dome in North Carolina — is 6707 feet. 



3. 



It is common to err in estimating the angle of a slope. To the eyes of most travelers, 

 a slope of 60° appears to be as steep as 80", and one of 80° to be at least 50°. In a front 



view of a declivity it is not possible to judge 

 rightly. A profile view should always be 

 obtained and carefully observed before regis- 

 tering an opinion. 



In Fig. 3 the bluff front facing the left 



would be ordinarily called a vertical precipice, 



while its angle of slope is actually about 



65° ; and the talus of broken stones at its base would seem at first sight to be 60°, 



when really 40°. 



V 



Fig. 4 represents a section of a volcanic mountain 3° in angle ; Fig. 5, another, of 7°, — 

 the average slope and form of Mount Kea, Hawaii ; Fig. 6, the same slope with the top 



7. 



8. 



rounded, as in Mount Loa ; Fig. 7, a slope of 15° ; Fig. 8, Jorullo, in Mexico, which has 

 one side 27° and the other 34°, as measured by N. S. Manross ; Fig. 9, a slope of 40°, — 



