18 



PHYSIOGRAPHIC GEOLOGY. 



slope of 60° appears to be as steep as 80°, and one of 30° 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 registering 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°. 



Fig. 4. 



T 



Fig. 



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

 7°, — the average slope and form of Mount Kea, Hawaii ; 6, the same slope with 



Fig. 7. Fig. 8. Fig. 9. 



the top rounded, as in Mount Loa; 7, a slope of 15 ; 8, Jorullo, in Mexico, 

 which has one side 27° and the other 34°, as measured by N. S. Manross ; 9, a 

 slope of 40°, — the steepest of volcanic cones. The lofty volcanoes of the Andes 

 are not steeper than in number 8, although frequently so pictured. 



"With a clinometer (see fig. 102) held between the eye and the mountain, 

 the angle of slope may be approximately measured. When no instrument is at 

 hand, it is easy to estimate with the eye the number of times a vertical as A B 

 in fig. 5 is contained in the semi-base B C ; and, this being ascertained, the angle 

 of slope may be easily calculated. The ratio 1 : 1 corresponds to the angle of 

 45° ; 1 : 2 to 33° 41V ', 1 : 3 to 26° 34' ; 1 : 4 to 18° 26' ; 1 : 5 to 11° 18£' ; 1 : 6 to 

 9° 28' ; 1:7 to 8° 8' ; 1:8 to 7° 7i' ; 1:9 to 6° 20£' ; 1 : 10 to 5° 42J' ; 1 : 12 to 

 4° 46'; 1:15 to 3° 49' ; 1 : 20 to 2° 52'. The inclinations corresponding to 

 several of these ratios are represented in the following cut. Fig. 10. 



