18 



PHYSIOGRAPHIC GEOLOGY. 



types of chains : (1) the broad and lofty plateau type ; (2) the narrow 



and lofty ridgy type, of which the Himalayas are another example ; 



(3) the broad and many-folded type, of which the Juras are another 



example. 



Illustrations. — It is common to err in estimating the angle of a slope. To the 

 eyes of most travellers, a 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 pro- 

 file view should always be obtained and carefully observed before registering an opinion. 



Fig. 3. 



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. 



Fig. 6. 



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 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 maybe easily calculated. 

 The ratio 1 : 1 corresponds to the angle 45°; 1 : 2 to 33° 41^' ; 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° 7£'; 1 : 9 to 6° 201'; 1:10 

 to 5° 42|'; 1: 12 to 4' 46'; 1: 15 to 3° 49'; 1:20 to 2° 52'. The inclinations correspond- 

 ing to several of these ratios are represented in the following cut. (Fig, 10.) 



