HEAT. 



681 



"When the beds are inclined between 5° and 30°, and are alike in hardness, 

 there is a tendency to make hills with a long back slope and bold front; but 

 with a much larger dip, the rocks, if hard, often outcrop in naked ledges. 



When the dipping strata are of unequal hardness, and lie in folds, there is 

 a wide diversity in the results on the features of elevation. 



Figs. 956, 957 represent the effects from the erosion of a synclinal elevation 

 consisting of alternations of hard and soft strata. The protection of the 



Figs. 956-961. 



softer beds by the harder is well shown. This is still further exhibited in 

 figs. 958-961. 



Anticlinal strata give rise to another series of forms, in part the reverse of 

 the preceding, and equally varied. Figs. 962-965 represent some of the sim- 

 pler cases. When the back of an anticlinal mountain is divided (as in figs. 

 962, 963, 964), the mountain loses the anticlinal feature, and the parts are 



Figs. 962-965. 



ndWhi 



65 



simply monoclinal ridges. In fig. 965, the anticlinal character is distinct in 

 the central portion, while lost in the parts either side. In fig. 965 to the 

 right, a common effect is shown of the protection afforded to softer layers by 

 even a vertical layer of hard rock : the vertical layer forms the axis of a low 

 ridge. 



The above are the simple results from the erosion of folded rocks. They 

 serve as a key to the complexities of features common through a large part 

 of the Appalachians, where synclinal and anticlinal axes, as Lesley states, are 

 in numberless complicated combinations, and are rendered doubly puzzling 

 by faults. See, further, pages 104-109, 404-408. 



V. HEAT. 



1. SOURCES OF HEAT. 



The crust of the Earth has three sources of heat: — (1) The Sun ; 

 (2) Chemical and mechanical action ; (3) The igneous condition 

 of the Earth's interior. 



