CHAPTER XIII 



ACTION OF WATER ON THE LAND 



305. Land Sculpture. The crests of the world ridges 

 upheaved by the internal energy of the cooling Earth in gently 

 undulating strata, or in the sharp broken anticlines of moun- 

 tain ranges, are subjected to erosion by solar energy acting 

 through various agencies. Earth energy is continually at 

 work raising the level of the elevated half of the globe, and 

 depressing the Abysmal Area. Sun energy acts as a leveller, 

 continually cutting down the high places and building up the 

 hollows with the resulting detritus or crushed fragments. The 

 process of uncovering old rocks by erosion of newer ones is 

 termed denudation. The rate at which it proceeds depends 

 to a very large extent on the chemical composition of the 

 rocks, on their tenacity, their dip, and joints ( 290), and 

 it is to the variety of these conditions that the* great variety 

 and character of the existing scenery of every part of the 

 world is due. 



306. Work of direct Sun-heat. One unit of heat (65) 

 when absorbed by one pound of an average rock raises 

 its temperature about 4, compared with i in the case of 

 water. In consequence of this low specific heat, although 

 the heat does not penetrate far ( 291), it greatly heats and 

 expands the superficial layer. At night the temperature 

 falls quickly by radiation and the chilled rock con- 

 tracts. In dry tropical regions the alternate heating and 

 chilling causes the surface layers to split off in angular 

 pieces or thin sheets, which, when the face of the rock is 



