SECTION II 



SURFACE AGENCIES 



The surface or superficial agents are those which act upon or 

 very near the surface of the ground and all of which are mani- 

 festations of solar energy. Their work may all be summed up in 

 two categories, the destruction and reconstruction of rock. These 

 two processes are complementary ; for since matter cannot be 

 destroyed, but can have only its position and its physical and 

 chemical relations changed, it is obvious that what is removed in 

 one place must be laid down in another. Thus, neither of these 

 processes can go on without the other, and reconstruction always 

 implies antecedent destruction to furnish the materials. Every- 

 where we find ceaseless cycles of change in progress, new combi- 

 nations of material being continually formed and older rocks 

 steadily worked over into newer. It is this circulation of matter 

 upon and within the crust of the earth that we have already com- 

 pared to the physiological changes in the body of a living organism. 



It is important to remember that the processes of rock destruc- 

 tion, which are grouped together under the general name of 

 denudation or erosion, are confined almost entirely to the land 

 surfaces, while those of reconstruction take place principally 

 beneath bodies of water. Some work of reconstruction is also 

 accomplished on the land, but this is of very minor importance. 



Since destruction necessarily precedes reconstruction, the con- 

 sideration of these processes will naturally begin with those of 

 destruction or denudation. The destructive agencies are : (i) the 

 atmosphere, (2) running water, (3) ice, (4) lakes, (5) the sea, 

 (6) plants and animals. 



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