41 



waves. Ice can do little work unless the rocks have 

 been previously ^affected by joints or other divisions. 

 The sun acts in combination with all the other denu- 

 dants; and in a greater or less degree, so do cold, heat, 

 wind, rain, rivers, and chemical action. The opera- 

 tions of the seven last-named agents are so intimately 

 connected, that in general they are classed together 

 under the names of subcerial or atmospheric denudation 

 and meteoric abrasion; but the individual work of each 

 is peculiar, and a short description of them will be 

 given hereafter. Although the names subserial and 

 atmospheric denudation are very generally used, 

 yet neither seems unobjectionable ; as, strictly speak- 

 ing, all denudation is subcerial, and all is more or less 

 directly due to atmospheric influences. Difference of 

 temperature causes the wind, the oceanic currents, 

 and the glaciers, also the evaporation that eventually 

 forms the rain and rivers. 



It should be borne in mind that ice and the sea as 

 agents have this relationship : they are, for the most 

 part, mechanical workers dislodging masses of rock, 

 which are by the former ground up against one 

 another, or against the sides and bottoms of the 

 valleys occupied by glaciers ; and by the latter broken, 

 by being rolled over one another. Meteoric abrasion, 

 on the other hand, ordinarily only disintegrates rock 

 surfaces, preparing matter to be carried away by the 

 sea, ice, wind, or rain and rivers. 



