CHAPTER VI. 



DENUDATION, OB GENERAL EROSION. 



Definition. Denudation is a term used to designate 

 the aggregate results of all erosive agents. Its correla- 

 tive is sedimentation. In the preceding pages we have 

 given the effects of erosion in many individual cases ; but 

 some general idea of the amount which has taken place, 

 under the action of all agents throughout all geological 

 times, and some very general estimate of geological time 

 based thereon, seem important, as a fitting preparation 

 for Part III, or Historical Geology, which deals especially 

 with time. 



Agents of Erosion. The possible agents of erosion 

 are 1. Rain and rivers. 2. Snow and ice. 3. Waves 

 and tides. 4. Oceanic currents. Of erosion by the last, 

 we have no observation. Oceanic currents run on a bed 

 and between banks of still water, and therefore produce 

 no erosion (page 48). We may probably leave them out 

 of account. Waves and tides are very powerful erosive 

 agents, but their action is confined wholly to the shore- 

 line. It has been estimated that, though so conspicuous, 

 their aggregate effect is certainly less than one fifth that 

 of rain and rivers. Snow is but a different form of rain, 

 and glaciers a different form of rivers ; therefore, in so 

 rough an estimate as we are about to make, we may safely 

 base our estimate upon the action of rain and rivers. 

 Our object, then, will be to give some very general idea 

 of the amount of denudation which has taken place in 

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