DENUDATION, OR GENERAL EROSION. 273 



CHAPTER VI. 



DENUDATION, OR GENERAL EROSION. 



As a fit ending of Part II, and preparation for Part III, in which 

 the idea of time is the underlying element, it seems appropriate to 

 make some rough estimate of the amount of general erosion which has 

 taken place in the history of the earth, and of geological time based 

 thereon. 



The term denudation is used by geologists to express the general 

 erosion which the earth-surface has suffered in geological times. The 

 correlative of denudation is sedimentation, and the amount of denuda- 

 tion is measured by the amount of stratified rocks. 



Agents of Denudation. — The agents of erosion, as we have already 

 seen in Part I, are : 1. Rivers, including under this head the whole 

 course of rainfall on its way back to the ocean whence it came ; 2. 

 Glaciers, including under this head not only glaciers proper, but moving 

 ice-sheets, such as now exist in polar regions, and in the Glacial epoch 

 extended far into now temperate regions, and also moving snow-fields, 

 for it is probable that all extensive snow-fields and snow-caps are in 

 motion ; 3. Waves and tides ; and, possibly, 4. Oceanic currents. 



Oceanic currents usually run on a led and between hanks of still 

 water, and therefore produce no erosion. It is possible, however, that 

 a rising sea-bottom may be eroded by this agent ; but as we have no 

 knowledge of such effects, we are compelled to omit this from our esti- 

 mate of the probable rate of denudation. The action of waves and tides 

 is violent and conspicuous ; yet these agents are so entirely confined to 

 the shore-line that their aggregate effect is but a small fraction of the 

 whole erosion. Prof. Phillips has shown * that, taking the coast-lines 

 of the world as 100,000 miles, and making the extravagant estimate 

 that the average erosion along this whole line is equal to that of the 

 English coast, or one foot per annum of a cliff one hundred feet high, 

 still the aggregate wave-erosion is far less than river-erosion, being 

 equivalent to a general land- surface erosion of only 2 }^ of an inch 

 per annum, or -^ of that which is now going on over the hydrographi- 

 cal basin of the Ganges, and \ of that going on in the basin of the 

 Mississippi. Glaciers and rivers, therefore, are the great agents of ero- 

 sion. The one takes the place of the other, according as falling water 

 takes the form of rain or snow ; both come under the general head of 

 circulating meteoric water. In a general estimate of the rate of denu- 

 dation we may, therefore, without sensible error, regard it as the work 

 of circulating meteoric water. 



* Life, its Origin, and Succession, p. 130. 

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