Fluviatile and Marine Gravels. 489 



foot a year, by Keade,* and 1/3500 of a foot a year by Salis- 

 bury.! By using the latter figure, and assuming that the con- 

 ditions within the Mississippi basin are representative for the 

 55,000,000 square miles of the earth's lands, 300 cubic miles of 

 land are removed each century by subaerial denudation. The 

 estimate of R. B. Dole, Chemist of the U. S. Geological Sur- 

 vey, that "the surface of the United States is now being 

 removed at the rate of 0'06 cubic miles a year,"J is probably a 

 fair representation of existing conditions, since the estimate 

 is based on a study of all the rivers of the United States. If 

 this rate be applied to the entire earth's surface, the figure 

 becomes in round numbers, 110 cubic miles a century. The 

 estimate of Murray§ is 370 cubic miles a century. The dis- 

 crepancy between these estimates is to a certain extent the 

 measure of our ignorance of the regime of running water under 

 various conditions. It is obvious that the average rate of removal 

 must include landslides as well as old-age streams whose action 

 in denuding the land is excessively slow, and must include 

 tropic and arctic lands as well as those of the temperate zone. 

 The lack of data for most of the world's rivers gives to all 

 estimates a very low degree of accuracy. 



The rate of cliff recession varies with the strength of waves 

 and the nature of the shore. Recession of shore line probably 

 reaches its maximum on the Yorkshire coast of England. At 

 Kilnsea the rate is 12 feet a year, and for 31 miles of coast 

 from Bridlington to the mouth of the Humber 9*75 feet a year, 

 or 975 feet a century. Rates of 300 to 600 feet a century are 

 not uncommon for portions of the English coast exposed to 

 vigorous waves. | On the other hand the west coast of England 

 recedes under wave attack probably less than one foot a cen- 

 tury and the recession of the world's shore line is probably to 

 be measured by a few feet per century. It is believed that the 

 estimate of 10 feet per century^f is safely within the average. 

 It is obvious that the rates attained under exceptionally favor- 

 able conditions no more represent coast erosion than does the 

 rapid denudation incident to cloudbursts and forest removal 

 represent subaerial erosion. On the other hand it should be 

 borne in mind also that the decreased supply of waste furnished 

 by streams of old-age lands relieves the waves of work in dis- 

 tributing and grinding debris and renders them free to attack 

 the land. If the rate of recession be taken as 10 feet per cen- 

 tury, the length of the coast line at 125,000 miles (including 



* Presidential Address, Liverpool Geol. Soc, 1884-85. 



f Physiography, p. 154, 1907. 



X Personal communication. 



§The Ocean, p. 44. 



I Wheeler, The Sea Coast. 



■ff Geikie, Textbook of Geology, 1903, vol. i, p. 567. 



