Chap. V. AND DENUDATION. 235 



paper appeared, Mr. A. Tylor had adduced 

 important evidence on sub-aerial denudation, 

 by showing that the amount of matter 

 brought down by rivers must infallibly lower 

 the level of their drainage-basins by many 

 feet in no immense lapse of time. This line 

 of argument has since been followed up in the 

 most interesting manner by Archibald Geikie, 

 Croll and others, in a series of valuable 

 memoirs.* For the sake of those who have 

 never attended to this subject, a single 

 instance may be here given, namely, that of 

 the Mississippi, which is chosen because the 

 amount of sediment brought down by this 

 great river has been investigated with especial 

 care by order of the United States Govern- 

 ment. The result is, as Mr. Croll shows, that 

 the mean level of its enormous area of 



* A. Tylor " On changes of the sea-level/' &c., ' Philosophical 

 Mag.' (Ser. 4th) vol. v., 1853, p. 258. Archibald Geikie, 

 Transactions Geolog. Soc. of Glasgow, vol. iii., p. 153 (read March, 

 1868). Croll "On Geological Time," 'Philosophical Mag.,' 

 May, August, and November, 1868. See also Croll, * Climate 

 and Time,' 1875, Chap. XX. For some recent information on 

 the amount of sediment brought down by rivers, see * Nature,' 

 Sept. 23rd, 1880. Mr. T. Mellard Keade has published some 

 interesting articles on the astonishing amount of matter 

 brought down in solution by rivers. See Address, Geolog. Soc., 

 Liverpool, 1876-77. 



