SEDIMENTARY DEPOSITS OF RIVERS. 487 



instance in point. We select the Thames, because 

 for centuries past it has been more closely watched, and 

 all its phenomena have been subjected to more minute 

 and accurate investigation than those of perhaps any 

 other river. Now, the amount of sediment discharged 

 by the Thames, annually, is estimated at 1,865,903 

 cubic feet, * besides matters washed away in solution. 

 This process of gradually washing away large quantities 

 of solid matter from the land into the sea, is known as 

 that of ' " Denudation ; " it is a work, as we have more 

 than once observed, always in operation, which is 

 mainly carried on under two forms : " Fluviatile denuda- 

 tion," which is the work of rivers; and "Pluvial 

 denudation," which is the work of rains. The other 

 minor forms may at present be left out of account. 

 It must be obvious that these processes, thus con- 

 tinuously going on, must, in course of time, inevitably 

 tend to profoundly modify the extent and condition of 

 the habitable surface of the globe; and in connection 

 with their probable results as regards our own country, 

 Professor Huxley, in his work on Physiography, quotes 

 an opinion of Professor Geikie, to the effect that, after 

 taking into account, as far as possible, the whole 

 quantity of matter which from every cause is annually 

 carried into the sea from the surface of the British 

 Islands, the latter " has calculated that at the present 

 rate of denudation, it would require about five and a 

 half million of years to reduce them to the level of 

 the sea." f 



But concurrently with the process of Denudation, 



* Modern Denudation, by Professor A. Geikie. See transactions of 

 Geological Society of Glasgow, Vol. iii., p. 153. 



j .Physiography, by Professor R. H. Huxley, F.R.S., 1887, p. 149. 



