380 Mr. J. Croll on Geological Time, and the probable 



the islands into smaller islands, and so on till the whole ulti- 

 mately disappears. 



There can be no doubt about the rate at which the American 

 continent is being denuded by subaerial agency. The sediment 

 carried down into the Gulf of Mexico assuredly comes all off the 

 land. It is not derived from the banks of the river itself, as 

 has been clearly shown by Mr. Tylor. ' ' The Mississippi/' he 

 says, " must draw its vast supplies of mud from its tributaries ; 

 for it could obtain them from no other source, unless we suppose 

 it abstracts them from its own plains. Certainly in many places 

 soil is being removed from one part or other of its plains ; but an 

 equal quantity must be added to some other part ; for the river 

 could not make a permanent inroad into its plains without enlar- 

 ging its channel. This it does not do, or it would be able to 

 carry off the winter freshets without overflowing, and the present 

 artificial bank would be unnecessary/' 



Every river running through an alluvial plain will cut a chan- 

 nel for itself of a definite capacity, which capacity will be deter- 

 mined by the volume and velocity of the river. If you attempt 

 to increase the size of the channel, it will silt up and assume its 

 former capacity. Or if you attempt to diminish its channel by 

 throwing in loose materials, the river will remove these. We 

 have a good example in the river Clyde of the tendency of a 

 river to preserve its normal size of channel. It is necessary for 

 commercial purposes that the channel of the river below the city 

 of Glasgow should be kept much deeper than the volume and 

 velocity of the river necessarily demands, and the consequence is 

 that it requires the continued efforts of several powerful dredging- 

 machines to counteract the tendency that this little river has to 

 silt up to its normal depth and size of channel. 



So long as the present order of things remains, the rate of 

 denudation will continue while land remains above the sea-level ; 

 and we have no warrant for supposing that the rate was during 

 past ages less than it is at the present day. It will not do as 

 an objection to say that, as a considerable amount of the sedi- 

 ment carried down by rivers is boulder-clay and other materials 

 belonging to the ice-age, the total amount removed by the rivers 

 is on that account greater than it would otherwise be. Were this 

 objection true, it would follow that, prior to the glacial period, 

 when it is assumed that there was no boulder-clay, the face of 

 the country must have consisted of bare rock; for in this case no 

 soil could have accumulated from the disintegration and decompo- 

 sition of the rocks, since, unless the rocks of a country disintegrate 

 more rapidly than the river-systems are able to carry the disinte- 

 grated materials to the sea, no surface-soil can form on that country. 

 The rate at which rivers carry down sediment is evidently not 



