ON CHANGES IN THE DISPOSITION 



to one of the sources of the so long mysterious fresh 

 water deposits. 



But the great visible effect of rivers consists in the 

 excavation of valleys ; correlatively also modifying the 

 original forms of the mountains. The ravine continues 

 to enlarge by the further destruction of its sides ; at 

 length becoming a narrow valley, and successively a 

 wider one., finally uniting at times to a plain or the 

 sea, in an almost imperceptible manner ; and thus a 

 system of successive valleys becomes shaped, if not 

 formed, in the course of a river and its tributary 

 streams ; replacing what was originally a system of 

 rocky ravines, or of valleys not always materially dif- 

 fering, often also containing lakes. Yet in this pro- 

 gress, other causes aid the mere action of the stream ; 

 the decreasing declivities of the hills being corroded 

 by the rains or other causes of disintegration, while 

 graivty aids the descent of alluvia, to be carried off by 

 the water, with all the rest, and to produce other work 

 for the still flowing river to perform. If "diluvian" 

 or general currents have been supposed to aid in these 

 excavations, I cannot perceive that any proof of this 

 has yet been produced, except in the single and rare 

 case of the bursting out of a lake; a source of destruc- 

 tion which I must hereafter notice, while some in- 

 stances were quoted in an early chapter. 



If it remains to trace the further process of destruc- 

 tion by a river, I must now consider it as having 

 ceased to act on the solid rock, since, henceforward, it 

 flows through the alluvia which it has transported from 

 the higher grounds. There is, thus, a process of pro- 

 duction, anticipating that of destruction : but these two 

 branches of the subject become here inseparable. 



The destructive operation in this case being that of 

 removing alluvia, these have been often supposed of 



