ON THE ACTION OF 11IVEKS. 443 



really diminished in height, is that of Tungasca in Sibe- 

 ria. We do not, however, assert but that there may be 

 others. So many causes different from those of erosion 

 may concur to lower a cascade, or even make it disappear 

 almost entirely, that we are rather astonished at the small 

 number of examples mentioned, than embarrassed by the 

 objections which these examples might present to the 

 opinion vrhich we are defending : for the fall of a part 

 of the rock which forms the cliff from which the cascade 

 is precipitated ; an abundant accumulation of debris at 

 the foot of the cliff ; a real destruction of the softer 

 deposits, forming part of the strata of the mountain 

 from which they fall, are sufficient causes for changing 

 the height of waterfalls. These causes must present 

 themselves pretty frequently ; but how different is their 

 action from that of erosion ? This, if it existed, would 

 extend from the source of the river to its mouth, and 

 would have a considerable influence upon the configura- 

 tion of the earth's surface. Those which we have men- 

 tioned have, on the contrary, an action so limited and so 

 local, as to be scarcely appreciable. 



3. Allowing, for the moment, that a river, possessed of 

 a vast erosive or disaggregating power, may have scoop- 

 ed out the valley in the bottom of which it at present 

 flows, in a state of feebleness very different from its ori-, 

 ginal state, we must account for the disposal of a vast mass 

 of earth and rock, which filled up the valley before the 

 river had removed it. It is not possible to suppose that 

 it has been transported into the sea, which is often 

 more than a hundred leagues from the valley; for 

 we know that when rivers, on reaching the plains, lose 

 their rapidity, they allow the matters to be precipi- 



