442 ON THE ACTION OF RUNNING WATERS. 



and often much greater vertical thickness. If, in order 

 to get rid of this difficulty, we admit a volume of water 

 incomparably larger than the present volume of the rivers 

 to which so great effects are attributed, we must admit 

 much more elevated and more extended mountains, to 

 give rise to so great a volume of water. 



Were we only detained by this hypothesis, and did 

 not direct observation oppose itself to the admission of 

 this disaggregating power and its effect, we might pass it 

 over ; but two other observations render the hypothesis 

 inadmissible. 



2. Historical records equally concur to prove that 

 the rivers possessed of the greatest power which can be 

 attributed to them, have no appreciable corroding action 

 upon the rocks on which they move. 



No one has maintained that the greater number of the 

 cascades, cataracts, or rapids, long known and mentioned 

 on account of their celebrity, have disappeared or have 

 even sensibly diminished, nor consequently that the na- 

 tural dike which the water had encountered in its course, 

 has been worn or even completely disrupted. We do 

 not find that cascades have changed into cataracts, and 

 these again into rapids. The cataracts of the Nile have 

 been spoken of from time immemorial, as always oppos- 

 ing an obstacle to the navigation of that river ; the same 

 is the case with those of the Danube, of the fall of the 

 Rhine at Schaffhausen, &c. The famous cascades of the 

 Alps and Pyrenees have been cited ever since writing 

 was in use ; and among all these examples we can scarcely 

 find two or three cascades that have been lowered, or 

 cataracts reduced in their level. 



The only cascade which we can point out as having 



