ON THE ACTION OF RIVERS. 445 



rivers the means of surmounting all these obstacles. 

 Now, observation would seem absolutely to prove the 

 contrary. 



We have remarked, that rapid rivers which, in 

 the bottom of valleys, fall in cascades, from rock to 

 rock, which beat with violence against the walls which 

 contain them, do not in any degree alter these rocks, and 

 that, far from corroding their surface, they allow it to be 

 covered with a rich coating of mosses, confervae, &c. 

 which could neither maintain itself, nor be formed at all, 

 were the least portion of the surface of these rocks con- 

 tinually or even only frequently removed. 



A much more striking fact is that which some of the 

 great rivers present, such as the Nile, the Orinoco, &c. 

 which flow in the equatorial regions. 



These powerful rivers, when they have arrived at 

 places where they are contracted, and, as it were, jam- 

 med in between two rocky walls, form impetuous cata- 

 racts. Their waters, endowed by the celerity of this fall 

 with the greatest erosive power that can be attributed 

 to this fluid, must necessarily have corroded, or at least 

 worn, the rocks which they have thus beat against 

 since the creation of our present Continent. Now, so far 

 from removing the surface, they cover it with a brownish 

 varnish of a peculiar nature. 



It appears, therefore, well established, that water alone 

 does not scoop those rocks, whose aggregation is complete, 

 or which are solid ; and that it does not wear them in 

 any way, whatever be its quantity of motion. 



We say water alone ; and we must insist on this dis- 

 tinction, in order to make the preceding facts agree with 

 other facts, which might seem contradictory. 



