440 ON THE ACTION OF RUNNING WATERS. 



own feeble means. But how diminutive and circumscrib- 

 ed are these changes produced in the configuration of the 

 globe, compared with the long and broad valleys which 

 furrow in vast numbers the immense surface of the earth, 

 and to the formation of which neither the torrents nor 

 great rivers which exist at the present day have in any 

 way contributed, as we shall presently demonstrate. 



2. Action of Rivers. 



THE action of rivers must be examined under two 

 very different circumstances, or at two different parts of 

 their course. 



First, When they are compressed between mountains, 

 whether at no great distance from their source, or even 

 at the middle of their course. 



Secondly, When they have reached broad valleys, 

 whose declivity is slight, or plains which commonly sur- 

 round their mouth. 



In the first case, these rivers partake of the impetu- 

 osity and power of torrents. They often run with ra- 

 pidity, and in great quantity, at the bottom of narrow 

 and deep valleys : they are as it were inclosed in chan- 

 nels, whose vertical walls appear as if cut by art. The 

 first idea which presents itself to all who have seen these 

 appearances for. the first time, and who are satisfied with 

 first impressions, is, that these streams, which are pret- 

 ty powerfuland always very impetuous, have dug these 

 deep grooves ; and if sometimes the hardness of the rocks 

 and the height of the precipices which form their sides, 



