PHYSIOGRAPHY. 



[chap. 



similarity of a system of drainage established in this way 

 to what is found in a large river-basin, that those who have 

 thought most upon the subject believe that one may be 

 taken to explain the other ; that, in point of fact, the pre- 

 sent rivers have gradually scooped out their own channels, 

 and that our river-valleys are, mainly, the result of work per- 

 formed by rain, rivers, and similar agents of denudation. 



Fig. 33. — Self-established drainage system. 



At first sight, it may seem incredible that a great river- 

 system, like that of the Thames, should have been shaped 

 by the action of instruments which seem so msignificant. 

 Yet the more one thinks upon it, the less are the difficulties 

 that beset such an explanation. No one can deny that 

 little water-courses may be eaten out of solid rock by a 

 running stream, for the very origin of such gulleys may 



