Mr. De la Beche on the Excavation of Valleys. 245 



bed ; but when a bank, a small hill, or the foot of a mountain, 

 opposes their progress, they assail it, and form cliffs, the ma- 

 terials of which, if soft, fall into the stream, or make undercliffs, 

 which are in time removed, and the work of destruction slowly- 

 continued (fig. 4'. a); or when the cliff thus formed is of harder 

 materials, blocks are accumulated in a talus at its base, and 

 the cliff' is in a great measure secured from further attack 

 (fig. 4. b.). There is scarcely a river of any considerable length 

 or breadth which does not afford examples of cliffs thus pro- 

 duced ; very frequently they overhang flat or gently sloping 

 land, on which the river has flowed while employed in cutting 

 the cliff". It is not a little curious to trace, in countries where 

 rivers wind considerably, the various obstacles that have deter- 

 mined the course of the stream, causing it to attack and de- 

 stroy the^ original more or less rounded forms of the bases of 

 the hills.* 



V. — Rivers escaping from Plains through Gorges. 



It is by no means uncommon to find plains of greater or less 

 extent bounded on all sides by high land, and through which 

 a principal river meanders, entering at one end by a valley, 

 and passing out through a gorge at the other, augmented by 

 tributary streams from the surrounding hills ; sometimes these 

 plains have no principal river passing through them, but only 

 numerous small streams descending from the heights, and 

 which uniting in the plain, pass out of it also through a gorge. 

 In such cases the plain often presents the appearance of a 

 drained lake, and such as all beds of existing lakes, if deprived 

 of their waters, would assume. Fig. 5. is intended to convey 

 a general idea of the interior of such drained lakes; b. repre- 

 senting the gorge through which the waters have passed du- 

 ring the gradual cutting down of the hill. 



The lake of Geneva would appear once to have been much 

 more extensive than at present, and to be only the remains of 

 a greater lake which has been partly drained by the cutting 

 down of the gorge at the Fort de 1' Eel use. The gorge at 

 Narni seems to have let out the waters of a lake, the ancient 

 bed of which now forms the plain of Terni. These examples 

 have principal rivers now running in them: the bed of the 

 Rhone runs through the drained part of the ancient lake, the 

 remainder of wliich constitutes the existing lake of Geneva, 

 and tlic Xera flows through tiie plain of Tcrni; and if the re- 

 spective gorges llirough wliicli the waters escaj^e were again 

 closed, tliuse rivers would again Ibrm lakes on the surface of 

 the plains *. rr^, 



• The great fci lilc plain of I'lurtiicc 5cciiis once to liavc been llic bed 



of 



