VIII RIVERS AND LAKES 327 



Reuss), the water acts more effectively than 

 in longitudinal valleys running along the 

 strike. Hence the lateral valleys have been 

 less deeply excavated than that of the Eeuss 

 itself, and the streams from them enter the 

 main valley by rapids or cascades. Again, 

 rivers running in transverse valleys cross 

 rocks which in many cases differ in hardness, 

 and of course they cut down the softer strata 

 more rapidly than the harder ones ; each ridge 

 of harder rock will therefore form a dam and 

 give rise to a rapid, or cataract. We often 

 as we ascend a river, after a comparatively 

 flat plain, find ourselves in a narrow defile, 

 down which the water rushes in an impet- 

 uous torrent, but at the summit of which, 

 to our surprise, we find another broad flat 

 valley. 



Another lesson which we learn from the 

 study of river valleys, is that, just as geological 

 structure was shown by Sir C. Lyell to be no 

 evidence- of cataclysms, but the result of slow 

 action; so also the excavation of valleys is 

 due mainly to the regular flow of rivers ; and 

 floods, though their effects are more sudden 



