Vill RIVERS AND LAKES Sat 
Reuss), the water acts more effectively than 
in longitudimal valleys running along the 
strike. Hence the lateral valleys have been 
less deeply excavated than that of the Reuss 
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 
