374 DYNAMICAL GEOLOGY.  [Boox IIT, _ 
their characteristic smoothed and polished surface down to and 
beneath the surface of the current. In descending the river I was 
astonished to observe that the famous rapids of the St. Lawrence are 
actually hemmed in by islets and steep banks of boulder-clay and 
not of solid rock. So little obvious erosion does the current perform 
even in its tumultuous billowy descent, that a raw scar of clay betoken- 
ing a recent slip is hardly to be seen. The banks are so grassed over 
or even covered with trees, as to prove how long they have remained 
undisturbed in their present condition. That very considerable local 

















Fia. 107.—Rooxy River CHANNEL WITH OLD Pot-HOLEs. 
destruction of these clay islands, however, has been caused by float- 
ing ice will be alluded to further on. 
Mere volume and rapidity of current, therefore, will not cause 
much erosion of the channel of a stream unless sediment be present 
in the water. A succession of lakes, by detaining the sediment, 
must necessarily enfeeble the direct excavating power of a river 
On the other hand, by the disintegrating action of the atmosphere, 
and by the operations of springs and frosts, loose detritus as well as 
portions of the river-banks are continually being launched into the 
currents, which as they roll along are thus supplied with fresh 
materials for erosion. | 

