122 GEOLOGY. 



The ceaseless repetition of the blows struck by the material in 

 suspension, or rolled on its bottom, hour after hour, day after day, 

 and year after year, will accomplish sensible results. In the long course 

 of the ages this process has excavated deep valleys. Concomitant 

 processes are largely concerned in making valleys wide, but the depth 

 of valleys cut in solid rock is chiefly the result of the impact and 

 friction of the sediment in transportation. 



The wear effected in this way is not proportional to the number 

 of blows struck. Since every pebble and every grain of sand carried 

 diminishes the velocity of a stream, and since with diminished velocity 

 the force of the blows struck is diminished, it follows that the blows 

 may become so weak, as the result of their multiplication, as to be 

 ineffective. The larger the load, therefore, which the stream carries, 

 the more the tools with which it has to work, but the less effectively 

 can it use them; and the load may be so far increased as to destroy 

 its corrasive power altogether. On the other hand, the smaller the 

 load of the stream the greater its velocity and the more effectively 

 will its tools be used; but their number may be so far reduced that 

 their aggregate effect is sHght. To accomplish the greatest results on 

 a bed of solid rock a stream must have tools to work with, but must 

 not be so heavily burdened as to interfere with its effective use of them. 



Whatever the cause of their unequal velocities swift and slow 

 streams corrade their valleys differently. The erosion of a swift stream 

 is chiefly at the bottom of its channel. The sluggish stream lowers 

 its channel less rapidly, while lateral erosion is relatively more impor- 

 tant. The result is that slow streams increase the width of their valleys 

 more than the depth, while swift streams increase the depth more 

 than the width. It follows that slow streams develop fiats, while 

 swift ones do not. Not only is a slow stream more Hkely to have a 

 flat, and therefore a better chance to meander, but it is more likely to 

 take advantage of opportunities in this hne, for a slow stream gets 

 out of the way for such obstacles as it may encounter, while a swift 

 stream is much more likely to get obstacles out of its way. 



Special phases of corrasion are introduced where waterfalls and 

 other peculiarities dependent on inequalities of rock resistance occur. 



Solution. — In most cases the solution effected by a stream is much 

 less important than its mechanical work. Only when conditions are 

 unfavorable to the latter, is solution the chief factor in the excavation 

 of a valley. This may be the case where a stream's bed is over soluble 



