VIII RIVERS AND LAKES 325 



continue, and the river would wander from 

 one part of its valley to another, spreading 

 the materials and forming' a river plain. At 

 length, as the rapidity still further diminished, 

 it would no longer have sufficient power even 

 to carry off the materials brought down. It 

 would form, therefore, a cone or delta, and 

 instead of meandering, would tend to divide 

 into different- branches. These three stages, 

 we may call those of — 



1. Deepening and widening ; 



2. Widening and levelling ; 



3. Filling up; 



and every place ip the second stage has passed 

 through the first ; every one in the third has 

 passed through the second. 



A velocity of 6 inches per second will lift 

 fine sand, 8 inches will move sand as coarse 

 as linseed, 12 inches will sweep along fine 

 gravel, 24 inches will roll along rounded 

 pebbles an inch diameter, and it requires 3 

 feet per second at the bottom to sweep along 

 angular stones of the size of an egg. 



When a river has so adjusted its slope that 

 it neither deepens its bed in the upper portion 



