146 GEOLOGY 



sand, because the former expose larger surfaces, relative to their 

 mass, to the water through which they sink. But even such par- 

 ticles, unless of extraordinary fineness, would presently reach the 

 bottom if acted on only by a horizontal current and gravity. Since 

 even sediment which is not of exceeding fineness is kept in suspen- 

 sion, it is clear that some other factor is involved. This is found, 

 in part at least, in the subordinate upward currents in a stream. 



Where a bowlder occurs in the bed of a stream (Fig. 110) a 

 part of the water which strikes it is forced up over it. If there are 

 many bowlders, the process is frequently repeated, and the number 

 of upward currents is great. Any roughness will serve the same 

 purpose, and every stream's bed is rough to a greater or less extent. 

 Roughnesses at the sides of a channel start currents which flow 

 toward the center, and the varying velocities of the different parts 

 of a stream serve a similar purpose. A river is therefore to be 

 looked upon as a multitude of currents, some rising from the bottom 

 toward the top, some descending from top to bottom, some diverg- 

 ing from the center toward the sides, and some converging from 

 the sides toward the center. 



It is, of course, true that the sum of the upward currents is 

 always less than the sum of the downward, so that the aggregate 

 motion of the water is down slope. Sediment in suspension is held 

 up chiefly by the upward currents, which, locally and temporarily, 

 overcome the effect of gravity. The particles in suspension ail 

 constantly tending to fall, and frequently falling; but before they 

 reach the bottom, many of them are seized and carried upward 

 by the subordinate currents, only to sink and be carried up again. 

 Even if they reach the bottom, as they frequently do, they ma> 

 picked up again. It is probable that every particle of sediment 

 of such size that it would sink readily in still water is dropped and 

 picked up many times in the course of any long river journey, and 

 its periods of rest often exceed its periods of movement. 



Corrasion 



The mechanical wear effected by running water is common. 

 So long as the materials to be moved are incoherent, it is easy t<> 

 understand how running water moves them. The water which 



