RIVERS SEEK THEIR BASE-LEVEL. 21 



materials. If heterogeneous material, such as ordinary earth, consisting 

 of grains of all sizes, from pebbles to the finest clay, be thrown into 

 still water, the coarse material sinks first to the bottom, and then the nest 

 finer, and the next, and so on, until the finest clay, falling last, covers 

 the whole. In running water the same sorting takes place even more 

 perfectly, only the different kinds of materials are not dropped upon 

 one another, but successively farther and farther down the stream in the 

 order of their fineness. This property we will call the sorting power of 

 water. Advantage is often taken of this property in the arts to separate 

 materials of different sizes or specific gravities. By this means grains 

 of gold are separated from the gravel with which it is mingled, and 

 emery or other powders are separated into various degrees of fineness. 



We will now apply the foregoing simple principles in the explana- 

 tion of all the phenomena of currents. 



1. — Relation of Current-Velocity to Erosion and Sedimentation. 



The force of a current is consumed in two ways, viz.. by transporta- 

 tion and erosion. If the current is full-loaded, its whole force is con- 

 sumed in transportation and none is left over for erosion. Such a river 

 will neither erode nor deposit. If under-loaded, the river will erode ; 

 if overloaded, it will deposit. Thus a current of pure water will erode 

 but little, because it carries no graving-tools. If we add sand and 

 gravel, the erosion will increase to a maximum, beyond which it again 

 decreases, because more and more force is consumed in carrying, and 

 less and less is left over for erosion ; until finally, in the full-loaded stream, 

 erosion ceases and sedimentation begins. Thus the Platte and Colorado 

 Eivers have about the same slope and velocity ; but while the Colorado 

 is deepening its channel, the Platte on the whole, remains about the 

 same level, sometimes cutting, sometimes depositing. The Colorado is 

 under-loaded, the Platte full-loaded. Again : the Feather Eiver, during 

 floods, is overloaded, and builds up by deposit. During low water it 

 scours out what was previously deposited, even though its velocity is 

 much less. 



2. — Fivers seel' their Base-Level. 



The level at which a river neither cuts nor deposits is called its 

 base-level. Every river is seeking this level. If above it, it seeks it 

 by cutting ; if below it, it seeks it by building up by sedimentation. 

 Eivers thus become delicate indicators of up or down movements of 

 land-surfaces. Suppose a continent to rise gradually and then remain 

 steady; all the rivers would immediately increase their velocity and 

 begin to cut. Xow. in making the resulting gorge, two processes are 

 going on, viz.. a cutting downward by the river, and a widening out by 

 cliff-crumbling and rain-wash. A V-shaped canon is thus usually 

 formed, whose shape, whether sharp or wide, will depend on the relative 



