NATURE AND FORMATION OF SOILS. 13 



The amount of these dissolved materials 

 though far less than that produced by mechan- 

 ical action is astonishing. That carried into 

 the Gulf of Mexico by the Mississippi River an- 

 nually reaches over 112,000,000 tons not all 

 derived from the river-bed, but taken up by the 

 water from the time it falls in rain till it reaches 

 the sea, whether it flows through the river and 

 its branches, or whether it comes from springs 

 or other underground sources. 



2. Mechanical A ction, The mechanical action 

 of water is threefold, (i) It disintegrates. (2) 

 It transports. (3) It assorts. 



The mechanical action of rain is due to the 

 friction produced by the drops in striking the 

 rocks, and by the abrasion of solid particles as 

 they are carried to lower levels. It forms into 

 little rills and gullies, washing out and carrying 

 with it as it goes all the loose material which it 

 can hold in suspension (Fig. 8). The amount 

 thus obtained depends partly upon the solubil- 

 ity of the rock over which it flows (though 

 even a granite would be slightly dissolved by 

 ordinary rain-water), and partly upon the vio- 

 lence of the precipitation, and the volume and 

 velocity of the stream. 



The velocity is affected by several influences, 

 but the greatest of them is the constant, never- 

 failing action of gravity. Hence, the steeper 

 the descent the greater the velocity. The 



