The Soil. 4L 



The chief agencies by which this is accomplished are, first by 

 Water, which acts in several ways. 



Mechanically, by liquid water The flow of water over the 

 surface of a rock abrades it slightly. The action is greatly in- 

 creased by the nibbing action of pebbles and gravel, urged on 

 by the current over the rock. In this way streams in the rapid 

 portions of a course carry away large quantities of sand, gravel, 

 etc., and deposit them in the lower and quieter portions of their 

 course as alluvial deposits. By glaciers Glaciers are slowly 

 moving masses of ice. In their descent, aided by fragments of 

 rock imbedded in them, they grind away the rock over which 

 they pass and the stream which flows from the base of a glacier 

 is always heavily charged with the finest mud, while the lowest 

 point reached by a glacier is marked by huge piles of rock frag- 

 ments of all sizes, carried down on the surface of the moving ice. 

 By alternate frost and thaw Ice occupies a greater volume than 

 the water from which it is formed. The increase in volume in 

 the act of freezing amounts to about 10 per cent and unless this 

 increase is allowed to occur water cannot freeze, however much 

 it be cooled. This is a powerful agency in the pulverization of 

 rocks. All rocks are more or less porous and absorb Vater. 

 During the warm part of a wet winter's day the crevices of a 

 rock become filled with water. If the temperature falls the 

 water begins to freeze, at first on the surface so that every crevice 

 becomes plugged with ice. As the liquid within continues to lose 

 heat it tends to solidify. This it can only do if it be allowed to 

 expand and in order to do this it must widen or lengthen the 

 crevice. When the next thaw comes, the enlarged crevice again 

 fills with water. The next freeze repeats the action and so the 

 process goes on until the hardest rock is broken into fragments. 



Chemically. Many minerals when exposed to the action of 

 water are acted upon in such a way as to load to their disintegra- 

 tion. A portion is often carried away in solution while the re- 

 mainder crumbles and is then easily moved by rain or running 

 water. In many rocks the cementing material, which holds the 



