38 ELEMENTARY AGRICULTURAL CHEMISTRY 



glaciers. Such deposits are known as glacial drift, and often 

 quite obscure the actual rock strata beneath. In this case the 

 transportation of the soil took place many ages ago. 



Wind also sometimes acts as a means of transporting sand, 

 volcanic ashes, &c., from a distance and depositing them in a 

 new position, there to form a soil. 



Formation of Soils.— In the formation of a soil the first 

 step is the mechanical breaking down of the rock into small 

 fragments. The chief agencies by which this is accomplished 

 are : 



1. Water, which acts in several ways. 



(a) Mechanically — 



(i) By Liquid IVater, — The flow of water over the surface 

 of a rock abrades it slightly. The action is greatly increased 

 by the attrition of pebbles, gravel, <fec., urged by the current 

 over the rock. In this way streams in the rapid portions of 

 their course carry away large quantities of sand, gravel, &c., and 

 deposit them in the lower and quieter portions of their course 

 as alluvial deposits. 



(ii) By Glaciers. — Glaciers are slowly moving masses of ice 

 formed by the compression of snow by its own weight. In 

 their descent glaciers, aided by fragments of rocks imbedded in 

 them, grind away the rock over which they pass, and the stream 

 which issues from the snout of a glacier is always heavily 

 charged with the finest mud, while the termination of a 

 glacier is marked by huge piles of rock fragnTents of all sizes 

 (" moraines ") carried down on the surface of the moving ice. 

 This agency has been very active in past ages, even in countries 

 — e.g.^ Britain — where no glaciers are now to be found. 



ih) 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 be allowed to occur water cannot freeze, 

 however much it be cooled. In the disintegration of rocks 

 this agency is very powerful. During the warm part of a wet 



