18 AGRICULTURE 



In considering the part played by water, 

 mention must also be made of its action as 

 an eroding agent. No one can visit a hilly 

 district where streams are abundant without 

 observing that they flow in well-defined 

 channels, and one may also sometimes find 

 that they have cut a narrow passage in the 

 solid rock. So long as the water is clear, 

 its power of deepening its passage is but 

 limited ; but directly a stream becomes 

 turbid, that is to say, as soon as it carries 

 solid material in suspension, its power of 

 erosion is greatly increased. Streams, which 

 when at summer level are comparatively 

 placid, may, during times of flood, become 

 raging torrents, carrying with them large 

 stones and rock debris. Movement of such 

 bodies over the stream's bed is accompanied 

 by great erosion, and the particles knocked off 

 the stones themselves, or detached from the 

 rocky bottom, are borne along by the water, 

 to be deposited under quieter conditions, 

 such as prevail in a lake or in the sea, or on 

 the fields (haughs) which the flood may 

 reach. The silt so deposited forms alluvial 

 soil, which in many cases is highly fertile. 



Another of the natural agencies at work 

 in disintegrating rocks and stones and pro- 

 ducing soil is oxygen. This gas, which is 



