6 AGRICULTURE FOR COMMON SCHOOLS 



into the streams. As it runs down over the land it washes 

 loose and carries along many particles of soil. When these 

 little streams come together in a larger stream the soil which 

 they have carried along makes the water muddy. The large 

 stream flows along, and if the rain has been a heavy one it 

 may overflow its banks in the low places and spread out over 

 the "bottom." When the water comes to a standstill the fine 

 particles of soil settle out, and after the water goes down they 

 remain behind as a thin layer over the land. Many creek and 

 river bottoms have been built up in this way. Such soil is 

 called alluvial soil. If the stream does not overflow, it may 

 carry the fine particles along until it reaches the gulf, bay or 

 ocean. Then, in the still water, they settle and form the deltas 

 which are common at the mouths of many large rivers. The 

 Mississippi and Amazon rivers deposit enormous amounts of 

 soil in this way. 



When this thin layer of mud is deposited over fields along 

 the streams it is generally beneficial, especially if the overflow 

 comes when there is no crop on the land. Farmers say that 

 an overflow is as good as a covering of manure. This is be- 

 cause the particles of soil are the very richest part of the land 

 from which they were washed. The Nile valley is enriched 

 every year by the overflow of the Nile which brings down 

 soil from its mountain sources. 



Glaciers have had much to do with transporting soil. The 

 country over which the glacier has passed has spread over it 

 after the ice melts a large amount of soil carried from other 

 regions. Sometimes this is as much as five hundred feet 

 deep, but on an average about thirty to fifty feet. Water 

 again has had a good deal to do with washing these drift 

 soils about and sorting them. Most of the gravel and sand 



