24 THE PRINCIPLES OP AGRICULTURE 



be deposited wherever obstructions occur, or be 

 discharged on the delta at the mouth. The 

 deposition of sediment in times of overflow 

 adds new vigor to the submerged lands. The 

 historic example of this is the Nile valley, 

 but all bottom lands which are subjected to 

 periodical overflows exhibit the same result. 

 Alluvial lands are formed from the deposition 

 of the sediment of water. 



39. In mountainous regions, snow and ice 

 carry away great quantities of rock and soil. 

 The most powerful transporters of soil are gla- 

 ciers, or moving masses of ice. Glaciers loosen 

 the rock and then grind and transport it. In 

 the glacial epoch, in which much of the north- 

 ern part of the northern hemisphere was cov- 

 ered with gigantic ice -sheets slowly moving to 

 the southward, enormous quantities of rock and 

 earth were transported, and deposited wherever 

 the ice melted. In eastern North America, the 

 ice -front advanced to the latitude of the Ohio 

 river, and the boulder -strewn fields and varied 

 soils to the northward of this latitude are the 

 legacy which the epoch left to the farmer. 



40. In all areas which are subjected to 

 periods of drought, the wind transports soils in 

 the form of dust, often in great amounts and 

 for long distances. In some parts of the world, 

 so much earth is carried by violent winds that 



