32 THE PRINCIPLES OF AGRICULTURE. 



When the streams and rivers are much swollen, they 

 wash out and bear away many rocks in their swift cur- 

 rent. As these are rolled and tumbled one upon an- 

 other, they are ground into soil, which is added to the 

 general deposit. The mountains and hills are thus grad- 

 ually carried away to fill the valleys below, and to extend 

 the coasts of the continent. The rich alluvial lands in 

 river valleys have been formed in this way. 



3. The Ocean. The movements of water on the shores 

 of the ocean produce an effect similar to that of rivers. 

 The flow and ebb of the tide, and the breaking of waves 

 on the beach, grind rocks into sand, or wash tip shells 

 from deeper water, grinding them into (hie powder. This 

 process has been going on since land first appeared above 

 water, forming the origin of our continent. 



The ocean has been continually forming beds of sand 

 and mud on or near its shores. These have been raised 

 above the surface, as the waters have receded, arid in 

 many cases have become consolidated into rock. Sand- 

 stone and limestone have been mostly produced in this 

 manner, the former from the sand beds, and the latter 

 from mud formed by the grinding of shells. A similar 

 effect has been produced by large lakes, some of which 

 have become entirely dry, leaving deposits of sand, lime, 

 or' other mineral matter. 



4. Ice. Ice forming in the cracks and crevices of 

 rocks, year after year, tends by its expansion to burst 

 the rocks or open crevices, thus admitting the atmos- 

 phere, and hastening the work of crumbling. 



Minute particles are also detached from the rocks by 

 the freezing and thawing of the moisture upon their 

 surface. If a number of clean stones are placed in a 

 pail of pure water, and the water is allowed to freeze 



