32 IIIK l'i;iN(lPLi:s of AGHICrLTlRK. 



When the streams and I'ivers are much swulloii, 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- 

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

 the coasts of the continent. The rich alluvial lauds in 

 river valleys have been formed in this way, 



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

 of the ocean ])roduce 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 up shells 

 from deeper water, grinding them into liuf powder. This 

 process has been going on since land lirst ai)peared above 

 water, forming the origin of our continent. 



The oeean lias l)cen continually forming l)eds of sand 

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

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

 many cases have become consolidated into rock. Sand- 

 stone and limestone have been mostly produced in tliis 

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

 from uiud b)rmed by the grinding of shells. A similar 

 effect has been produced by large lakes, some of which 

 have become entirely dry, leaving deposits of snud. lime, 

 or other mineral matter. 



4. Jce. — Ice forniiug in the cracks and cre\ices of 

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

 the rocks or open crevices, thus admitting the atmos- 

 ]>her(>, and hastiuiing the work of ei'uiuMiug. 



]\Iiuute particles are also detached from the rocks by 

 the freezing and thawing of the moistun* upon their 

 surface. If a numl)ei" of clean stones are pliiecd in a 

 j)ail of pur(> water, and the water is allowed to freeze 



