lmw SOILS ARE MADB 69 



and the force exerted u ta nendous, >trong glass bot- 

 tles are burst l»\ treating water, and area ir«>n vessels 



■ in to pieces. The surfaces oi rocki expos 

 i in temperate elimatei are gradually being 

 brokm to pieces by freezing water. Tins action of water 

 is called mechanical to distinguish it from the dm 



.. which wo shall consider presently. The mechani- 

 cal action of freeaing water cannot, oi course, form soils 



in warm eoun! 



Running water also helps to form soils. Notice the 

 in a creek or river bed. They are all worn smooth 

 by the action of the water. Once these stones were 

 rough and jagged in shape, hut the running water has 

 tumbled them about, grinding them together until all 

 the rough edges are worn olf. The small particles which 

 from the grinding are deposited by the water and 

 goto form soils. 



70. Chemical Action of Water. — Rain water dissolves 

 many salts from the soil and washes the soil from ex- 

 posed hillsides, but it compensates for this mischief by 

 tiding in the formation of fresh soil. Bain water falling 

 on the surface of rocks gradually dissolves away the 

 portions soluble in water, and the insoluble parts crum- 

 ble away and form soils. The great caves in limestone 

 regions are the n suit of ehemical action of water. Lime- 

 is not very soluble in ordinary water, but water 

 that has carbon dioxide gas dissolved in it — soda water 

 is ordinary water containing carbon dioxide gas— dis- 

 solves limestone much better than ordinary water. Rain 

 water dissolves the carbon dioxide of the air, and also 

 some that is formed from the organic matter in the soil. 



