SOILS 



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rocks cool again. This causes tension in the rocks 

 and little cracks and checks appear in them just as 

 in the heated plate, only more slowly. This check- 

 ing may also be brought about by a cool shower 

 falling on the sun heated rocks just as the cool 

 water cracked the warm glass. Many rocks if ex- 

 amined closely will be found to be composed of sev- 

 eral materials. These materials do not expand and 

 contract alike when heated and cooled and the ten- 

 dency for them to check is greater even than that 

 of the plate. This is the case with most rocks. 



Work of Rain. Rain falling on the rocks may 

 dissolve a part of them just as it dissolved the rock 

 salt ; or, working into the small cracks made by the 

 sun, may wash out loosened particles ; or, during 

 cold weather it may freeze in the cracks and by its 

 expansion chip off small pieces; or, getting into 

 large cracks and freezing, may split the rock just 

 as freezing water splits a water pitcher or the water 

 pipes. 



Work of Moving Water. Visit some neighbor- 

 ing beach or the banks of some rapid stream. See 

 how the waves are rolling the sand and pebbles up 

 and down the beach, grinding them together, round- 

 ing their corners and edges, throwing them up into 

 sand beds, and carrying off the finer particles to 

 deposit elsewhere. Now visit a quiet cove or inlet 

 and see how the quiet water is laying down the fine 

 particles, making a clay bed. Notice also how the 

 water plants along the border are helping. They 

 act as an immense strainer, collecting the suspended 

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