SECTION IX. HOW THE SOIL WAS FORMED. 

 KINDS OF SOILS 



IF we hammer a small piece of stone, we can usually 

 change it into a powder. The tiny particles that make 

 up this powder are often like some of the grains of the 

 soil that may be found near the stone. What is now soil, 

 in ages past was solid rock. Far mightier forces than the 

 heaviest of hammers cracked and ground these ancient 

 rocks for thousands of years, until they crumbled into sand 

 and soil. As the earth's surface cooled, and shrunk, and 

 wrinkled, the rocks cracked. Water standing in these 

 -cracks and tiny rough places froze. In freezing, the water 

 expanded, and thus broke off great and tiny pieces of rock. 



Air and water, just as they eat slowly into iron, forming 

 iron rust, so changed and dissolved some of the cementing 

 material in the rock. Then the remaining parts of the 

 large rocks crumbled. 



Water grinds rocks into soil. Streams of water rolled 

 the sharp-edged pieces of rock against each other, grind- 

 ing off the sharp points, making sand of the fragments, and 

 leaving rounded stones and pebbles. When a boy wishes 

 nice, smooth stones for his sling shot, he knows he will find 

 them in the bed of a stream. While searching for smooth 

 stones, he walks over a sand-bar. This sand-bar shows how 

 soil was formed. It is made partly of fine gravel, partly 



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