WHERE THE ROCK PARTICLES COME FROM 97 



masses over one twelfth of an inch in diameter. Very 

 large stones are called rocks. Stones form a surface cover 

 to prevent evaporation from the soil beneath them, and so 

 far they are helpful to farming. A boy sometimes likes 

 to turn over the flat stones in a field to see the insects 

 scamper away from beneath it. They had been drawn to 

 the place by the moisture under the stone. 



BOTTLE OF SOIL SIFTED. 

 Each pile is in front of the sieve through which it came. 



57. Where Do the Rock Particles Come From ? Except 

 for the size of their particles, there is little difference, 

 as far as appearances go, between sand, silt, clay, gravel 

 and stones. It is possible to go to a ledge, and, detaching 

 a large, rocky mass, to crush it with a hammer, first into 

 stones, then into gravel, next into sand, then into silt, 

 and finally into clay. In fact, when rocks are crushed 

 for road building or concrete work, this is what the crusher 

 actually does. (The crusher, of course, makes all the 

 different sizes at one blow.) 



Now there are natural forces that produce the same 



