10 



ELEMENTS OF FARM PRACTICE 



large cracks will be seen in the rocks. These cracks are 

 made by frost, by alternate expansion and contrac- 

 tion caused by heat and cooling, or by the force of 

 growing roots. When these crocks are formed little 

 particles of rock are broken off. A strong wind will blow 

 these particles about over the rocks, make them finer and 

 wear off other particles. Rain and water running over 

 the rocks do the same thing. The wind and water tend to 

 gather the soil particles into crevices in the rock and other 

 sheltered places. When several of these little particles 



Figure 2. — Soil well filled with humns. 



have been gathered in one place, there is the beginning of 

 a little patch of soil. When this little patch of soil becomes 

 moist from rain or melting snow, and the warm sun shines 

 on it, some kind of a plant, like moss, will start to grow. 

 At first these little patches are very small and plants can 

 grow only a very little while. When the plants die they 

 are added to the soil. Then other small pieces of rock are 

 added and still other plants grow and die and are added 

 to the little patch of soil. This has been going on for many 

 thousands of years, so that nearly the whole surface of the 

 earth is covered with soil. 



