92 THE NEW AGRICULTURE. 



causing them to crumble away, or even making successive crusts 

 of the stone fall off. 



" Again, rocks which are at one time well soaked with rain, 

 and at another time are liable to be dried by the sun's rays and by 

 wind, are apt to crumble away. If then it be true, as it is, that a 

 general wasting of the surface of the land goes on, you may nat- 

 urally ask why this should be. Out of the crumbled stones all 

 soil is made, and on the formation and renewal of the soil we 

 depend for our daily food. 



" Take up a handful of soil from any field or garden, and look 

 at it attentively. What is it made of ? You see little pieces of 

 crumbling stone, particles of sand and clay, perhaps a few vege- 

 table fibers ; and the whole soil has a dark color from the decayed 

 remains of plants and animals diffused through it. Now let us 

 try to learn how these different materials have been brought to- 

 gether. 



" Every drop of rain which falls upon the land helps to alter the 

 surface. You have followed the chemical action of rain when it 

 dissolves parts of rocks. It is by the . constant repetition of the 

 process, drop after drop, and shower after shower, for years to- 

 gether, that the rocks become so wasted and worn. But the rain 

 has also a mechanical action. 



" Watch what happens when the first pattering drops of a show- 

 er begin to fall upon a smooth surface of sand, such as that of a 

 beach. Each drop makes a little dent or impression. It thus 

 forces aside the grains of sand. On sloping ground, where the 

 drops can run together and flow downward, they are able to push 

 or carry the particles of sand or clay along. This is called a 

 mechanical action ; while the actual solution of the particles, as 

 you would dissolve sugar or salt, a chemical action. Each drop of 

 rain may act in either or both of these ways. 



