THE SOIL. 39 



tower at the entrance of the hole, but in time is 

 knocked over and becomes mixed with the upper 

 layers. Soil thus brought to the surface is fresh 

 and productive, and takes the place of that which 

 has become exhausted. 



The crayfish holes, too, benefit the soil. They 

 permit a free circulation of air to greater depths 

 than would otherwise be possible. The falling-in 

 of their walls produces a loose column of earth 

 down which the roots of plants easily grow for 

 fresh supplies of food and water. 



Ants are very common as earth-mixing animals. 

 A colony of ants will bring to the surface during a 

 single season enough earth to form a mound several 

 inches high, and two or three feet in diameter; 

 in many cases, they make much larger ones. They 

 carry into their galleries animal and vegetable 

 matter which finally decays and becomes a part of 

 the soil. 



Some rodents, and other small fur-bearing ani- 

 mals, dig in the ground not a little. The mole lives 

 in the ground all the time, and its entire make-up 

 is well suited to the life it leads. It tunnels the 

 ground in every direction, making it loose and 

 open for the free circulation of air and water. 

 Gophers, mice, prairie dogs, and many such ani- 

 mals dig in the ground more or less and do valu- 

 able work in stirring the soil. 



The most valuable of all animals in soil-making 

 is the common earthworm, or angleworm. It is a 



