4 AGRICULTURE FOR COMMON SCHOOLS 



corners and become smaller and smaller until they are noth- 

 ing but sand. The parts rubbed off are very small and are 

 carried into still water where they sink to the bottom, and 

 when the stream changes its course they appear as real soil. 



Perhaps the most noticeable action of water is when it 

 works in the form of ice. We have seen cakes of ice all cov- 

 ered with mud floating down stream in the spring. We have 

 seen, too, where ice cakes have struck trees and knocked off 

 the bark. We are told by geologists that centuries ago all of 

 the North American continent as far south as the Ohio and 

 Missouri rivers was covered by ice to a great depth. The ice 

 moved slowly down from the north like a great river and 

 carried everything before it. It gathered up great rocks, and 

 shoved them along and rubbed them over other rocks. Of 

 course, when these rocks were ground against each other 

 pieces were broken off which were carried along and ground 

 on other rocks, until great quantities were crushed into fine 

 powder which we now call soil. All over the region covered 

 by the glaciers are to be found large boulders which escaped 

 crushing and were left behind when the ice melted. These 

 boulders are nearly always rounded, owing to their being 

 rolled over and over and coming in contact with other rocks. 



4. Animals. — While the dead bodies of animals contribute 

 something toward the making of soil, the work of living ani- 

 mals is more noticeable. When such animals as gophers, 

 prairie-dogs, wood-chucks, rabbits, crayfish, earthworms and 

 ants burrow in the earth and throw out raw subsoil and small 

 pieces of rock they are helping to make soil, for this raw 

 material will be acted upon by freezing and thawing, and 

 the roots of plants, and very soon will become, like the 

 surface soil, fit to produce crops. 



