THE SOIL 



309 



dust is placed under the microscope the individual shells 

 can be seen. The extensive chalk deposits in the United 

 States and England were formed of these tiny animals. 



The marble of the New England States was made 

 by the limestone deep in the earth being heated 

 to a certain temperature and 

 then cooled; afterward the sand 

 rock on top of it was washed 

 away and the marble was ex- 

 posed to view and now man can 

 quarry it for building purposes. 



The formation of sand rock 

 from water deposits, limestone 

 from deposits of shells, and with 

 a deposit of clay occasionally 

 between the rock layers, has 

 gone on for millions of years, 

 so that now rock strata formed 

 of water deposits are many miles 

 deep. During all these past 

 ages the earth was being pre- 

 pared for the habitation of man, who has now studied the 

 structure and composition of the various kinds of rocks 

 and has learned their relation to soils and agriculture. 



The disintegration of all kinds of rock near the earth's 

 surface is still going on and soil is being formed. By 

 a knowledge of the kind of rock on the surface in any 

 locality, one can tell much of the nature of the soil and 

 what should be done to make it fertile and keep it pro- 

 ductive. A region that has limestone near the surface 

 is usually very productive, while the soils in sandstone 

 regions are not very fertile. It is very expensive to make 

 sandy and clay soils fertile in a hilly or rolling country; 



CHALK 



Magnified. 



