SOILS 281 



the development of the science of agriculture a man is 

 able to produce much more on an acre of ground than 

 formerly. The surplus that he does not need for his own 

 uses is exchanged for the manufactured products of his 

 city brother, and both the manufacturer and the farmer 

 profit thereby. 



The Crust of the Earth. - - The crust of the earth varies 

 greatly in composition. Many different kinds of rocks 



FIG. 240. Sedimentary Rock. 

 Horizontal strata near Quebec, Canada, showing layers of sediment. 



are found on or near the surface of the earth. These are 

 covered with a variety of soils, the soil usually containing 

 in each case the same elements and compounds as the 

 rocks below it. 



The rocks of the earth's crust are divided into three 

 classes : igneous, sedimentary, and metamorphic, in 

 accordance with the manner of their formation. 



Igneous rocks are those which at some time in their 



