88 ELEMENTARY SCIENCE 



banks, or where there is a freshly made railroad cutting, 

 you may see, as in the picture, how the underlying rock 

 and subsoil gradually merge into the soil which covers 

 them. In such places you may readily see how soil is 

 made in part by the decay of plants above and in part by 

 the decay of rock beneath. The death of things helps 

 produce it, while it, in turn, helps to produce new living 

 things again; it nourishes the plants; they draw from it 

 materials which they use in growth, and which they trans- 

 form from what was lifeless and inorganic into what is 

 living and organic. You have learned in geography that 

 the wealth of countries depends a good deal upon their 

 "mineral resources," an expression which makes you think 

 of gold and silver and iron and other valuable metals. 

 But the most valuable mineral resource of all is the soil. 



To understand how soil has been formed, we must think 

 of the earth as it was before there was any soil. It is 

 believed that at one time water and rock formed the entire 

 surface of the earth. There was no soil and there was no 

 vegetation. But this condition could not continue long. 

 There are forces constantly at work in nature which cause 

 even the hardest rock to crumble at the surface, and thus 

 the formation of soil begins. The operation of these 

 forces on rock is called rock weathering, and soil has been 

 called "rock-meal" or "rock-flour" because it is chiefly 

 composed of substances which were once hard rock. The 

 organic material in soil, which comes chiefly from the decay 

 of fallen leaves and twigs, is called humus or mould, it is 

 dark-colored. Rich surface soil, especially in forests, is 

 largely composed of humus. 



You have learned that rock means any hard, inorganic 

 part of the earth. So the loose rock (gravel, sand, etc/ 



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