516 WITHIN THE EARTH'S CRUST 



deep below the surface, are exposed and can be mined. 

 Mountains are therefore the great regions for the mining of 

 metals. 



In this country mining is a most important industry in 

 the Sierra Nevada Mountains and in the Appalachian re- 

 gion. In one are found great quantities of copper, silver, 



PLACER MINING IN THE SIERRAS 

 The sand is washed from the gold by huge streams of water. 



and gold, and in the other iron and coal. In the old Lauren- 

 tian Mountain region, near the Great Lakes, much copper 

 is found. The Alps and the Pyrenees are among those 

 mountains that have few minerals. 



The Story of Coal. We have learned that warm, moist 

 air is necessary for the activities of the bacteria of decay. 

 Where there is too much water and not enough air the con- 

 ditions are not favorable for complete decay. When plants 



