90 



Our Surroundings 



all of which consist largely of the elements carbon, hydrogen, 

 and oxygen. Wood also contains various minerals. The flame 

 we see in the process of combustion is due to the burning of 

 vapors or gases which are released from the wood by heat. 



Coal. Coal is composed largely of carbon. Ages ago it 

 was formed from large trees and other luxuriant growths of 

 vegetation that died and became covered with water in the swampy 

 places where they grew. As water kept the air from their tis- 



United States Geological Survey. 



BRINGING COAL FROM THE MINE 



sues, they did not readily decay. Eventually they became cov- 

 ered with earth, and new forests with smaller forms of plants 

 grew over them. These, in turn, died and were covered by 

 rocky materials. This continued for ages, until the pressure 

 and heat due to this covering became great enough to change 

 the vegetable matter into coal. The vegetable matter under 

 the greatest pressure and heat became hard, or anthracite, 

 coal; while that under less pressure and heat became soft, or 



