206 A TEXT-BOOK OP BOTANY 



first stages in coal-formation. During the Coal-measures 

 there were very extensive areas of swampy land covered 

 with a luxuriant vegetation, consisting principally of ferns, 

 equisetums, and lycopods. The dead bodies of these 

 plants accumulated in immense deposits in the swamp 

 waters; and when a sinking of the land brought it under 

 water, sediments were deposited upon the accumulated 

 vegetation and it was gradually changed into coal. Suc- 

 cessive risings and sinkings of the 'land surface brought 

 about an alternation of vegetation and sediments, and so 

 the coal lies in beds of varying thickness. The ferns, 

 equisetums, and lycopods are often spoken of as peculiarly 

 useless plants; but when one considers the part they 

 played in coal-formation, and the importance of coal in our 

 civilization, it is evident that no plants have done more for 

 human welfare. 



The different kinds of coal depend upon the amount and 

 kind of changes in this old buried vegetation. For example, 

 hard coal (anthracite) has been changed most, containing 

 eighty-five per cent or more of carbon; while soft (bitumi- 

 nous) coal contains only fifty to seventy-five per cent of 

 carbon. It will be remembered that green- plants take 

 carbon dioxide from the air and use the carbon in building 

 their bodies ( 14).' Therefore, the enormous amount of 

 carbon contained in coal deposits was in the main drawn 

 from the air by plants. When coal is burned now there 

 is made a tardy return of carbon dioxide to the air for that 

 which was taken from it millions of years ago. 



The coal-fields of the United States are the greatest in 

 the world that are now being worked; but the coal-fields 

 of China are probably even greater. The coal of the United 

 States is all soft coal, except in the mountain region of 

 Pennsylvania, wh,ere the bituminous coal has been changed 

 into anthracite. 



