206 A TEXT-BOOK OF 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 les 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, where the bituminous coal has been changed 
into anthracite. 
