COAL 341 



and perhaps as luxuriant as the tropical forests of to-day. They 

 have survived by adjusting themselves to very different life 

 conditions from those of Carboniferous times, and by adopting 

 life habits which remove them as far as possible from competi- 

 tion with the prevailing vegetation forms of to-day (trees, grasses, 

 herbs, etc.). The degenerate, saprophytic gametophytes of Ly co- 

 podium illustrate well how far such changes of life habits may 

 extend. 



330. Coal. Coal was formed during a number of periods in 

 the earth's history, but the most extensive deposits were laid 

 down during the Carboniferous Age (frequently called the coal 

 age), forming the so-called coal measures. The luxuriant pterido- 

 phyte vegetation of tree ferns, horsetails, and club mosses formed 

 deposits in swamps over immense areas, probably in much the 

 same way as peat is being formed to-day. Such plant deposits 

 from time to time became covered with sediment by the sink- 

 ing of the land. And since the land alternately rose and sank, 

 successive layers or beds of plant remains were laid down. 

 These remains became finally buried under heavy deposits of 

 sediment, which pressed them into compact beds of the car- 

 bonaceous matter called coal. 



Coal is of two sorts: (1) soft or bituminous coal, which is 

 hardly more than half pure carbon, the rest being composed of 

 a variety of carbon compounds, and (2) hard or anthracite coal, 

 which may be 90 per cent pure carbon. Hard coal represents a 

 greater degree of change than soft coal, the oils and other products 

 having been driven off under pressure by the heat of the earth. 

 The coal beds vary in thickness from small layers of only a few 

 inches to deposits a hundred feet deep. Those of the United 

 States cover several hundred thousand square miles, of which 

 perhaps fifty thousand square miles are being worked. Vast as 

 are these coal beds in the United States, there are deposits in 

 other lands, as in China, of even greater extent. The coal supply 

 of China is estimated as enough to last the world seven hundred 

 years. The total deposits of pteridophyte vegetation were very 



