THE PENNSYLVANIAN PERIOD 379 



If, however, the tissues decompose under water, where the air 

 is excluded, the changes are quite different (Fig. 399). There 

 is not enough oxygen present to form much carbon dioxide 

 and water. The principal products are a carbon-hydrogen 

 gas, known as marsh gas, and other compounds which contain 

 less carbon. While the bulk of the hydrogen and oxygen 

 are thus removed, the carbon is only moderately reduced, 

 and thus it comes to form a proportionately larger part of the 

 solid mass which is left. The result of this process is coal. 

 (See curves in Fig. 399.) 



For the formation of coal, then, two things are needed : 

 abundant vegetation, and decay under water. In forests 

 we have the first condition, but not the second. In the sea, 

 decay takes place under 

 water, but the vegetation 

 is not usually deposited in 

 great quantity. In swamps 

 and marshes, however, both 

 conditions are favorable. 

 That coal has actually come 

 from marsh deposits is 

 plainly indicated by many 



facts. The seams of COal FIG. 400. Petrified stump and roots 

 are basin-shaped, being of a tree uncovered in a coal mine in 

 thickest near the middle, 



and thinning out into mere black soils at the edges ; this is 

 just the shape of existing marshes. Again, we find the old 

 stumps and rootlets embedded in the clay beneath the coal 

 (Fig. 400), showing that the vegetation grew where the coal 

 now lies ; and remains of aquatic animals in the midst of the 

 coal tell of the presence of water while it was being deposited. 

 Coal, then, is nothing but the half-decomposed vegetable 

 matter of swamps, long buried by later sediments, compressed 

 by their weight, and converted into a hard rock. 



Varieties of coal. The varieties of coal mark stages in 

 the process by which the volatile components are gradually 



