THE PENNSYLVANIAN PERIOD 635 



carbon dioxide, the oxygen is exhausted twice as rapidly as the 

 carbon. If the hydrogen and the oxygen of the wood combine, 

 the result is to increase the proportion of carbon remaining. 



While the exact quantitative relations of the reactions which 

 take place are not known, and are probably not constant, the follow- 

 ing table 1 suggests certain changes which might take place, and the 

 products which would remain at certain stages: 



( 20 H 2 O ) 

 C 72 H 120 O GO (cellulose) ] 8 CO 2 [ =C 6 ., H ?2 O 24 (peat). 



(2CH 4 ) 

 6H 2 O ) 

 C 62 H 72 O, 4 (peat) -j 4 CO, [ =C 57 H 56 O 10 (brown coal). 



(6H 2 O 

 1 4 CO, 

 ( CH 4 



3H 2 O ) 



C 57 H^ O, (brown coal) ] CO., [ =C 54 H 42 O 5 (bituminous coal). 



(2CH 4 ) 



(2H 2 ) 



C 54 H 42 O 5 (bituminous coal) ] CO 2 > =C 48 H 18 O (anthracite coal). 



(5CH" 4 ) 



From this table it will be seen that the process which converts 

 vegetable matter into coal is characterized by progressive changes 

 in the nature of the chemical decomposition. The elimination of 

 hydrogen and oxygen in the form of water probably is the domi- 

 nating chemical change in the production of peat from cellulose. 

 Second in importance at this stage is the removal of oxygen in 

 the form of carbon dioxide, while the liberation of methane (CH 4 ) 

 is of still less importance. As the alteration of the peaty material 

 progresses through successive stages to anthracite coal, less and 

 less water and carbon dioxide are given off, and there is a steady 

 increase in the proportion of methane which is freed. Laboratory 

 investigations have shown that while carbon dioxide may con- 

 stitute an important part of the free gas held within the pores of 

 some of the Cretaceous coals, the gas which escapes from the more 

 advanced stages of Pennsylvanian anthracite coal is largely methane. 

 The burial of the peat compresses it, and the physical change result- 

 ing is a part of the process of coal-making. 



If the coal-beds represent Carboniferous swamps, as they are 

 believed to, we have still to inquire into the conditions under 



1 Prepared by Rollin T. Chamberlin. 



