452 PENNSYLVANIAN PERIOD 



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 (H 2 O) probably is the dominant change in the 

 production of peat from cellulose. Second in importance at this 

 stage is the removal of oxygen in the form of CC>2, while the libera- 

 tion of methane (CH 4 ) is of still less importance. As the alteration 

 of the peaty material progresses through successive stages to coal, 

 less and less water and carbon dioxide are given off, and there is an 

 increase in the proportion of CH4 set free. Laboratory investiga- 

 tions have shown that while CO2 may constitute an important part 

 of the free gas held in the pores of some of the Cretaceous coals, the 

 gas which escapes from the more advanced stages of Pennsylvanian 

 anthracite coal is largely CH 4 . The burial of the peat compresses 

 it, and the physical change resulting is a part of the process of coal- 

 making. 



If coal-beds represent former swamps, as they are believed to, 

 we have still to inquire into the conditions under which such ex- 

 tensive swamps existed, and to seek the explanation of their recur- 

 rence (one for each coal-bed) in many regions. 



The first condition for a swamp is lack of drainage, and the 

 second a sufficient, but not an excessive amount of water. Enough 

 to stop the growth of vegetation would be excessive, and too little 

 to preserve it after its growth and death, would be insufficient. 



During the widespread movements which affected the eastern 

 interior at the close of the Mississippian period, great areas appear 

 to have emerged from the sea. Early in the Pennsylvanian period, 

 considerable tracts which were not submerged stood so low as to 

 be ill-drained, or undrained, and constituted marshes. Climatic 

 conditions were such as to permit the growth of abundant vegetation 

 in the marshes, where, after death, the vegetable matter underwent 

 changes of the nature suggested above. The marshes were thus 

 converted into peat bogs. Some of the great coal-swamps prob- 

 ably came into existence along shores, and some in shallow inland 

 basins or undrained areas. 



Each coal-bed represents the accumulated vegetable growth 

 of a long period. It would appear that the growth and accumula- 

 tion of vegetation was repeatedly brought to an end by subsidence 

 which let the water (sea, lake, or aggrading stream) in over the 

 marshes, drowning the plants, and burying the organic matter which 



