THE PENNSYLVANIAN PERIOD. 571 



largely confined to the constituent elements of the vegetable matter. 

 The weight of the sediment would compress the vegetable tissues, 

 and it is believed that the result would be its ultimate conversion into 

 coal. The various grades of coal, bituminous, anthracite, etc., are 

 thought to represent different stages in the process of change. 



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

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

 swamps as extensive as the coal-beds existed, and to seek the explana- 

 tion of their frequent recurrence (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 from ready decay after its growth and death, would be insufficient. 



In the course of the widespread, but for the most part very gentle, 

 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. The conditions of climate and moisture 

 appear to have been such as to allow the abundant growth of vegeta- 

 tion in the marshes. On falling into the shallow water, or upon the 

 water-soaked bottom, the vegetable matter underwent changes of 

 the nature indicated above. The ill-drained areas were thus con- 

 verted into peat bogs, in which, in many places, little inorganic 

 matter was mixed with the organic. The longer the life of the bog, 

 the deeper the peat became. The rate of accumulation, though de- 

 pending on climate and on the abundance of vegetation, is always 

 slow. Under very favorable conditions, a foot of peat may accumulate 

 in ten years or even less, but the usual rate is probably much slower. 

 Peat bogs are now in existence in which the depth of the accumulated 

 organic matter is 40 or 50 feet, but the length of time involved in 

 the accumulation is not known. 



Some of the great swamps probably came into existence along 

 the sea-shores, and some in shallow basins or undrained areas remote 

 from the sea, for fresh-water shells are found in association with some 

 coal-beds, and marine fossils in association with others. 



The history of a coal-bed. — Each coal-bed represents the accu- 

 mulated vegetable growth of a long period. It would appear that 



