636 GEOLOGY 



which such extensive swamps 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 prompt decay after its growth and death, would 

 be insufficient. 



Summary. In the course of the wide- spread 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 sub- 

 merged stood so low as to be ill-drained, or undrained, and con- 

 stituted marshes. Climatic conditions were such as to permit the 

 growth of abundant vegetation in the marshes. On falling into the 

 shallow water, the vegetable matter underwent changes of the nature 

 suggested above. The marshes were thus converted into peat bogs. 

 Some of the great coal-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 with others. 



Each coal-bed represents the accumulated vegetable growth 

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

 tion of vegetation was often brought to an end by subsidence which 

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

 drowning the plants, and burying the organic matter which had 

 already accumulated under deposits of mud, sand, etc., which the 

 submergence brought in its train. A second coal-bed in the sumo 

 region points to the recurrence of swamp conditions, and moan- 

 either (a) that after submergence and burial of the organic matter 

 slight emergence reproduced the conditions for bogs; or (/>) that 

 by sedimentation the sea or lake bottom where the first bo.u had 

 been was built up to the water-level, restoring swamp com lit ions. 



The number of coal-beds is often great. In Pennsylvania it 

 frequently (but not everywhere) exceeds 20; in Alabama, .V> (not 

 all workable) have been enumerated; in Nova Scotia, the number. 

 including some dirt-beds, is said to be about 80; but in the Mi 



