164 HISTORICAL GEOLOGY 



Pottsville basin of deposition expanded and extended over much of 

 the interior coal fields region. Though not so well known, the best 

 evidence also points to considerable submergence of the western 

 United States in the early part of the period. 



Middle and Late Pennsylvanian. — During the middle Penn- 

 sylvanian the geographic conditions were essentially as shown on 

 the paleogeographic map (Fig. 96), except that true marine waters 

 then also covered the temporary land area in Wyoming, Colorado, 

 and New Mexico. In the west, marine conditions prevailed till the 

 close of the period, while in the east true marine, estuarine, lacus- 

 trine, marsh or bog, and possibly even land conditions alternated 

 repeatedly and more or less locally in the basins of deposition. 

 Since such remarkable physical geography conditions favored the 

 accumulation of the world's greatest coal beds, the} T deserve more 

 detailed discussion. " Perhaps the most perfect resemblance to 

 coal-forming condition is that now found on such coastal plains as 

 that of southern Florida and the Dismal Swamp of Virginia and 

 North Carolina. Both of these areas are very Jevel, though with 

 slight depressions in which there is either standing water or swamp 

 conditions. In both regions there is such general interference with 

 free drainage that there are extensive areas of swamp, and in both 

 there are beds of vegetable accumulations. In each of these areas 

 there is a general absence of sediment and therefore a marked 

 variety of vegetable deposit. If either of these areas were sub- 

 merged beneath the sea, the vegetable remains would be buried 

 and a further step made toward the formation of a coal bed. 

 Reelevation, making a coastal plain, would permit the accumula- 

 tion of another coal bed above the first, and this process might be 

 continued again and again." x It is, however, not necessary to 

 assume repeated elevation and subsidence of swamp areas in order 

 to account for numerous coal beds one above another in a given 

 region. A general subsidence, often intermittent (with possibly 

 some upward movements), would occasionally cause the luxuriant 

 vegetation of a great swamp area to be killed and allow the depo- 

 sition of sediment over the site. Then the filling of the shallow 

 water with sediment would allow another bog to be formed, etc. 

 In the coal field of Nova Scotia there are 76 distinct coal beds; in 

 Alabama 35; in Pennsylvania at least 20; and in Illinois 9. Each 

 of these coal beds represents an ancient swamp in which grew a 

 1 H. Ries: Economic Geology, 1910, p. 9. 



