TRANSGRESSIONS OF THE SEA 271 



beds which are* absent, and the maximum thickness indicated on the 

 map is that reported for the beds remaining in the Alabama basins. It 

 is probable that 1,500 feet or more of additional sediments would be 

 required to carry the section in this region up to the base of the Alle- 

 gheny. A thickness of 7,000 feet would be hardly greater than that of 

 some of the Millstone Grit sections of Great Britain and considerably 

 less than the Waldenburg group of the Lower Silesian region. 



Development of the Basin 



general features 



By reference to the map it will be noticed that the curve of the bottom 

 of the Pottsville basin steepens rapidly toward the eastward and flattens 

 or becomes irregular in passing toward the northwest. The latter is the 

 great region of encroachment or overlap. 



One of the facts first to be disclosed by the study of the Pottsville 

 floras is that the thick sections along the eastern border of the Appala- 

 chian coal region contain floras distinctly older than those present in 

 the lowest beds of the thin northwestern sections, and that the character- 

 istic floras of the thin sections occur in their natural order in the upper 

 part only of the thick sections.* For example, the flora of the Sharon 

 coal, whose horizon is near the level of the base of the Pottsville in the 

 bituminous regions of Pennsylvania and Ohio, is in the upper part of 

 the deep sections of the Pottsville in the Southern Anthracite field or in 

 southern West Virginia,f while in northern Tennessee it is above the 

 Wartburg sandstone. In other words, the lower Pottsville beds were 

 never deposited in the bituminous regions of Pennsylvania, Ohio, or in 

 northwestern West Virginia. 



The nature and extent of the overlap within the surviving Coal Meas- 

 ures areas are best expressed in tne accompanying cross-sections of the 

 coal fields on the lines A B, C D, E F, and G H, the exaggerated vertical 

 scale being nearly 2,250 feet to the inch. The correlation of the main 

 divisions of the Pottsville from region to region are chiefly based on the 

 paleobotany of the sections. 



POCAHONTAS DEPOSITION 



The oldest Pennsylvanian formation yet recognized is the Pocahontas 

 formation of the Great Flat Top region. It is the lowermost Pottsville. 

 In the Pocahontas quadrangle % it is reported to have a thickness of 



*Bull. Geol. Soc. Am., vol. 6, 1895, pp. 319, 320. 



Twentieth Ann. Rept. U. S. Geol. Survey, pt. 2, 1900, pp. 819-823. 

 fBull. Geol. Soc. Am., vol. 6, 1895, p. 319. 

 J Geol. Atlas of the United States, folio no. 15. 



