428 STEVENSON— THE FORMATION OF COAL BEDS. [Nov. i. 



increasing southwardly, so that in that direction the thickness of the 

 deposits increases until eastern Alabama is reached, where one 

 finds progressive overlap and each deposit has its attenuated out- 

 crop beyond that of its predecessor. Toward the close of the 

 Mississippian, Alleghania was becoming better defined; all of the 

 formations are present in most of it but they are very thin, nowhere 

 more than 400 feet thick at the north, less than one fifth as much 

 as in the anthracite region. Whether or not the Shenango shales 

 of western Pennsylvania are synchronous with the upper red beds 

 of the Mauch Chunk region cannot be determined, as a gap of 60 

 miles exists, from which the beds have been removed. Fossils are 

 rare and insufficient for correlation ; they make evident, however, 

 that marine conditions prevailed in Alleghania, for the individuals 

 obtained in southwestern Pennsylvania are large, well developed and 

 thoroughly characteristic. The water was probably too shallow and 

 too variable in distribution to permit abundant life; the sun cracks, 

 ripple marks and other features indicate that these fine muds were 

 spread out on mud flats, with constantly shifting areas of tidal 

 waters. It is certain that withdrawal of the sea was continuous on 

 the western side, so that before the close of Mississippian, the Ohio 

 basin had become dry land and Alleghania had become once more 

 a distinct ridge, dividing the basin longitudinally from New York 

 to central Tennessee. But the withdrawal affected almost the 

 whole of the Appalachian basin; and this withdrawal may have 

 been caused by extensive deformation of the surface. While 

 Alleghania was raised at the west, there was rejuvenation of Ap- 

 palachia at the east. In the later Mississippian, the streams had 

 reached base level along the borders, for only fine muds were carried 

 into the basin; but the Pottsville opens with coarse deposits from 

 Pennsylvania to Alabama. 



The distribution and character of the Pottsville deposits' seem 

 to place beyond doubt the assertion that at the beginning of the 

 Pennsylvanian the whole basin, excepting at the southwest corner, 



'p. White, "Deposition of the Appalachian Pottsville," Bull. Geol. Soc. 

 Amer., Vol. 15, 1905, pp. 267-282; J. J. Stevenson, "Carboniferous of the 

 Appalachian Basin," ibid., Vol. 18, 1907, pp. 142-150. 



