191 1-] STEVENSON— FORMATION OF COAL BEDS. 43 



the great marsh, which became the Pittsburg coal bed, crept up the 

 shore — and this perhaps to the very close of the epoch. Thus it is, 

 that though giving origin to many subordinate seams, the great bed 

 diminishes westward. The Pittsburg coal bed began at the east 

 and advanced westwardly. There is evidence in the distribution of 

 sandstones and shales that a delta formation at the east pushed out 

 into the basin, so that conditions favorable to coal-making existed 

 first on the east side of the great basin. His summary is : 



( I ) The great bituminous trough west from the Alleghanies does 

 not owe its basin-shape primarily to the Appalachian revolution. 

 (2) The coal measures of this basin were not united to those of 

 Indiana and Illinois at any time posterior to the Lower Coal Meas- 

 ures (Allegheny) and probably were always distinct. (3) The 

 Upper Coal Measures (Alonongahela) extended as far west as the 

 Muskingum river in Ohio. (4) Throughout the Upper Coal ]\Ieas- 

 ures epoch, the general condition was that of subsidence, interrupted 

 by longer or shorter intervals of repose. During subsidence, the 

 Pittsburgh marsh crept up the shore, and in each of the longer 

 intervals of repose it pushed out upon the advancing land, thus giving 

 rise to the successive beds of the Upper Coal Measures. (5) The 

 Pittsburgh marsh had its origin at the east. 



Two years later,"*" after further studies in West \'irginia, he 

 ofifered additional arguments in favor of his suggestions and ex- 

 tended the scope of his hypothesis. The Appalachian basin at the 

 beginning of the Upper Coal Pleasures was closely landlocked, com- 

 municating with the ocean at the southwest by a comparatively nar- 

 row outlet. At the east and southeast, rivers brought in their loads 

 of detritus to be spread over the bottom of the basin ; on the opposite 

 side, few and sluggish streams flowed from the low Cincinnati fold. 

 During periods of repose, deltas were formed and the marsh ad- 

 vanced on the newly formed land. If the period of repose were long 

 enough to permit the filling of the bay. the marsh would extend 

 across if begun on one side, or to the middle if passing out from all 

 sides. The basin in West Virginia was never so filled with detritus 

 as to permit coal beds to cross it. The Appalachian basin was never 



'""On the Alleged Parallelism of Coal Beds," Proc. Aiiici: Phil. Soc, 

 Vol. XIV., 1875, pp. 283-295. 



43 



