742 



PRINCIPLES OF STRATIGRAPHY 



horizons possible. The formation is thickest in the eastern Appa- 

 lachians and thins away westward by failure of the lower beds and 

 overlap of the higher. The lowest beds are found in two localities in 

 the eastern area, near Pottsville, Pennsylvania, and in the Poca- 

 hontas region of West Virginia and Virginia. From these two cen- 

 ters the deposits spread northward, westward and southwestward, 

 each later division overlapping the preceding ones, until in western 



Fig. 155. Diagram showing replacing overlap of a nonmarine series over a 

 marine one. 



Pennsylvania and Ohio only the highest members of the series are 

 present. Since the source of these beds can only be in the eastern 

 Appalachians, where alone rocks capable of furnishing such ma- 

 terial exist, the overlap becomes one away from the source of sup- 

 ply. Such an overlap might perhaps be produced under water if 

 the overlapping marine sediments belonged to a spreading sub- 

 marine delta. In this case the beds would become increasingly finer 

 around the periphery of the submerged delta, and they would merge 



Fig. 156. Diagram showing the relationship of the nonmarine Catskill and 

 the marine Chcmnng in eastern North America. A Devonic ex- 

 ample of replacing overlap. 



into deposits of purely marine origin. Nor would the delta be of a 

 size comparable to that of the Pottsville, which by its character, its 

 coal beds and its overlap relations is shown to be of subaerial ori- 

 gin. There are to be sure one or more reported horizons of marine 

 faunas, a brachiopod fauna having been found in the middle Potts- 

 ville (Horsepen) as far north as Sewell, on New River. If these 

 organisms really indicate marine conditions and not secondary in- 

 clusions, this can only mean a momentary invasion of the delta 

 area from a neighboring arm of the sea. Naiadites and "Spirorbis" 

 have been found in the Lower Lykens division of the anthracite 



