WHITE.] UPPER LIMIT OF THE POTTS VlLLE FORMATION. 823 



yet subject to erosion in other areas, eventually resulted in the sub- 

 mergence of those portions of the coast where thinner Pottsville sec- 

 tions are now found, each of which contains its own paleontolog-ic rec- 

 ords marking- the time of the submergence of that localit3^ 



The relatively later ages of the lower beds of the Pottsville sections 

 which we meet in approaching the northern and western margins of 

 the Appalachian coal fields constitute conclusive evidence of the west- 

 ward and northward transgression of the Carboniferous sea during 

 Pottsville time. 



UPPER IiI3IIT OF THE POTTSVIELE rORMATIO:N^. 



THE FLORA AT THE BASE OF THE LOWER COAL MEASURES IN 

 THE SOUTHERN ANTHRACITE FIELD. 



As may readily be inferred from an examination of the columnar 

 sections of the Coal Measures of the Southern Anthracite field, as 

 developed, for example, in the Tremont-Lincoln district,^ the alterna- 

 tion of conglomerates is so continuous from the typical ''Serai con- 

 glomerate " into the Lower Coal Measures that it was found in general 

 to be *' impossible to assign a well-defined permanent horizon of sepa- 

 ration '' between the two formations. Not onh' are the sandstones 

 exceedingly variable as to their thickness and limits, but their uncer- 

 tain and often indefinite exposures poorly serve the needs of precise 

 delimitation, such as is required for a detailed instrumental mineral 

 survey. Accordingly, for reasons of expediency and necessity, the 

 boundary was arbitrarih" fixed by Rogers "at the bottom of the first 

 or lowest considerable coal seam." " Under the more refined and exact 

 methods of the succeeding geologists, who have made thorough exami- 

 nation of the region by the aid of abundant prospecting, the position 

 of the lowest workable coal, or its supposed equivalent, has been locally 

 ascertained throughout the greater portion of the field, and the cor- 

 relations have been extended also to the Middle Anthracite fields.'' 



The practicabilit}" of this method is largely due to the occurrence of 

 a coal generally of workable thickness over the greater portion of the 

 fields not far above the great plexus of conglomerate plates at the 

 top of the series. However, as may be supposed, where several thin 

 coals are present in a given section between the lowest workable coal 

 and the main conglomerates of the Pottsville, or where of several thin 

 coals none is workal>le, the boundary is uncertain. In general, and 

 especially in the central portion of the field, including the vicinity of 

 the type section, it is possible to trace the Twin coal, adopted by 



' Atlas Southern Anthracite Field, Pt. IVB, columnar-section sheet x; also Pt. II, columnar-sfction 

 sheet vi; and Pt. I, coluninai-sectior sheet i. 



2Ge()l. Pennsylvania, Vol. II, Pt. I, laW, p. 17. 



3 Ann. Rept. Geol. Survey Pennsylvania, 1886, Pt. Ill, p. 9:«; Summary Final Report, Vol. Ill, Pt. I, 

 pp. 18.54, 1920. 



