WHITE.] LOWER LIMIT OF THE POTTSVILLE FORMATION. 831 



at Mill Creek Gap, on the north side of the Hecksctierville Valley 

 Basin, in which case the latter should be of Coal Measures age rather 

 than Pottsville age, as has generall}^ been assumed. The reference of 

 several of the thin coals not far below the Twin or Buck Mountain 

 horizon to the Lower Coal Measures is not discordant with the opinion 

 that the}" are but splits from the Buck Mountain bed,^ though I do 

 not so regard them. It appears more probable, however, that they 

 are distinct and earlier beds, whose geographic extent may not be 

 great, and whose individual correlation, in any event, is uncertain. 



The difference between Ihe positions of the conventional formation 

 limit in the Southern as compared with that in the Northern Anthra- 

 cite held is no doubt due to the continued deposition, with exceed- 

 ingly slight intermissions, of heavy conglomerates above the main 

 plexus of egg conglomerates in the Southern Anthracite field, which, 

 in turn, is the result of the nearness of the latter to the abundant and 

 rapid suppl}^ of coarse sediments. 



LOWER LIMIT OF THE F0RMATI0:N^. 



Owing to the transitional character of the passage beds from the 

 typical red shale of the Mauch Chunk to the typical gray conglomerate 

 phase of the Pottsville, as illustrated in PL CLXXXll, the discover}^ 

 of a constant and recognizable boundarj^ is a much more difficult matter 

 than would at first appear, if indeed it is not an impossibility. Having 

 observed that the upper beds of the red shale are, like the interca- 

 lated conglomerates, irregular, unstable, and subject to disappearance 

 by wedging or pinching out,^ the practice of selecting some arbitrarj^ 

 boundary in the conglomeratic upper portion of the red shale was 

 inaugurated l)y the first State geological survey, and has been followed 

 b}" the geologists of the second surve}'. As the result of this usage in 

 'the type region, where the transition is the most gradual, "the fixing 

 of a precise limit between the two formations becomes, in man}^ 

 instances, a matter of individual preference and judgment,"^ and it fol- 

 lows, not only that the thickness assigned to the sections varies with the 

 geologists, but that it is often necessary to hold the published colunmar 

 section in hand in order to find the arbitrary boundar3\ To this ele- 

 ment of uncertainty is probably due a portion of the apparent varia- 

 tions in the recorded thickness of the formation at certain points, as 

 compared to that at other places, since in some sections several hun- 

 dreds of feet below the topmost red shale have been included in the 

 Pottsville formation, while in other regions the line has been drawn at 

 or near the last stratum of red shale. Fortunately, toward the west- 

 ern end, and along the northern border of the Southern Anthracite 



1 Summary Final Report, Geol. Survey Pennsylvania, Vol. Ill, Pt. I, p. 2083. 



2 Rogers, Geol. Pennsylvania, Vol. II, Pt. I, pp. 22, 25. 



3 Smith, Summary Final Report, Vol. Ill, Pt. I, p. 1921. 



