WHITE] UPPER LIMIT OF THE POTTS VILLE FORMATION. 829 



in one of the collieries a short distance west of the latter gap, was not 

 discovered in a tunnel extending 400 feet below coal B, is in reality- 

 only a bottom split of the Buck Mountain coal. 



THE PALEONTOLOGIC UPPER LIMIT OE THE POTTSVILLE. 



The important fact embodied in the preceding lists of plants from 

 the roof shales of the Twin coal, in both the Southern and the West- 

 ern Middle Anthracite fields, is that they represent a typical and dis- 

 tinctive Coal Measures flora. The small element that this flora has in 

 conunon with that of the Potts ville formation comprises species whose 

 precursors, for the most part difi'ering in their forms and phases from 

 the normal types, have made their appearance only toward the close of 

 Pottsville time. Compared, as a whole, with the flora of Lykens coal 

 No. 1. about 300 feet below the Twin bed in the Lincoln region, or 

 bed L, 380 feet below the Twin in the type section, the species of 

 the latter coal are so different as even to suggest the existence of a 

 time break between the intervening beds. As an argument against 

 such a supposition, 1 have, however, only to cite the plants from Ijeds 

 N and M, whose floras, not less than 210 feet below the Twin coal 

 at Pottsville, clearly presage the development of the Lower Coal Meas- 

 ures plant life by the introduction of a number of Coal Measures types 

 of ferns, notwithstanding the generally stronger paleontologic bond 

 which attaches these beds to the Upper Lykens division of the Potts- 

 ville formation. The transition already indicated in beds N and M 

 seems to have been entirely completed within the time represented by 

 the succeeding 200 feet of the section. 



It is necessar}" in this connection to note the relations of the flora of 

 the Twin coal to those of the Lower Coal Measures in other regions. 

 If we compare that flora with those accompanying the lower coals of 

 the Allegheny series ^ in the Northern States, we find that its composi- 

 tion, range, and development point definitely to a level as high as that 

 of the well-known plant beds at Mazon Creek, Illinois; and that the 

 horizon of the Twin coal should be nearly as high as that of the 

 plants described from Henry County, Missouri,^ and Cannelton, Penn- 

 sylvania. Compared with the better -known floras of the anthracite 

 fields, the plants from the coal in question appear to indicate a level 

 certainly not lower than that of coal C in the -Northern Anthracite 

 field, while it is perhaps safe to say that they are nearly as young as 

 coal D (the Marcy coal) in the vicinity of Pittston. 



According to the evidence of the plants, the beds of the basal por- 

 tion of the Alleghen}^ series, between the top of the Homewood sand- 

 stone and the Morris coal at Mazon Creek — an interval, probably 

 including the Brookville coal, between the same sandstone and a level 

 probably as high as the Clarion coal in the bituminous basins of western 



1 Bull. Geol. Soc. America, Vol. XI, p. 149. SMon. U. S. Geol. Survey, Vol. XXXVII. 



