756 FLORAL ZONES OF THE POTTS VILLE FORMATION. 



bituminous basin.s a nunib(>r of terms are used for individual terranes 

 or local groups, which have often erroneously and without harmony 

 been correlated with portions of, or with the whole of, the Pottsville 

 formation. The extra-anthracitic nomenclature is not, however, 

 involved in the immediate consideration of the formation in the type 

 region. '"Formation" is here used in a broad sense in preference to 

 "series," in view of the subordinate rank of the collective terranes in 

 the geologic column, their biologic unity, and their lack of individual 

 persistence or continuity. 



In general, the Pottsville formation has been understood as wholly 

 or in part representing the Millstone grit of Nova Scotia, New Bruns- 

 wick, and the Old World. This correlation is founded chiefly on the 

 lithologic similarity and the coincidental occurrence of the two forma- 

 tions at the l)ase of the Productive Coal Measures. In the case of the 

 Pottsville the correlation has rested entirely on the order of strati- 

 graphic occurrence and the lithology, a method of coordination that, 

 as will be shown later, has resulted in the reference of a portion of 

 the formation in the Southern States, where it contains the most 

 valual)le coals of the Southern Appalachian districts, to the Productive 

 Coal Measures. It has, moreover, been the custom to consider the 

 lithologic representative in each State or region as contemporaneous 

 with and equivalent in toto to the lithologic member or group in ever}" 

 other region, including the type section. The studies, now in progress, 

 of the plant fossils of the terranes in different regions, correlated by 

 lithology with the Pottsville formation, clearly show the fallacy of 

 reuartling the lithologic section in each region as covering the same 

 time interval as that covered by every other section. They also show 

 that in certain regions considerable thicknesses of beds which, on 

 account of the lithology, are referred to the post-Pottsville Coal Meas- 

 ures contain the well-marked and distinctly characteristic floras of 

 various horizons in other sections which have been determined, on the 

 lithologic basis, as Pottsville. It is clear that under such circumstances 

 the correlation, especially between separate basins, must be bv means 

 of comparative paleontology. 



AGENCY OF FOSSIL PLANTS IN THE CORRELATION OF THE 



TERRANES. 



From tlie foregoing it will be seen that the existing condition is one 

 in wliicli we have, under numerous names, a large number of terranes 

 of supi)osed Pottsville age in both the interior or Mississippi basin 

 and the (>astern l»asins, the final correlation of which is largely or 

 often wholly dependent on the results of paleontologic studv. Since 

 the organic remains in these beds are comi)osed prinlominantly, if not 

 exclusively, of plant fragments, the foremost (piestions involved in any 

 correlation with the Peinisylvaniansectionare: Has the typical Pottsville 



