914 FLORAL ZONES OF THE POTTSVILLE FORMATION. 



3. Tho irrogularity and the lack of selection in the materials inter- 

 larded with the upper beds of red and green shales (Mauch Chunk) 

 appear to l)e natural results of the submergence and somewhat imper- 

 fect working-over of an intermittently subsiding coastal plain under 

 the action of strong and varying detritus-laden currents. 



4r. No evidence of a marked or general iniconformity between the 

 Pottsville and Mauch Chunk is noticeable in this regicm, though at 

 various points within several hundred feet of strata beds of small 

 bowlders or coarse conglomerates are imposed, in a knife-edge contact, 

 on the distinctly uneven surfaces of olive-green mud beds. 



5. The conditions are such that it is impossible to determine upon 

 a persistent stratigraphic basal line of division which can be traced 

 or recognized throughout the basin. Different geologists have taken 

 different horizons. In this report the topmost bed of tvpical red 

 shale or sandstone in the section is arlntrarily taken as the upper 

 limit of the ]Mauch Chunk. This is the usage of geologists in the 

 bituminous fields, w^here, it should be noted, an unconformity probably 

 exists in most areas. 



6. The upper limit of the Pottsville formation has, for reasons of 

 necessity or practicability, been placed by the various geologists and 

 surveys of Pennsylvania at the base of the lowest '* considerable coal,'' 

 which usually occurs not far above the main plexus of massive conglom- 

 erates at the top of the Pottsville formation. Such a horizon, though 

 usuallv traceable for a distance of several nliles, is not always definite 

 where, as happens in portions of the field, the distinctly conglomeratic 

 character of the terranes continues into the Coal Measures and a num- 

 ber of thin coals are interspersed. Yet this mode of delimitation, 

 employed in conjunction with the kno^edge of the stratigraphic rela- 

 tions of the low coals to the main upper group of conglomerates, has 

 probably rarely led to any consideralile vertical error throughout the 

 central portion of the field, including the vicinity of the type section; 

 and, from the standpoint of field practicability, it is probably the 

 most satisfactory method of definition at present available. 



7. The flora in the roof of the Buck Mountain coal, or its supposed 

 equivalents, at the very base of the Lower Coal Measures at Pottsville 

 is a typical Coal Measures flora, very distinct from the floras typical 

 of the Pottsville formation, although a few of its speci(>s appear in the 

 upper 250 feet of beds in the latter, which contain a mixed flora. 

 It is even slightly later than that of the l)asal l)eds of the "Lower Coal 

 Measures"^ in the Northern Anthracite field or of the Allegheny 



1 The term Lower Coal Measures is used in the anthracite fields in the original sense as proposed by 

 Rogers for the series next above the Scral (Pottsville) eonglomerate. It is similarly applied in the 

 northern bituminous basins, where it is, in part at least, synonymous with Allegheny series, Desmoines 

 series, etc. It is, however, shown by the fossil plant-s to be as a whole somewhat later than tlie 

 Lt>wer Coal Pleasures of the Old World. See Bulletin of the Geological Society of America, Vol. XI, 

 pp. 1 1.^178. 



