WH1TF.J RELATIVE HORIZONS OF BASAL BEDS. 821 



Susquehanna Gap through the north wall of the Northern Anthracite 

 field, seem to vary according- to the horizon adopted as the upper limit 

 of the formation at that point. But whatever the thickness of the 

 formation, it is certain that the plant-bearing shales beneath the main 

 cong'lomerate and within 20 feet of the supposed representative of the 

 Mauch Chunk formation are younger than the zone of Lykens coals 

 Nos. 3 and 2, and it is very probable that they are higher than Lykens 

 coal No. 1 or bed L in the type section. In short, although these sec- 

 tions vary greath^ by the expansion of the different divisions, there 

 can be no doubt that, generally speaking, the thin sections of the Potts- 

 ville formation in the bituminous basins in Pennsylvania and Ohio, or 

 along the western margin of the Appalachian trough in Tennessee and 

 Kentucky, contain only the j^ounger beds of the formation, the oldest 

 beds of which appear to be present only in the deepest sections along 

 the eastern margin of the trough in the Virginia region and in the 

 Southern Anthracite field. ^ 



The existence of the older floras in the lower portions only of the 

 thick sections, or, in other words, the equivalence of the very thin 

 sections to the upper portions only of the very thick sections, suggests 

 alternative hypotheses in explanation of the conditions attending the 

 sedimentation of the Pottsville formation. First, the lower portions of 

 the ver}^ thick sections may be regarded as laid down in Mauch Chunk 

 time and contemporaneous with the latest red shale or other Lower 

 Carboniferous sediments in other regions, in which case the basal 

 boundary of the Pottsville in those regions may be diagonal in time 

 without unconformity. The second hypothesis assumes a case of 

 overlap, by which the upper and relatively thinner northern and 

 western deposits W' ere spread beyond the limits of the deeper eastern 

 basins in which the thicker deposits were accumulated. In the latter 

 case the unconformity may or may not extend throughout the field. 



At present we have no conclusive, proof that the oldest Pottsville 

 beds are s^'nchronous with any portion of the marine Lower Carbon- 

 iferous. This does not of itself, however, necessarily preclude the 

 possibility, or even the probability, of a partial contemporaneity, 

 since the conditions governing the deposition of the typical Pottsville 

 sediments were those directly or indirectly causing the expulsion 

 from the same region of the Lowel* Carboniferous faunas and 

 promoting the introduction of the earliest Coal Measures iuA^erte- 

 brate types. Moreover, the upper red shales of the Mauch Chunk 

 formation in the anthracite regions are almost entirely destitute of 

 marine molluscan fossils. Evidence in proof of such contemporaneity 



'The lower horizons of the extraordinarily thick section of the Pottsville formation in Alabama 

 have not been paleontologically studied. It is not, therefore, known whether the oldest floras of the 

 Flat Top coal field or of the Southern Anthracite field are present in the lower part of the section in 

 Alabama. 



