822 FLORAL ZONES OF THE POTTS VILLE FORMATION. 



may be sought in the Missis.sippiaii reo-lons, whence the slight indica- 

 tions at present in hand have been derived. 



Distinct proof of unconf ormit}' between the Pottsville and the Mauch 

 Chunk formations is ahiiost unknown in the anthracite fields, except 

 beneath thc^ thin sections of the Pottsville in the Northern Anthracite 

 coal field. Still, such an unconformity has generally been supposed 

 to exist throughout the greater part of the extent of the formation. It 

 must be admitted that the existence of the thick transition series 

 throughout the central portion of the Southern Anthracite field is 

 opposed to such a lack of continuitv of the Mauch Chunk and the 

 Pottsville, notwithstanding the large number of local irregularities, 

 such as those indicated in the section, PI. CLXXXII, all of which 

 may be explained hy shifting current action. ]My own observation in 

 the Northern field convinces me of the discordant relations of the two 

 formations, the lower of which, though very well developed on the 

 eastern border, appears to be almost extinguished beneath Campbell 

 Ledge, on the western margin, less than 5 miles distant. The discord- 

 ance is frequently observable beneath the thin developments of the 

 Pottsville formation in the more northern and western regions of the 

 Appalachian province. Nevertheless, I hesitate to agree with those 

 geologists who believe the unconformity to exist even beneath the 

 thickest sections of the Pottsville. Such a conclusion seems to be 

 dependent on the still prevalent hypothesis of the absolute time equiva- 

 lence of the entire Pottsville in all sections and all regions. 



For my own part I am slightly inclined to regard the lowest beds of 

 the formation in the deepest sections, such as that in the Flat Top 

 region in Virginia and West Virginia, in which the transition series is 

 not conglomeratic, or that in the Southern Anthracite field, which is 

 conspicuouslv conglomeratic, as continuous with the Mauch Chunk 

 formation, with whose latest typical sediments in other regions the 

 oldest Pottsville beds may be contemporaneous. From such a begin- 

 ning the formation expanded to an enormous thickness of materials, 

 supplied, for the most part, through the transportative agency of 

 destructive wave and current action directed against not very distant 

 coastal plain detrital accumulations, under the favoring conditions of a 

 general, though frequently interrupted, submergence. Brief periods 

 of stability or even slight reactive uplifts, in conjunction with bar- 

 forming currents, ma}' have assisted in producing lagoons, coastal 

 swamps, or other conditions suited either to the accumulation of 

 vegetable matter or to the deposition of thin beds of argillaceous 

 material, while at the same time afiording opportunities for the fur- 

 ther extension of the frontal submarine Pottsville shelf. The gradual, 

 though intermittent, subsidence of the bottom, which made possible 

 the continued piling up of Pottsville strata over ))road areas in these 

 regions, while for a time marginal portions of the Mauch Chunk were 



