EARLY POCAHONTAS AXIS 275 



that the beds of the Sewell formation, lying above the Bon Air- Raleigh 

 sandstone, swept far bej^ond the present western boundary of the coal 

 field in southern Tennessee, while the Pottsville beds above the Sewell 

 must have been of great thickness in addition. 



In the region of the most southerly of the cross sections here presented, 

 the Briceville shale, which corresponds to a portion only of the Sewell, 

 is reported * to thin from 500 to 600 feet at its eastern border to about 

 200 feet at its last full exposure, less than half way across the coal field, 

 here almost at its narrowest. In this connection it is proper to suggest 

 that a westward gradation of the Briceville into sandstone recognized as 

 Wartburg, or possibly its diminution alone may account for the approach 

 or even the contact of the Wartburg and Lee to the northwest, and the 

 inclusion of the Corbin (Wartburg) in the Lee along the northwestern 

 border of the field near the southern Kentucky line. Further evidence 

 is needed for the satisfactory determination of this point. 



POSITION OF EARLY POTTSVILLE AXIS 



The existence of an early axial Pottsville basin near the eastern border 

 of the present Appalachian coal region has been clearly shown ; but con- 

 cerning the more exact geography and hydrography of this axial basin 

 much remains to be learned. It seems probable that we approach most 

 nearly to this narrow basin at the eastern border of the Great Flat Top coal 

 field in Virginia and in the Southern Anthracite field in Pennsylvania. 

 The evidence in support of this view consists in (1) the presence at these 

 points of the oldest recognized Pottsville floras, and (2) the existence of 

 transition series. Reference has already been made to the thick transi- 

 tional beds at the base of the Southern Anthracite field.f Proximity to 

 the eastern land in this vicinity is indicated not only by the gradual 

 passage from red shales and greenish sandstones to gray coal-bearing 

 conglomerates, but by the large size of some of the boulders in the con- 

 glomerates. In the Pocahontas region the sedimentation was continu- 

 ous though relatively rapid from the red and olive shales of the Mauch 

 Chunk through the Lower and Middle Pottsville without the introduc- 

 tion of conglomeratic sediments. In this region there is no unconform- 

 ity between the Mississippian and the Pennsylvanian.J Farther to the 

 southwest, toward the Tennessee line, although the total Pottsville sedi- 



*See Arthur Keith : Briceville folio, no. 33. 



fSee Twentieth Ann. Rept. U. S. Geol. Survey, pt. 2, p. 831. 



\ In his earliest papers on the subject the writer suggested that the lower portions of some of the 

 thick eastern sections of the Pottsville might have been laid down near or in deltas contempora- 

 neously with the highest red or green Mauch Chunk beds of some other region. This still 

 appears possible; but that such a condition continued for any considerable length of time or 

 beyond the earliest Pottsville is very improbable. 



XXXVI— Bull. Geol. Soc. Am., Vol. 15, 1903 



