SUMMARY 281 



gress in Kentucky and southern West Virginia. It is probable that the 

 erogenic movement to the eastward was more or less progressive. 



5. There appears to have been relative steadiness during the greater 

 part of Sewell time (Upper Pottsville, in part), though sedimentation 

 was very rapid in the Southern Appalachian region ; but subsidence evi- 

 dently continued, and the period including the Sharon conglomerate 

 marks another great encroachment when the sea overspread the present 

 bituminous region of Ohio and Pennsylvania, transgressing the eroded 

 and somewhat uneven Mississippian floor until the Connoquenessing 

 sandstone came into contact with the Pocono or basal Mississippian here 

 and there along the northern shores or the flank of the Cincinnati arch 

 to the northwest. The conclusion of the Homewood stage — that is, the 

 close of the Pottsville — probably saw the shoreline removed some dis- 

 tance be}^ond the present limits of the coal fields in the northern region * 

 though there is room for doubt as to whether it crossed the Cincinnati 

 axis in Ohio and thus made connection with the Michigan coal field. It 

 is nighty probable that by this time the Pottsville sea swept across the 

 Cincinnati arch in southern Kentucky and Tennessee, so as to connect 

 with the interior region, which may already have been reached farther 

 south, westward from central Alabama, at a time perhaps as early as the 

 Bon Air stage. 



Continuity of sedimentation from the Mauch Chunk to the Pottsville 

 without abruptness of lithological change is evident only at the base of 

 the Pocahontas formation on the eastern border of the Great Flat Top 

 coal field in southwestern Virginia, f Both northward and southward 

 from this district the lower and middle Pottsville, when, present, are 

 generally characterized by massive conglomerates, though often the con- 

 tact is not sharp between the latter and the green or red shales of the 

 Mississippian. The conglomeratic phase of the older Pottsville is con- 

 spicuous near the Tennessee line and in the Southern Anthracite coal 

 field. 



The Raleigh-Bon Air stage marks an interval of most widely distrib- 

 uted sandstones or conglomerates throughout the greater part of the 

 limits of the at that time much restricted basin. It is doubtful, however, 

 whether the northern portion of the basin, including the present Southern 

 Anthracite field, was as yet accessible to marine molluscan life. The 



*As has been elsewhere stated, the Mercer group of the northern region, with its refractory fine 

 clays, limestones, coals, and iron ores, undoubtedly represents a duration greatly disproportion- 

 ate with its thickness. The Mercer group aud Connoquenessing sandstones appear to represent 

 by far the greater part of the Kanawha formation in southern West Virginia, and a thickness 

 probably still greater in the eastern Kentucky-Virginia region. See Science, vol. xvii, p. 942. 



f The Flat Top district is, as compared with other thick sections of the Pottsville, particularly 

 notable for the almost complete absence of conglomerates in the Pennsylvanian. 



