44 J. J. STEVENSON PENNSYLVANIA ANTHRACITE. 



the folding diminishes and the axes are insignificant long before the 

 plateau area of northern Pennsylvania and southern New York has been 

 reached; the structure becomes simpler as the evidences of disturbance 

 become less in that direction. Thus, between the Alleghany and the Kit- 

 tatinny, the many and great folds of Maryland and of the southern coun- 

 ties of Pennsylvania become fewer and less bold, the synclinals broader 

 and deeper; so that instead of the single area of Coal Measures, that of 

 Broad Top (itself due to the northward decrease of three strong anticlin- 

 als), one finds the several anthracite fields in northeastern Pennsylvania. 

 The same general statement holds good with respect to the other basins. 

 Southward the condition is similar except in the strip east from the Alle- 

 ghany mountain, and yet the difference is very noteworthy. The degree 

 of disturbance in the region west from the Alleghany steadily decreases 

 southward along the strike, and the basins soon become merged ; at least, 

 the folds limiting them become so gentle as to be followed with some 

 difficulty. Even the magnificent anticlinal representing the Alleghany 

 alongside of the Cumberland coal-field in Maryland, after attaining its 

 maximum in Randolph county of West Virginia, quickl}'' diminishes so as 

 to cause comparatively small interruptions in the dip on the lower New 

 river — just enough to keep the Coal Measures from passing under that 

 stream for a long distance. The contrast between the southern and the 

 northern conditions is very great. At the north the basins are canoe- 

 shaped and become shallower, the bottom rising so that lower and lower 

 rocks appear in succession as the higher ones pass into the air. At the 

 south, in West Virginia, the anticlinals are lowered so that the higher 

 rocks pass over them. Thus the detached prongs of the Upper Coal 

 Measures in southern Pennsylvania become united as the Pittsburg 

 coal bed crosses the diminishing anticlinals. The Chestnut hill anti- 

 clinal attains its maximum in southern Pennsylvania, near the old Na- 

 tional road, where the upper Chemung, to a thickness of several hundred 

 feet, is exposed at the summit, about 2,500 feet above tide. But at 

 barely 40 miles away, in West Virginia, the Pottsville crosses this fold 

 at 989 feet above tide, while at 20 miles further the Pittsburg coal-bed 

 also crosses the axis at a slightly greater altitude. The depression of 

 the fold in this interval is not less than 2,300 feet. It is probably much 

 more, because the Carboniferous groups thicken rapidly in that direc- 

 tion. There is then a decrease of disturbance southward toward the 

 border of Kentucky and Tennessee in these so-called bituminous basins. 

 The conditions in the strip between the Alleghany and Kittatinny 

 mountains are quite difi'erent. Extensive coal areas exist further north- 

 ward, measured along the strike, than in the other region, for the great 

 eastward curve of the Blue ridge in the northerly portion makes possible 

 broader and less abrupt folds ; but from central Pennsylvania southward 



