66 J. J. STEVENSON CARBONIFEROUS OF APPALACHIAN BASIN 



In the early reports on the geological survey of Pennsylvania Professor 

 Henry D. Eogers used a numerical scale to designate the formations, the 

 Pottsville being XII and the Coal Measures above being XIII ; but in the 

 fourth report he divided Formation XIII into the Allegheny and Monon- 

 gahela series, drawing the line between them at the lowest rock bed seen 

 at Pittsburg, or nearly at the place of the Ames limestone. In the next 

 year he abandoned the geographical terms, using only XIII and desig- 

 nating the lower coals by letters.* 



The numerical method was adopted in Virginia by Professor William 

 B. Eogers, the Pottsville being designated by 12 in his second report. 

 Two years later he divided the measures into the Lower and the Upper 

 Coal series, separated by a considerable thickness of barren measures, and 

 in his fifth report he gives 



Lower Coal group, or Formation XIII ; 



Lower shale and sandstone group, or Formation XIV ; 



Upper Coal group, or Formation XV ; 



the Mahoning sandstone being included in the Lower group, f 

 In 1856 Professor J. P. Lesley offered this grouping: 



Barren measures, 

 Upper series, 

 Barren measures, 

 Mahoning sandstone, 

 Lower series, 



with the Pittsburg coal as the highest member of Number 3 and the upper 

 limit of Number 2 indefinite.]: 



Professor Eogers's final report of the geology of Pennsylvania ap- 

 peared in 1858 and contained this arrangement : 



Upper Barren ^ Greene County 



Waynesburg group and Pittsburg eoal and limestone j group. 



Lower Barren measures ") 



Mahoning sandstone } Mkldle or Barren group - 



Freeport group 



Freeport sandstone [ Lower group. 



Clarion group 



The plane between the Allegheny and Monongahela of the previous 



* H. D. Rogers : Second Ann. Kept. Geol. Explor. of Pennsylvania, p. 71 ; Third Rept., 

 p. G2 ; Fourth Rept., p. 150. 



f W. B. Rogers : Rept. Geol. Virginia for 1838, p. 84 ; Rept. for 1839, p. 98 ; Rept. 

 for 1840, p. 76 et seq. 



I J. P. Lesley : Manual of Coal and its Topography, pp. 94, 11G. 



