The Coal Measures above the Pottsville have been grouped in various ways by those who have studied the Appalachian basin.
In the early reports on the geological survey of Pennsylvania Professor Henry D. Rogers 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 Monongahela 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 designating the lower coals by letters.*
The numerical method was adopted in Virginia by Professor William B. Rogers, 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 . . .