96 J. J. STEVENSON CARBONIFEKOUS CF APPALACHIAN BASIN 



insignificant. At one time the beds of this formation were continuous 

 eastward to beyond the Alleghenies, as fragments remain in Maryland 

 and west central Pennsylvania. The extreme thickness, as found in the 

 southwest comer of Pennsylvania at the West Virginia line, and deter- 

 mined by oil-well records, is a little less than 1,300 feet. The thickness 

 decreases greatly toward the north, the bottom 475 feet becoming about 

 165 feet at the most northerly exposure and the succeeding 240 feet is 

 reduced to 150 feet at its northernmost exposure, nearly 30 miles south 

 from that of the lower interval. There is a similar decrease in a north- 

 westward direction, and toward the southwest one finds the bottom 700 

 feet of the thickest area reduced to barely 500 feet in Tyler of West 

 Virginia, 35 miles away. Nothing can be determined respecting con- 

 ditions toward the east, as erosion due to great anticlines prevents com- 

 parison with the fragments east from the Alleghenies in the deep basins 

 of Broad Top and Maryland. 



In the original description of this formation as it is in Pennsylvania, 

 Stevenson divided it into the Washington County and the Greene County 

 group, placing the plane on top of the Upper Washington limestone. 

 Aside from the convenience of a division in a column of such length 

 and complexity, one must recognize in the physical conditions good rea- 

 sons for this separation. These, as will be seen, appear only in part 

 along Dunkard creek, where Doctor White's studies led him afterwards 

 to group the whole succession into one formation, the Dunkard. They 

 are best shown farther north, in central Greene county, where they justify 

 a return to the original grouping and to the recognition of the Washing-, 

 ton and Greene formations as of equal rank with the Monongahela and 

 others below. 



As the Dunkard colunm is almost as long as the total of Allegheny, 

 Conemaugh, and Monongahela in the northern part of the field, the char- 

 acteristic deposits are numerous, most of them exhibiting peculiarities 

 deserving of notice. In ascending order they are : 



Washington formation : 

 Cassville shale. 

 Waynesburg sandstone. 

 Waynesburg A coal bed. 

 Colvin limestone. 

 Waynesburg B coal bed. 

 Little Washington coal bed. 

 Washington sandstone. 

 Washington coal bed. 

 Lower Washington limestone. 



