1912.] STEVENSON— THE FORMATION OF COAL BEDS. 433 



River but comparative uniformity thence to the eastern outcrop. In 

 Muskingum and Guernsey counties of Ohio, the thickness increases 

 from 315 to 340 feet but, 40 miles farther, at the Ohio it is 500 feet. 

 In Pennsylvania and western Maryland the average is very nearly 

 600 feet. How thick the formation was in the northern anthracite 

 field cannot be determined, as the top of the section has been 

 removed, but the beds there are not less than 1,000 feet thick. 

 The conditions were the same throughout the whole period of 

 deposit, for variations in the upper half are similar to those in the 

 lower half. Apparently there was slower subsidence along the 

 western border than elsewhere, but no proof of overlapping or of 

 regression can be found, as the outcrop is usually at a considerable 

 distance from the border of deposit. There was a singular uni- 

 formity of conditions from north to south along the middle line 

 of the bituminous area, a distance of more than 200 miles. At the 

 most northerly outcrop in Pennsylvania, the thickness is approxi- 

 mately 600 feet and thence southward along the whole line it varies 

 from 575 to 600 feet. Beyond Doddridge county of West Virginia, 

 ■definite measurements cannot be made as coal beds and limestones 

 alike have disappeared; but at Huntingdon, 100 miles beyond, and 

 just west from the middle line the thickness is said to be 660 feet. 

 At the extreme southeastern outcrop in West Virginia, the interval 

 lias been reported as 800 feet but some doubt remains as to the 

 upper limit. Conditions during the Athens resembled those during 

 the Pottsville, in that the trough of sedimentation with greater sub- 

 sidence still existed at the east. But there was no longer dififer- 

 •ential subsidence toward the south and there is much reason for 

 believing that there was notable contraction of the area of deposi- 

 tion in that direction, so that the Conemaugh may have extended, 

 at most, only a short distance into Tennessee. ^^ 



But when one reaches the Wheeling he finds a notable change. 

 Going eastward from the western outcrop in Ohio, the thickness 



" The measurements of the Conemaugh have been taken from J. J. 

 Stevenson, Ohio Geol. Survey, Vol. III., 1879; Second Geol. Serv. of Penn- 

 sylvania, Reports K, 1876, KK, 1877, T2, 1882; I. C. White, U. S. Survey, 

 Bull. 65, 1891 ; West Virginia Geol. Surv., Vol. I., 1899. 



