West of the Alleghany Mountains. 251 



disappearing, which in their turn thin out as do the lime- 

 stones on the west side of the basin. This is evidence of 

 clear, quiet waters on the west, while rivers or shore cur- 

 rents carried in their freight of sand and mud from the east 

 and northeast. 



After the formation of Goal X, which, as has been stated, 

 may have been synchronous with the Uniontmun of Pennsyl- 

 vania, the conditions were more nearly alike throughout the 

 basin until the formation of the Waynesburg , though at the 

 east and west, alternations of limestone and sandstone during 

 the greater portion of the interval give evidence of neighbor- 

 ing shores. The limestone is still greatest in the central 

 portions, but is easily traceable in strata or nodules as far 

 west as Barnesville, north as Unionport, and east as Elk Lick 

 creek, all on the line of final outcrop. 



On the west side of the basin, we find the Redstone, Se- 

 wickly, VIII c and IX successively merged into the Pittsburg. 

 On the east side we find the Redstone disappearing and the 

 Sewickly brought nearer to the Pittsburg by so much as the 

 Redstone was distant from it, while the interval between 

 the Sewickly and the Uniontown (X ?) is reduced at the 

 most easterly exposure to barely one-third of what it is 

 nearer the central portion, on the Monongahela. We have 

 thus evidence of a series of gradual subsidences, separated 

 by intervals of repose, during each of which a lid of coal 

 was formed over all or a part of the basin. These subsi- 

 dences could not have been paroxysmal, for we find that as 

 the shore-line sank, the great Pittsburg marsh crept up the 

 shore, continually from the beginning of the epoch until long 

 after the formation of Coal IX, perhaps until the very close 

 of the epoch. Thus it is that, although giving origin to so 

 many subordinate seams, the great coal bed diminishes in 

 thickness when followed west from the Ohio, or east from 

 the immediate valley of the Monongahela. 



It is highly probable that the Pittsburg was begun on the 

 east and advanced westward ly. We have ample evidence in 



