OUTLINE OF ROCK FORMATIONS. 65 



They cany not fewer than nine workable coal seams in Pennsjdvania and 

 six well-defined seams in Ohio These are thrown into three groups, known 

 as the Clarion, the Kittanniug, and the Freeport, each of which in Pemisyl- 

 vaiiia carries three coal seams. The outcrops are around the borders of 

 the coal basin, but the formation probably underlies the interior portion. 



Conemaugh, or Lower Barren Coal Measures. AboVe the LoWer ProduCtivC Coal 



Measures there are 300 to 600 feet of sandstones and shales in which the 

 few coal seams that appear are thin and wanting in persistenc}'. For 

 this reason, and because of a similar series at a higher horizon, they are 

 known as the Lower Barren Measures. One coal seam, the Mahoning, is 

 \^-orked in Columbiana County, Ohio. These barren measures outcrop 

 extensively in southeastern Ohio at a distance of 30 to 50 miles back from 

 the Ohio River, but come to the border of the Ohio Valley in eastern Ohio 

 and western Pemisylvania. 



Monongahela, or Upper Productive Coal Measures. The LoWCr BarrCU MeaSUreS arC 



overlain by the Upper Productive Coal Measures, which carry the widely 

 known Pittsburg coal (the most valuable seam of this great field), and a 

 less important seam, known as the Meigs Creek coal. The thickness of 

 these nieasures has been estimated by Orton to be 250 to 300 feet, though 

 the upper limits are not well defined. The extent is much g'reater than the 

 limits of the productive portion of the Pittsburg coal. 



Dunkard beds (Permian?). — Above the Upper Productive Measui'es there is a 

 formation which attains, where best developed, a thickness of several 

 hundred feet, and which has long been known as the Upper Bai'ren Coal 

 Measures. But since the fossil plants of this formation are of Permian 

 rather than Coal Measures type, the formation can scarcely be retained as 

 a part of the Coal Measu.res. The name Dunkard, taken from a creek in 

 southwestern Pennsylvania, where the beds are well developed, has been 

 substituted for the former name. This formation, as interpreted by Orton, 

 covers only a small area in Ohio, being confined chiefly to Belmont and 

 Monroe counties, but it occupies a large area in West Virginia between 

 the Ohio and Monongahela rivers, and encroaches slightly on southwestern 

 Pennsylvania. 



This is apparently the newest rock formation in the region under dis- 

 cussion. Only the residuary clays and a few beds of gravel at higli levels 

 are present to bear witness to the several long periods that iritervened 

 between Carboniferous and Glacial times. 



