MONONGAHELA FORMATION 35 



to be of local importance. It is from 30 to 80 feet above the "Main" 

 coal of the Pittsburg and its place is almost invariably marked by coal 

 or by richly carbonaceous shale. In many localities where it is wanting 

 its absence is due to erosion during deposition of an overlying sand- 

 stone, continuous downward with the Pittsburg. Sometimes it is of 

 workable thickness in southern Pennsylvania and adjacent part of West 

 Virginia near the Monongahela river. The identification of the bed 

 is very clear in Pennsylvania, West Virginia, and Belmont county of 

 Ohio, but the reference of isolated bits of coal in southern Ohio is tenta- 

 tive to the last degree. They may be only "rider" coals to the Pittsburg. 

 A feature of the Eedstone in much of Pennsylvania is the occurrence of 

 broad clay veins in localities where the underlying Pittsburg coal bed 

 is undisturbed. The bed is roofed ordinarily by shale or sandstone and 

 is often only an inch or two above its limestone. At one place in 

 Washington county, Pennsylvania, it has a limestone roof. 



The Fishpot limestone (J. J. Stevenson, 1876) has been regarded as 

 equivalent to the Sewickley limestone (P. and W. G. Piatt, 1877), 

 but the relations of the latter limestone are very uncertain and in all 

 probability it is not the same with the Fishpot. This limestone is 

 separated at most by 5 or 6 feet from the Lower Sewickley coal bed, and 

 occasionally the two deposits are apparently in contact. It seems to be 

 wanting east from the AUeghenies as well as in the eastern basins and 

 along the northern outcrop in Pennsylvania; but it seems to be per- 

 sistent, though extremely variable, in most of the area west from Chest- 

 nut ridge across Pennsylvania into northern Ohio as well as southward 

 for 30 miles in West Virginia. It is thick in some of the Pennsylvania 

 basins and thin or uncertain in others. It becomes very thick in the 

 basins of Fayette and Westmoreland as well as in the adjacent portions 

 of Washington and Greene, sometimes 30 feet, and it is equally im- 

 portant on the Ohio river. This deposit is confined to the northern 

 part of the basin and, like most of the other limestones in the forma- 

 tion, seems to be wholly wanting in the interior part. Its interval rarely 

 carries any limestone in Ohio south from northern Monroe coimty. 



The Sewickley coal bed (H. D. Eogers, 1858) includes two coal beds; 

 the lower bed rests on the Fishpot limestone or is separated from it 

 by thin clay, while the upper bed overlies the Sewickley sandstone and 

 is almost directly under the great Benwood limestone. One seldom sees 

 both beds well defined in a single section. The type sections obtained 

 by the geologists of the First survey as well as by those of the Second 

 survey were obtained in southern Fayette and Greene counties near 

 the West Virginia line, where the Sewickley sandstone is unusually 



