434 INDEX To THE STRATIGEAPHY OF NORTH AMERICA. 



Section in Mercer County, Pa. 



Feet. 



1. Homewood sandstone ["Tionesta," "Johnson Run," and "Piedmont"] 50 



2. Shales ! 5 



3. Iron ore [in western Pennsylvania and across the southern Ohio line into northeastern Ken- 



tucky] 2 



4. Mercer upper limestone [marine fossils] 2J 



5. Mercer upper coal [place of Mount Savage coal and fire clays in Pennsylvania and Maryland] . 2 J 



6 . Shales [locally in northern Pennsylvania inclosing a ' ' Middle Alton " coal] 25 



7. Iron ore [somewhat more persistent than No. 3, over much the same areas] 2 



8. Mercer lower limestone [marine MoUusca; more persistent than the Upper Mercer limestone 



but disappearing eastward] 2J 



9. Shales 10 



10. Mercer lower coal 2J 



11. Shales 10 



12. Iron ore 1 



13. Shales 5 



14. Connoquenessing upper sandstone ["Massillon" sandstone in part; present in nearly all 



sections] 40 



15. Shales with iron ore 10 



16. Quakertown coal [generally recognized] ,. 2 



17. Shales [in places lacking] ■ 40 



18. Connoquenessing lower sandstone [lacking in portions of the Allegheny Valley, locally along 



the Allegheny Front, and in some of the small northeastern residual areas; "Massillon" 



sandstone in part] 30 



19. Sharon iron shales [locally including a thin " Upper Sharon " coal] 30 



20. Sharon coal 4 



21 . Fire clay and shales 5 



22. Sharon conglomerate [usually conglomerate, at least in part, and extremely variable in 



thickness or absent entirely; perhaps equals the Olean conglomerate of McKean and 

 Warren counties, Pa.; usually lacking together with Nos. 19 to 21, where No. 18 is defi- 

 cient] 20 



Contact with Mississippian. 



301 



Tlie average tliickness of this part of tlie PottsviUe in tlie bituminous regions of K 17 and 

 K 18 is less than 250 feet. As might be expected in a region of encroachment and overlap, 

 the lower members are in places lacking. This lack is especially noticeable along portions of 

 the northern border of the field in Ohio and Pennsylvania; but it is also evident locally in the 

 interior regions, especially in the areas east of Allegheny River, in the Allegheny Front, and 

 in the Broad Top field. It is not recognized beneath the deeper portions of the main basal 

 syncline. 



The Homewood sandstone member, like the others, is. highly variable, being in some places 

 represented only by a few feet of shales. The limestones and coals are likewise subject to 

 disappearance, the limestones being rarely represented near the eastern escarpment. The 

 coals are all workable at one point or another. 



For the upper PottsviUe in the region under consideration, as described above, I. C. White ""'^ 

 has recently revived Lesley's term Beaver River group, though excluding therefrom the Sharon 

 conglomerate member, which was shown by Chance and D'lnvilliers to be of Pennsylvanian age. 



Allegheny formation. — The Allegheny, as differentiated in 1875" by Franklin Piatt,"" is 

 a formation whose limitation is largely dictated by convenience. It embraces the softer sedi- 

 ments of more quiescent waters intervening between the arenaceous invading PottsviUe and 

 the Mahoning and other sandstone and shale members of the overlying Conemaugh formation, 

 or "Lower Barren Measures." It is remarkable for its good coals, on account of which it was 

 for a time known as the "Lower Productive Measures," No. XIII of Lesley. As compared 

 with the PottsviUe the members of the Allegheny are relatively regular and continuous, and 

 the occurrence in them of marine MoUusca is comparatively common. 



The principal members of the Allegheny formation, which in the bituminous regions does 

 not usually exceed 300 feet in thickness, are as follows: 



" For other earlier and abandoned classifications see Stevenson, J. J., Bull. Geol. Soc. America, vol. 17, 1906, 

 pp. 65-69. 



