432 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



was formerly supposed to contain the last representatives of distinctly marine MoUusca in the 

 Appalachian trough, but recent discoveries indicate marine conditions at a level somewhat 

 higher. Not far from the horizon of the Ames member there have been found reptilian bones 

 alhed to early representatives of the group characteristic in general of the Permian. They 

 may be regarded as forerunners of the Permian fauna. 



All sediments of Conemaugh or later age appear to have been removed from the portion 

 of the Appalachian trough south of northeastern Kentucky, though the presence of the marine 

 MoUusca implies continuity of these sediments down the basin to connect with the open sea. 



Monongahela formation. — The Monongahela is distinguished by its relatively large propor- 

 tion of coal and limestone, the latter composing over one-third of the sediments in some districts. 

 The formation, as stratigraphically recognized in Pennsylvania, Ohio, western Maryland, and 

 West Virginia, averages about 325 feet or less in thickness. Its coals, including the great 

 Pittsburg coal at its base, are of notable thickness and value. As with the Conemaugh, 

 red sediments become more prominent toward the south and its hmestones, generally blue and 

 in places containing "fresh- water" MoUusca, thin and disappear in the same direction. 



The Monongahela and the Conemaugh are not fuUy differentiated in the anthracite regions. 



Dunkard group. — The Dunkard group, which succeeds the Monongahela and which in 

 lithologic characters and mineral content does not differ notably from that formation, is dis- 

 cussed in Chapter XI (pp. 492-494). The reference of its lower portion (below the Washington 

 coal) to the Permian is not wholly beyond question. 



DETAILED DISCUSSION OF FORMATIONS. 



The composition and stratigraphy of the several formations of the Appalachian trough 

 will now be considered. In order to give precedence to the series and regions more thoroughly 

 known, the geographic order of treatment will be from north to south. Proceeding thus, we 

 shall begin at the northwest border of the Pennsylvanian trough, encountering first the younger 

 formations in the regions of late PottsviUe overlap. Subsequently, working eastward and 

 southward, we shall reach the thicker developments of the formations and the minor areas 

 of the oldest formations lying along or near the earher axis of the basin. 



OHIO AND PENNSYLVANIA. 



General features. — Map sections K 17 and K 18 include the north end of the great bitu- 

 minous coal field in Ohio and Pennsylvania, several small isolated residual areas to the 

 northeast, the Broad Top field, and the anthracite region. With the exception of the 

 anthracite fields the coal measures of this strip are a unit not only in the classification applied 

 but also as to essential continuity and parallelism of the strata. 



The great bituminous coal field of these States forms a part of a plateau correlated with 

 the Cretaceous or later base-leveling and subsequent elevation. Toward the north and west 

 the strata, rising more rapidly than the plateau surface, outcrop diagonally and are cut by 

 erosion into an intricate fringe of ixregiilar lobes, tongues, and isolated patches that it is 

 impracticable to indicate on the map scale. The eastern border of the plateau coal field is 

 defined by the stronger anticlines to which reference has already been made. 



The Broad Top coal field in Huntingdon, Fulton, and Bedford counties, in southern 

 Pennsylvania, is a small remnant in a deep compound fold, in the region of close plication. 

 In general the alteration of the coals increases toward the regions of greater thrust pressure; 

 and so from ordinary domestic and gas fuels we pass through coking and steam coals to semi- 

 bituminous coals at the eastern escarpment (Allegheny Front) of the great plateau field and 

 in the Broad Top field. Small residual areas of semianthracite in Wyoming and SuUivan 

 counties survive in other very shallow folds between the plateau and anthracite regions. 



The anthracite fields, three in number, in eastern Pennsylvania, consist of a group of 

 residual troughs lying in northeast-southwest synclines. The structure is very complex, the 

 strata being locally overturned, especially toward the southern edge of the region, whUe subor- 

 dinate folds comiect many of the synclines in varying degrees. '^^^ 



