THE ONONDAGA SEA IN THE ALLEGHENY REGION 101 



of the limestones of Onondaga age in Ohio, Indiana, and Illinois. 

 Its occurrence in typical Onondaga limestone only in an area which 

 is nearly adjacent to the region of the shaly facies of the formation 

 suggests that the latter type of sediments furnished its normal and 

 most congenial habitat. Spirifer acuminatUs, on the other hand, 

 does not extend very far to the southward into the .region occupied 

 by the argillaceous facies of the Onondaga. Other Onondaga 

 species, however, like Odontocephalus aegeria, appear to be equally 

 adapted and distributed in both types of sediment. 



Some of the stratigraphic data relating to this fauna may be 

 very briefly summarized as follows: 



The calcareous shales holding this fauna are generally preceded 

 in the sections by the Oriskany sandstone and always followed 

 by the dark fissile and comparatively barren shales of the Marcel- 

 lus. These two limiting formations exhibit in general essentially 

 the same lithologic characters throughout Pennsylvania, Mary- 

 land, West Virginia, and much of Virginia as in New York. Both 

 are, however, much thicker in this more southerly region than in 

 the type region of the Onondaga limestone in New York. In the 

 Helderberg mountain region the Onondaga and the Hamilton 

 faunas are separated by 300 feet of comparatively barren dark 

 Marcellus shale, and in western New York by about half this thick- 

 ness, while in Pennsylvania and southward these shales often have 

 a thickness of more than 500 feet. 



While the succession from the Onondaga fauna to the Marcellus 

 fauna above is a uniform one throughout most of the Allegheny 

 region, as it is in New York, the succession at the base of the fauna 

 is not everywhere precisely the same. In most of the territory the 

 Onondaga beds rest upon the Oriskany, but in some of the Penn- 

 sylvania sections they immediately follow beds representing the 

 Esopus shale. In respect to its underlying formation, however, the 

 Onondaga shows less variation than in New York, where, in different 

 areas, it is found to follow the Manlius, Oriskany, Esopus, and 

 Schoharie. Thus, we find that this fauna occupies in the Allegheny 

 region the same relative position in the succession of faunas as the 

 Onondaga fauna does in the standard sections of New York. The 

 stratigraphic evidence, therefore, coincides with the paleontologic 



