48 The Middle Devoxiax Deposits of Maryland 



Monterey and Eomney to the Avest of Staunton, Virginia, where at its 

 extreme the black shales rest on an irregiilar, probably eroded surface of 

 the Oriskany.' This feature was observed at many places in Maryland. 

 The estimates of the thickness of the Eomney formation vary from about 

 GOO to 1650 feet. 



SUBDIVISIONS 



The Eomney shale shows everywhere in Marv^laud three more or le.^'s 

 sharply defined lithologic divisions which are characterized by distinctive 

 faunas. Where the lithologic differentiation is least prominent the three 

 types of faunas found in the Eomney shale are still limited to the same 

 relative parts of the section as farther north. The lowest of these faunas 

 is confined to a sedimentary type which includes, mainlj'-, alternating 

 drab or olive green and black shales with occasional thin bands of impure 

 limestone. Following these is a series of generally fissile black shales 

 with comparatively few drab or light-colored shales. The uppermost 

 division includes drab or sandy shales and sandstones with the Hamilton 

 fauna. These divisions are named, in ascending order, the Onondaga, 

 Mareellus and Hamilton members respectively. 



Onondaga Shale Meinher 



Character and Thickness. — This member is more persistent in its 

 lithologic characters than the other two divisions of the Eomney. After 

 the latter have become nearly indistinguishable on either a lithologic or 

 faunal basis to the south of Maiyland, the drab or dark shales with lime- 

 stone bands, representing the Onondaga shale, and their distinctive 

 faunas are still easily recognized. 



The Onondaga shale is prevailingly lighter in color than the overlying 

 Mareellus shale while many of the beds are blocky. Upon weathering it 

 usually becomes buff or greenish brown, and breaks into irregular frag- 

 ments, resembling in this respect, the upper part of the Hamilton member, 

 and finally disintegrates into clay. Interbedded with the lighter strata, 

 however, arc beds of black, or dark brown, fissile shale, which resemble the 

 shale of the overlying Mareellus very closely. Thin beds of dark, argilla- 

 ceous limestone occur at several horizons and vary much in thickness and 



' Am. Geol., Vol. X. 1892, p. 16. 



