. JMaryland Geological Survey 53 



Ea:posures of Onondaga Member 



Exposure at 21st Bridge. — There is probably no locality in Mary- 

 land where the relations of the Onondaga, Marcellus, and Hamilton mem- 

 bers of the Eomney can be seen to better advantage than at 21st Bridge, 

 1 mile east of Keyser. The three cuts adjacent to the intersection of the 

 Baltimore and Ohio and Western Maryland railroads, together expose 

 nearly all of the two basal members and a portion of the Hamilton. The 

 lower portion of the Onondaga and the basal Oriskany sandstone are well 

 exposed in the west cut where the section shows the following beds : 



Feet 

 Drab, rather hard, slightly sandy shale with some calcareous concretions 



and bands 45 



Lead-gray, fissile soft shale, Orhiculoidea abundant ,. . 40 



Oriskany sandstone 30 



The highest beds of the Onondaga do not appear ii:i the west cut but 

 are exposed in the east cut of the Western Marjdand Eailroad at 21st 

 Bridge. These highest beds include at the toi> about 8 feet of dark 

 argillaceous limestone in strata 6 to 20 inches thick, interbedded with 

 dark shale. They were estimated by Prosser to lie about 168 feet above 

 the base of the Eomney, giving a total thickness of about 170 feet for the 

 Onondaga. This limestone holds Agoniatites expansus suggesting that it 

 may represent a horizon near that of the " Agoniatite limestone " fauna 

 of New York which, according to J. M. Clarke,^ was introduced into 

 western New York before the cessation of Onondaga limestone depo- 

 sition. With the exception of .4. expanstis, however, the fauna of this 

 zone bears little resemblance to the assemblage recorded from the " Ago- 

 niatite limestone " of New York. 



The following species have been recognized in a collection made by 



Swartz from a bed about 100 feet above the base of the Eomney in a section 



ascending the hill east of the river one-half mile south of 31st Bridge: 



Pholidops cf. areolata 

 Dalmanella lenticularis 

 Goniophora sp. undet. 

 Panenka sp. undet. 

 Agoniatites expansus. 

 Phacops crlstata 



' Bull. New York State Mus., No. 49, 1901, p. 137. 



