168 The Lower Devonian Deposits of Maryland 



II. Section at 21st Bridge 

 An excellent section of the Oriskany formation and of the New Scotland 

 member of the Helderberg is seen in the axis of a small anticline that is 

 cut by the Potomac Eiver opposite 31st Bridge, Maryland, on the West 

 Virginia side, along the tracks of the Baltimore and Ohio Eailroad. The 

 New Scotland shale is well exposed. The New Scotland limestone appears 

 thicker than usual leading Schuchert to estimate its thickness at 44 feet * 

 at this place. This appearance is, however, misleading as some of the beds 

 are duplicated by a thrust fault seen along the river so that it is probable 

 that the thickness of the beds is the same as at Keyser and Dawson. 



Oriskany Formation 



Vertical 



Shriver Chert Member ^^''^^^^^^ 



Siliceous shale containing much black chert in nodules. 



Heldekbekg Formation 

 New Scotland Member 



Soft, bluish, argillaceous shales, with some harder layers and occa- 

 sional manganese-phosphatic nodules. Chonetes helderbergicB, Meristella 

 arcuata, Trematospira multistriata, Spirifer macropleurus, Schuchertella 

 woolworthana, Stropheodonta becki, etc 20.0 



Massive gray limestone with bands of chert, becoming thin-bedded 

 above, with partings of shale; characterized by Spirifer macropleurus 

 and containing Hindia sphwroidalis, Edriocrinus pocilliformis, Dalma- 

 nella perelegans, Rhipidomella oblata, Eaionia singularis, E. peculiaris, 

 E. medians, Anoplotheca concava, Trematospira multistriata, Parazyga 

 dewyi, Spirifer perlamellosus, S. macropleurus, S. cyclopterus, Platyceras 

 spirale, Phacops logani, etc. 



Concealed. 



III. Section at Daivson 

 An excellent section of the New Scotland and Coeymans limestone is 

 exposed at the south end of Fort Hill, near Dawson, along the tracks of the 

 Baltimore and Ohio Railroad. The Oriskany sandstone forms the " back- 

 bone " at the south end of the mountain and is succeeded by a small 

 covered interval. Immediately east of this the limestone beds of the New 

 Scotland are seen and below them the Coeymans and upper part of the 



' Proc. U. S. Nat. Mus., vol. xxvi, 1903, p. 422. 



