W. B. Clark — Potomac River Section, etc. 365 



Aet. XLIII. — The Potomac River Section of the Middle 

 Atlantic Coast Eocene ; by Wm. Bullock Clakk. 



Our knowledge of the Tertiary geology and paleontology of 

 the middle Atlantic slope has been largely increased since the 

 days of Conrad and Eogers, yet few fields afford better oppor- 

 tunities for continued observation, and in none is there greater 

 need of a careful revision of results. Very divergent opinions 

 have prevailed and to-day find expression in the different 

 interpretations of the data. 



General Features of the Formation. 



The Eocene strata of the middle Atlantic slope, described by 

 Darton* under the name of the Pamunkey Formation, form a 

 belt of varying width extending from northeast to southwest 

 somewhat to the west of the center of the coastal plain. This 

 belt has been traced almost continuously from the southern 

 portion of Newcastle County, Delaware, to the valley of the 

 Nottaway river in southern Virginia, and although at times 

 buried beneath later deposits it presents fine exposures along 

 all the leading stream channels, while not infrequently broad 

 expanses of the formation outcrop at the surface in the inter- 

 vening country. 



Lithologic Characters.— -The deposits are typically glauco- 

 nitic and are found in their un weathered state either as dark 

 gray or green sands or clays. The glauconite varies in 

 amount all the way from very nearly pure beds of that sub- 

 stance to deposits in which the arenaceous and argillaceous 

 elements predominate, although the strata are generally very 

 homogeneous for considerable thicknesses. At some horizons 

 the shells of organisms are found commingled with the glauco- 

 nitic materials in such numbers as largely to make up the 

 beds, producing what is known as a green-sand marl. These 

 beds are at times indurated, forming true limestone bands. 

 This latter phase is seen typically developed both at Fort Wash- 

 ington, Maryland, and Aquia Creek, Virginia, interstratified 

 with the unconsolidated green-sand layers. 



When the glauconite is weathered the deposits lose their 

 characteristic gray or green color, and generally become lighter 

 gray with reddish or reddish-brown streaks or bands, or may 

 become entirely of the latter color. In this condition they are 

 often cemented into a ferruginous sandstone. This change 



*Bull. Geol. Soc. Amer., vol. ii, p. 411, 1891. 



