of the Middle Atlantic Coast Eocene. 369 



No. 13. This bed consists of a light grey glauconitic sand 

 somewhat weathered and tilled with shells of Venericardia 

 jplanicosta. It reaches three feet in thickness. This zone is 

 very persistent and has been found outcropping in several of 

 the ravines to the east and south of Potomac Creek, as well as 

 in the bluff upon the river front. 



No. 14. Overlying the Venericardia layer is a bed of green- 

 ish-grey argillaceous sand some four feet in thickness, which 

 contains a great number of bands filled with gypsum crystals. 

 No fossils were observed. 



No. 15. This bed consists of greenish-grey argillaceous sand 

 in which the glauconite grains have been extensively weathered. 

 No fossils were found. The bed has a thickness of twenty- 

 five feet. 



No. 16. In this zone have been placed the green-sand strata 

 intervening between the upper layers of the Eocene in the 

 Potomac Creek section and the base of the Woodstock section. 



Comparatively little is known regarding this portion of the 

 series, as no satisfactory outcrops appear on the river bluffs, 

 although the strata are found in an unfossiliferous condition 

 in some of the ravines to the west of the Woodstock area. 

 The estimated thickness of these beds is fifty feet. 



No. IT. The highly glauconitic beds at Woodstock, Virginia 

 and Pope's Creek, Maryland are grouped together in one zone, 

 as no satisfactory separation could be made. The deposits are 

 very homogeneous, although an inconstant indurated layer 

 appears about six feet above the base of the Woodstock sec- 

 tion with a band of Venericardia jplanicosta below it, while a 

 thin bed of Ostrea sellojformis also occurs in the lower part of 

 the zone, although evidently not always at the same horizon. 

 Otherwise the fossils are the same throughout, so far as observed. 

 The most common forms are Protocardia virgmiana, Cytherea 

 subimpressa, Corbula nasuta, Corbula o?iiscus, Ostrea sellce- 

 formis, Pectunculus idoneus, Leda improceva, Leda jpavva^ 

 Nucula mag?iifica, Lucina dartoni, Lucina uhleri, Lucina 

 whitei, Ringicula dalli and Cylichna venusta. 



P aleontological characteristics. — The paleontological charac- 

 teristics of the several zones indicate two very distinct faunal 

 stages in the middle Atlantic slope Eocene, the first typically 

 developed in zones 2 to 9, and the second in zone 17. The 

 characteristics of zone 1 and of zones 10 to 16 cannot be 

 readily made out in the Potomac River area on account of the 

 extensive weathering of the beds, although in some of the 

 adjoining districts there is an intermingling of some of the 

 forms of the two stages in the beds intervening between 9 and 

 17. To the two faunal divisions, the names of Aquia Creek Stage 

 and Woodstock Stage have been already given by the writer.* 



* Johns Hopkins Univ. Circulars, vol. xv, p. 3, 1895. 



