.MAi£Yi>A\i) (;i;<ti.i)(;ic \i, sri[\i;v G5 



Other species are restricted to (Jiiu or the <jtlier ot the suhstages of the 

 Nanjemoy and are mentioned hcyond. Additional to these are the forms 

 previously referred to as round in liotli divisions of the Pamunkey 

 group. 



TIIR POTAPACO .\IK.MHi;ll OU SUHHTAGE. 



The l?()t;i]i;i(() nicinhcr, so-called from the early name of Port Tobacco 

 Creek, wliich is a conniplioii of tlic word I'otapaeo found on the Smith 

 and other early niaj)s, is composed of greensand, often very argillaceous 

 nnd at times gypseous. Tlie clayey character of the member, especially 

 in tlie lower bed, is in marked contrast to the more highly glauconitic 

 nature of the Aquia lonnalion. The thickness of the member is about 

 GO to 65 feet. 



The Potapaco substage embraces Zones 10 to l.^. The following 

 species are restricted in range to it: 



Cypraea umithl Aldrich. rcripJonta sp. 



SoIe7i lisbonensis Aldrich. Ceriopont rnlcropora Goldfuss. 



(?) Lxcina astartiformis Aldrich. 



The following zones constitute the sululivisions of the Potapaco in 

 the Potomac area, some of which can be recognized over wide areas: 



Zone 10. — The greenish-gray sand which overlies the Turritella rock 

 is more argillaceous than the underlying or overlying beds of the Eocene. 

 The glauconite grains have been much weathered and nearly all trace 

 of the shell substance has been removed from the few forms recognized. 

 To the northeast of the Potomac area, throughout the central portion 

 of Southern Maryland, this bed becomes a well-defined clay, as at Upper 

 Marlboro, and has been referred to as the Marlboro clay. The casts 

 found at the Potomac Creek bluff are chiefly those of a Meretrix, prob- 

 ably Meretrix ovata var. ovata. An indurated layer, near the middle of 

 the zone, contains Calyptraplwriis trinodiferus ; below this is the repre- 

 sentative of the red clay Avhich occurs typically about Upper Marlboro. 

 ISTo fossils were observed at the Aquia Creek bluff. The bed is about 25 

 feet in thickness. 



Zone 11. — This zone is composed of a thin, indurated layer of argil- 

 laceous greensand, 1 to 2 feet in thickness. It is well developed at the 

 Potomac Creek bluff, where it contains Tenericardia potapacoensis, and 

 is the lowest horizon at which this species has been found. 



