>tAUVI,.\.\D OHOI.OCICAI- SLUVEY 59 



Several species have been ['(umd which arc coiiiiikmi lo all the substagee 



of ( he I'aiiiiiiikcy. They arc: 



Voliililil/ten pclrosiin (Conrad). Ci>rbiilit imixcim Conrad. 



Cnli/ptnu'it iipvrtit (Solniuler). Tcllina I'irrjiniiinn Cliirk. 



Ciii/ii/i/s ii/ifii/ih'x Meyer iiliil Alilrieli. Modiolux nliihuineunin Aldricll. 



Slill (iIIkt I'liniis ;iic loiiiid in both the Aquia and Xanjemoy stages, 

 althougli iin( iccogiiizcd in ail the substages. The following species 

 liave been I'uimd in the Piscataway, Paspotansa, and Potapaco substages, 

 but are not known in the Woodstock, viz.: 



Titr/iii/illiien bellii Conrad. Lunatiu nuirijlimdlrn Conrad. 



Vijlh-liuii roiiisiii Clark. Oxtrea (Ori/phiieoiitreii) romer (.Mortf>ii). 



The following have been found in the Paspotansa and Potapaco sub- 

 stages, viz.: 



Calyptrap/ioriis trlnodifervn Conrad. DenUdiiim mississippie7iiiis Conrad. 



The following form ranges from the Paspotansa through the Potapaco 

 and Woodstock substages, viz.: 



Fecten choctapensis Aldrieli. 



The following forms have been found only in the Piscataway, Pas- 

 potansa and Woodstock, viz. : 



Myliobatis copeanus Clark. Lucina uhhri Clark. 



Odontaspi.i cuspidata (A2;assiz). Cucullaea gigantea Conrad. 



Strepsidura stibscalarina Heilprin. ^^odosa7-ia bacillum Defrance. 



Corbula subengonala Dall. Pohjmorphina gibba d'Orbiguy. 



Protocardia lenitt Conrad. Trnncatxllna imgeriona. (d'Orbigny). 



The Aquia Formation or Stage. 

 The Aquia formation, so-called from Aqnia Creek, which enters the 

 Potomac river from the Virginia side about fifty miles below Washing- 

 ton, is composed chiefly of greensands and greensand marls, at times 

 highly calcareous and less frequently argillaceous. The various litho- 

 logical and paleontological characteristics are clearly shown in the de- 

 tailed sections which follow. The deposits reach abont 100 feet in thick- 

 ness, gradually thickening eastward. The name Aquia was originally 

 employed' to embrace the faunal stage represented by Zones 2 to 9. 



'Johns Hopkins Univ. Circ, 1895, p. 3; Bull. 141, U. S. Geol. Survey, 1S96. p. 39. 



