58 THE EOCEXE DEPOSITS OF MARYLAXD 



of the upper member has been shown to persist very generally through- 

 out the area, while the beds above and below are sufficiently distinct as 

 regards their lithologic and faunal characteristics to be distinguished 

 readily everywhere by the geologist in the field. It is therefore evident 

 that every interest will be subserved by the establishment of two forma- 

 tions, and they will be thus recognized both in the text and on the 

 accompanying map. Each of these formations also represents a well- 

 defined paleontological stage and each is again divided into two clearly 

 defined members or substages, and the latter again subdivided locally 

 into zones. 



FOEMATIONS AND FAUNAE STAGES.' 



The two formational units of the Eocene of Maryland are known as 

 the Aquia foemation and the Kanjemoy formation, names derived 

 from localities in the Potomac valley where the deposits of each division 

 are typically developed. Each is characterized by a well-marked fauna, 

 representing a clearly-defined paleontological stage.^ 



The formations are in each instance divided into two members which 

 represent an equal number of paleontological substages. The Aquia 

 FOEMATION is divided into a basal Piscataway member or suhstage, and an 

 overlying Paspotansa memter or substage; and the Nanjemoy forma- 

 tion is divided into a lower Potapaco member or substage and an upper 

 Woodstoch member or substage. 



These various divisions are shown in the following table: 



Group. Formations or Stages. Members or Substages. 



C Nanjemoy. j Woodstock. 



I 1 Potapaco. 



Pamunkey. -\ 



( Paspotansa. 

 [^ Aquia. | Piscataway. 



' In discussing tlie various divisions of tlie Maryland Eocene deposits a distinction is 

 drawn between stratigrapbic and paleontologic units ; the former are designated as 

 formations and members, the latter as stages and substages. As their limits are the 

 same the same name is employed for each. 



-The senior author in an earlier publication (U. S. G. S., Bull. 141, p. :39) divided the 

 Piimunkey on the basis of its contained fauna into the Aquia and Woodstock stages. 

 Later investigation has shown that the fauna from the Woodstock beds constitutes 

 simply a substage, and that this fauna together with the fauna more lately discovered 

 in the underlying beds, and called the Potapaco substage, comprises a larger division 

 now termed the Nanjemoy stage and formation. 



