84 THE EOCEXE DEPOSITS OF ZHAETLAXD 



American Eocene horizons. Tlie sequence of stages from above down- 

 wards as given by him is as follows : 



Jacks onian, 



Claibornianj 



Chickasawan, 



Midwayan. 



Paleontological Criteria. 



A comparison of the ^laryland Eocene faunas with those represented 

 in the Gulf area, which have primarily afforded the basis for the classi- 

 fication above given, shows that the Maryland Atlantic Coast Eocene 

 contains a great many species not represented in the Gulf, or of which 

 the exact range there has not been well enough determined for com- 

 parison (158 out of 207), while the majority of identical forms (49 in 

 all) are of wide geological range, and are thus of little value in deter- 

 mining the exact age of the deposits. There are enough distinctive 

 species, however, as will be seen from a study of the table, to show 

 that the Maryland Eocene must represent beyond any doubt the 

 Chickasawan with both its upper and lower substages,^ and that from 

 the Potomac basin southward through Virginia we also have in all 

 probability the Lower Claibornian as well, with its Osirea seUaeformis 

 zone. 



Whether higher or lower stages of the Eocene are represented in the 

 Middle Atlantic Slope is a question that cannot be satisfactorily an- 

 swered. There are no paleontological data that bear on this point, and 

 the other evidence which may be brought forward cannot be regarded as 

 conclusive. The non-fossiliferous zone at the base of the Aquia forma- 

 tion may or may not be older than the Chickasawan, and the highest 

 strata recognized as Eocene in central and southern Virginia have not 

 been sufficiently studied to show whether they are or are not younger 

 than the lower Claibornian. 



The following table contains the forms common to the Maryland and 

 Gulf areas: 



1 TheUpper C/nckasawan as here used includes the Hatchetigbee and Woods Bluff (or 

 Bashi) substages, while the Lower Chickasaican includes the Bells Landing (or Tusca- 

 homa), Greggs Landing and Nanafalia substages as differentiated in Alabama and 

 Mississippi. 



