374 W. B. Clark— Potomac River Section, etc. 



upper portion of the series in the latter area may have existed 

 contemporaneously with older forms outside the region. With- 

 out a much fuller knowledge of the characteristics of the 

 Eocene fauna in the intermediate district, this cannot be defi- 

 nitely determined, although it seems highly probable. 



Considering all the facts, the writer is strongly of the opin- 

 ion that the Eocene deposits of the middle Atlantic slope 

 represent the greater portion of the Eocene series of the Gulf, 

 its upper members alone excepted. Compared with the sec- 

 tion recognized by Prof. E. A. Smith, in the Alabama area, it 

 undoubtedly comprises all or the major part of the Lignitic, 

 Buhrstone and Claiborne and, possibly, also portions of higher 

 horizons. This reference does not, however, necessarily involve 

 the assumption that the basal beds of the Potomac section are the 

 exact equivalent of the basal beds of the Lignitic, since deposi- 

 tion may have commenced in the one area somewhat earlier 

 than in the other, although the difference was probably not 

 great. 



Conclusions. — 1. The Eocene deposits of the middle Atlantic 

 slope constitute a single geological unit already described under 

 the name of the Pamunkey formation. 



2. The deposits are remarkably homogeneous, consisting 

 typically of glauconitic sands and clays which reach a thickness 

 of nearly 300 feet. 



3. Two clearly defined faunal zones are found, viz : the 

 Aquia Creek Stage and the Woodstock- Stage. 



4. The Aquia Creek fossiliferous zone is approximately 

 middle, or middle and upper Lignitic, the Woodstock zone 

 middle, or middle and upper Claiborne. If restricted respec- 

 tively to the Bells Landing sub-stage of the Lignitic and the 

 Ostrea selleeformis bed of the Claiborne, as seems hardly prob- 

 able for the reasons above cited, the 600 feet between those 

 zones in the central Alabama area would then be represented 

 by only 125 feet or perhaps considerably less in the Potomac 

 region. The upper beds of the Woodstock stage might then 

 perhaps represent the upper portion of the Claiborne while 

 the beds below the Aquia Creek fossiliferous zone would 

 stand as the approximate equivalent of the lower Lignitic, with- 

 out however necessarily assuming that the basal beds of the 

 Potomac section are the exact equivalents of the basal beds of 

 the Lignitic. 



5. The middle Atlantic slope Eocene, therefore, represents 

 in a broad way all or the major part of the Lignitic, Buhr- 

 stone and Claiborne of Smith, and, when the physical conditions 

 affecting range and migration of species are considered, perhaps 

 even more. Both the geological and paleontological criteria 

 are wholly inadequate for establishing the great number of 

 local subdivisions recognized in the Gulf area, and in fact the 

 sequence of forms indicates that no such differentiation of the 

 fauna took place. 



