OLDER POTOMAC OF VIRGINIA AND MARYLAXD. 575 



Tho Potomac of ]\[arvlan(l difiVrs from thai nf \'ii-iiiiiia in llir appai- 

 ent al)SoiU'e of llic Mount A'ci'iioii iiKMiihcr and in tlio apparent confine- 

 menl of the \(|uia d'cck to the l"oi1 l'"oott' locality. It (lifToi's in another 

 respect — the absence in Virginia of the Raritan, which is so conspicuous 

 in Maryland. It has i)een stated in pi-eceding panics of this paper that 

 in ^'irginia tlu' Lower Potomac has Ijeen divided by Professor Ward 

 into the following members, mentioned in ascending ordei-: (1) James 

 River, (2) Rappahannock, (li) Mount Vernon, (1) Acjuia Creek. The 

 Rappahannock is identical w-ith the beds I have named FicdiM-icksburg, 

 and the Aquia Creek with the Brooke beds as given in Monograph X^^ 

 The Maryland C.eological Stirvey divides the formation, in ascending 

 order, into (Ij Patuxent, (2) Arundel, (8) Patapsco, and (4) Raritan. 

 In neithei- of these divisions is there any cjuestion of a division of the 

 formation into an upper and a lower meml^er, while I have constantly 

 referred to the Potomac of Virginia as Lower Potomac. It is, then, 

 necessary to explain what is meant ))y Lower Potomac. In my opinion, 

 the change in the character of the plants, in passing from the liase to the 

 summit of the heterogeneous mass of nonmarine deposits, called by 

 most writers Potomac, necessitates a subdivision of it into a lower and 

 an upper member, if the Raritan is regarded as Potomac. Other sub- 

 divisions may be made that are based on changes in lithological char- 

 acter or on want of continuity, etc., but the more the plants are studied 

 the more clearly it appears that there is only one great break in the 

 continuity of the flora, and that is in the passing from the imderlying 

 beds into those called Raritan or Amboy cla3^s. Judging from the 

 plants, the Raritan member forms the Upper Potomac, if it is Potomac, 

 while all below is included in the Lower Potomac. 



The main difference between the Potomac of Virginia and that of 

 Maiyland lies in the fact that the Raritan is absent in the former, while 

 it is in force in the latter State. 



It is not contended that there are not considerable changes in the 

 flora of the Lower Potomac in passing to higher beds. This change 

 is a gradual one, caused by the diminution of old types and the increase 

 and introduction of more modern ones, but there is no wholesale change, 

 and the flora shows essential continuity. Some of the changes have 

 already been indicated in pointing out the differences between the Rap- 

 pahannock or James River member and the higher Mount Vernon and 



