OLDER POTOMAC OF VIRGINIA AND MARYLAND. 575 



The Potomac of Maryland differs from that of Virginia in the appar- 

 ent absence of the Mount Vernon member and in the apparent confine- 

 ment of the Aquia Creek to the Fort Foote locality. It differs in another 

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

 in Maryland. It has been stated in preceding pages of this paper that 

 in Virginia the Lower Potomac has been divided by Professor Ward 

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

 River, (2) Rappahannock, (3) Mount Vernon, (4) Aquia Creek. The 

 Rappahannock is identical with the beds I have named Fredericksburg, 

 and the Aquia Creek with the Brooke beds as given in Monograph XV. 

 The Maryland Geological Survey divides the formation, in ascending 

 order, into (1) Patuxent, (2) Arundel, (3) Patapsco, and (4) Raritan. 

 In neither of these divisions is there any question of a division of the 

 formation into an upper and a lower member, while I have constantly 

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

 necessarj^ to explain what is meant by Lower Potomac. In my opinion, 

 the change in the character of the plants, in passing from the base 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 ma}^ 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 underlying 

 beds into those called Raritan or Amboy clays. 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 

 Mar3dand 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 



