576 MESOZOIC FLORAS OF UNITED STATES. 



Aquia Creek groups. While many of the old types, much dimmished 

 in numbers, pass up into the Mount Vernon and Aquia Creek horizons, 

 in number of individuals thej^ are to some extent subordinate to more 

 modern types. The case is very different in passing up into the Rari- 

 tan or Amboy clays. Here there is a wholesale change. Few, if an}^, 

 of the older types persist. Even those more modern types that were 

 introduced in the Mount Vernon and Aquia Creek groups disappear. 

 A great number of wholly new plants, more recent in character than 

 were the most modern of the Aquia Creek strata, appear, and dicoty- 

 ledons overwhelmingly predominate. In a word, in passing into the 

 Raritan strata we find the flora wholly changed. This being the case, 

 the question may be asked. Why give the name Potomac to this group? 

 It has, it is true, in common with the underlying strata, a nonmarine 

 or estuarine character in the deposits, and this seems to be the reason 

 for making it a member of the Potomac. Professor Marsh thought 

 that he had, from the vertebrate fauna found in the Arundel member, 

 proved that its age is Jurassic. He, with most writers on the subject, 

 included the Raritan in the Potomac, but, unlike others, he went further, 

 and maintained that the whole formation is Jurassic. It might with 

 reason be maintained that Professor Mai^sh's conclusion as to the age 

 of the whole formation is the logical one. If the continuity of the for- 

 mation is sufficient to make the Raritan a member of the Potomac, 

 and if the age of the lower portion of the Potomac is Jurassic, it might 

 be claimed that the Raritan must be Jurassic. Hardly anyone would 

 now maintain such an age for it. 



As indicated above. Professor Marsh maintained that the whole 

 of the Potomac is Jurassic in age. This was based on the evidence 

 of vertebrate fossils found in the Arundel of Maryland. So far as I 

 am informed, no one is disposed to go as far as Professor Marsh for the 

 whole formation, but some agree with him in regarding at least the 

 lower portion of the Potomac as Jurassic, and Professor Clark and Mr. 

 Bibl^ins regard the age of the Patuxent and Arundel as possibly Juras- 

 sic. The question of the Jurassic or Lower Cretaceous age of the Lower 

 Potomac hinges upon the position of the Wealden formation. 



After a study of the Lower Potomac plants described in Monograph 

 XV, I expressed the opinion that they indicated a Lower Cretaceous 



