57() MESOZOK' FLORAS OF FM TFD STATFS. 



Aquia ("reek groups. Wliile iiianv of tlio old types, nuicli diniinishod 

 ill miniboi-s, pass \ip into the Mount Vernon and Acjnia ("reek horizons, 

 in ninnlxn' of indivi(hials tliev are to some extent suhoi'dinate to more 

 niod(n'n types. The case is very (hrferent in passing;' up into the Pvari- 

 tan or Aniboy clays. Here there is a wholesale change. Few, if any, 

 of the older types persist. Even those nior(> modern types that were 

 introduced in the Mount Vernon and Acjuia (,'reek groups disappear. 

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

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

 ledons overwhelmingly predominate. In a wf)rd, in passing into the 

 Raritan strata we find the flora wholly changed. This lieing 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 Potom.ac. 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, l)ut, unlike others, ha went further, 

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

 reason he maintained that Professor Marsh'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 aliiove, 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 

 lo\\'er portion of the Potomac as Jurassic, and Professor Clark and Mr. 

 Bilibins 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 ojiinion that they indicated a Lower Cretaceous 



