FLORA UF ULDEK PUTCBIAC FOILMATIUN. 3()9 



and T intended to leave it so t li;H if the strati<:ra|)liy and tlio animal remains requin d 

 its reference to the Jurassic the plants would not present any serious ohstaeles to 

 such a reference. 



Ill Ills r(>j)ly, tiiidcr (l;it(> of May 24, 1888, ho inakos a substantial 

 (•<»iitril)uti(Mi to \\\c (lisciissioii, wliicli should he piihhshod. Ho says: 



• 1 (lid not alleni|>l to express the evidence in the form of percent ai;;es, because 



I thou<;lil that this l'oi-m niiiiht fi'ive undue weight to those types that are represented 

 by a consideiahle number of species which are, however, found at but few jijaccs, 

 and hav(> very h'w iiuli\iduals. I was disposed to j^i\e more weii^ht ti) sucli a 

 species as Diooriiti's Huchianus than would appear from its single species, for this 

 form is widely diffused and immense in the number of individuals. The same is 

 true of others i}f the species identical with known Neocomian forms. 



You nii.<;ht have made out the case hir anti(iiiity even sti'oni^er, if vou liad called 

 attention to the large number of peculiar types, such as the broad-leaved conifers, 

 and others, which are so largely developed in the Potomac, but show no trace in 

 the Cenomanian. 1 think your exposition of the evidence is a very just one, and I 

 do not untlerstand you as connnitted to a Jurassic age. 



In auothor lottor. (hitod June 1-i, 1888, he further .sa^'s: 



I received a letter from Doctor Newi)errv not long since about the Potomac floia 

 and its age. He seemed to think that you argued for the Jurassic age of the Potomac, 

 and this seems to be Mr. McGee's notion also. 1 do not untlerstand your paper so 

 to argue. It is plain that it goes to show that the sum of the evidence from the 

 plants, as it now stands, points to the Wealden or Lower Neoconuan age of the beds, 

 but that there is no evidence incom])atil)le with an Upper Jurassic age. 



This in my ojjinion is the correct view, with the modification that I would make 

 the age range through the Urgonian. 



I do not think that Professor Marsh's dinosaurs mean anything more than 

 Wealden. The Wealden vertebrate fauna is in ])art dinosaurian. Professor Marsh 

 said that a numl)er of the sj^ecies were allied to those of his Atlantosaurus beds, 

 and these he called Wealden. Doctor Newberry says that all of Professor Marsh's 

 Potomac species are new, and hence do not necessarily prove Jurassic age. He 

 (Xewberiyi maintains either that the Maryland and \'irginia IxhIs are different or 

 that they are not older than Lower Cretaceous [Neocomian 1 sup])osc he means]. 



Prof. P. R. Uhler, who is the best informed person now living in rela- 

 tion to the early geological work of Maryland, and especially as to the locali- 

 ties at which the cycadean trunks collected by Tyson were found, made in 

 1888 the following statement on this subject, which may be relied upon: 



Rarest, of great value, and still unr<'j)resented in any other collection, are the 

 stumps of Cycads presented to the Academy by Mr. P. T. T^'son. All of these w ci'o 

 taken from the Upper Jurassic clays of Maryland. One specimen cam(> from the 



MON XLVIII — 0.5 24 



