FLORA OF OLDER POTOMAC FORMATION. 369 



and I intended to leave it so that if the stratigraphy and the animal remains required 

 its reference to the Jurassic the plants would not present anj^ serious obstacles to 

 such a reference. 



In his reply, under date of May 24, 1888, he makes a substantial 

 contribution to the discussion, which should be published. He says: 



I did not attempt to express the evidence in the form of percentages, because 

 I thought that this form might give undue weight to those types that are represented 

 by a considerable number of species which are, however, found at but few places, 

 and have very few individuals. I was disposed to give more weight to such a 

 species as Dioonites Buchianus 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 of the species identical with known Neocomian forms. 



You might have made out the case for antiquity even stronger, if you had 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. I think your exposition of the evidence is a very just one, and I 

 do not understand you as committed to a Jurassic age. 



In another letter, dated June 14, 1888, he further says: 



I received a letter from Doctor Newberry not long since about the Potomac flora 

 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. I do not understand 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 Neocomian age of the beds, 

 but that there is no evidence incompatible with an Upper Jurassic age. 



This in my opinion 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 part dinosaurian. Professor Marsh 

 said that a number of the species 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 

 (Newberry) maintains either that the Maryland and Virginia beds are different or 

 that they are not older than Lower Cretaceous [Neocomian I suppose 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 cohected by Tyson were found, made in 

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



Rarest, of great value, and still unrepresented in any other collection, are the 

 stumps of Cycads presented to the Academy by Mr. P. T. Tyson. All of these were 

 taken from the Upper Jurassic clays of Maryland. One specimen came from the 



MON XLVIII — 05 24 



