394 ■ MKSOZOIC FLORAS OF UNITED STATES. 



The first of \hvsv papers was promptly replied to by Dr. Artiiur 

 Holliek," who was i)rol)al)ly the best informed person as to the age of the 

 Block Ishind lieds. In view of my prolonged studies of the whole series of 

 beds of whieh those of Block Island constituted only one link in the chain 

 from Staten Island to Xantucket, the age of which I had so closely worked 

 down from the vegetable remains, I also felt called upon to reply, and this I 

 did in Xoveml)er,'' ])efore the appearance of the third and more elaborate 

 paper of Professor Marsh. I did not care to discuss the age of the beds in 

 Maryland from which the only vertebrate remains had Ijcen oljtained. and 

 confined myself to showing that the Block Island deposits, which he classed 

 along with these as Jurassic, were much higher in the series. 



'Sir. Gilbert, professing no expert knowledge of paleontology, wrote 

 wholly in the interest of method,'' and said: 



The number of persons to whom the local question of correlation is im])ortant 

 mav not be large, but the whole body of geologists and paleontologists are concerned 

 with the methods and i)rmciples of correlation, and an excellent opjiortunity seems 

 to be here afforded for the comparison of vertelirate with botanic evidence. I 

 therefore write to express the hope that when Prof. Marsh continues the subject, 

 as he has promised to do, he set forth the grounds for the conclusion he has 

 announced with so much confidence. His article states, in effect, that through a 

 comparison of vertebrates from the Potomac formation with vertebrates from other 

 formations he has inferred the Jurassic age of the Potomac; but he gives no hint 

 of the character of his evidence or the course of his reasoning, so that the conclusion 

 has at present only the authority of his statement, without opportunity for verifica- 

 tion. 



Mr. Hill' defended the Cretaceous age of the Wealden, to wdiich 

 Professor Marsh admitted that the Potomac might belong. It is easy to 

 see how this was vital to Mr. Hill, because it would certainly place the 

 Comanche series of Texas, which is admitted to go down even lower than 

 the oldest Potomac, in the Jurassic, and Mr. Hill, as we have seen (p. 341), 

 had long abandoned that position. 



Mr. Marcou's contril)ution' ought scarcely to Ije included in this 

 series, as the Potomac is not mentioned, and it is devoted to sustaining 



« The geology of Block Island, by Arthur Holliek: Science, N. S., Vol. IV, October 16, 1896. pp. .571-572. 

 '' Age of the Island series: Science, n. s., Vol. IV, November 20, 1896, pp. 757-760. 

 <■ Age of the Potomac formation, by 0. K. Oilbert: Ibid., December 11, 1896, pp. 87.5-877. 

 'I A question of cIa.ssification, by Robert T. Hill: Ibid., December 18, 1896, pp 918-920. 

 ' The Jurassic Wealden (Tithonian) of England, by -Jules Marcou: Ibid., Vol. V, January 22, 1897, pp. 

 149-152. 



