394 MESOZOIC FLORAS OF UNITED STATES. 



The first of these papers was promptly rephed to by Dr. Arthur 

 Holhck," who was probably the best informed person as to the age of the 

 Block Island beds. In view of m}^ prolonged studies of the whole series of 

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

 from Staten Island to Nantucket, 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 November,* before 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 been obtained, and 

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

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



Mr. 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 important 

 raeij not be large, but the whole bodj^ of geologists and paleontologists are concerned 

 with the methods and principles of correlation, and an excellent opportunity seems 

 to be here afforded for the comparison of vertebrate 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 iaf erred 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 authoritj'' of his statement, without opportunity for verifica- 

 tion. 



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

 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, vv'hich 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 contribution' ought scarcely to be included in this 

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



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

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

 Age of the Potomac formation, by G. K. Gilbert: Ibid., December 11, 1896, pp. 875-877. 

 rf A question of classification, by Robert T. Hill: Ibid., December 18, 1896, pp 918-920. 

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

 149-152. 



