FLORA OF OLDER POTOxMAC FORMATION. 367 



It is not strange that Mr. Knowlton finds Cupressinoxylon, for I find at least 

 six abundant and widely diffused species of Sequoia, determined, without taking 

 the cones into consideration. I think it highly probable that the genera Sequoia, 

 Athrotaxopsis, and Sphenolepidium had wood closely alike, and that the trees grew 

 in forests and furnislxed most of the vast quantity of lignite that occurs in the 

 Potomac. 



After Professor Fontaine had left Washington it occurred to Pro- 

 fessor Marsh that a paper on the flora of the Potomac formation and its 

 geological significance ought to be presented to the National Academy 

 of Sciences at its April meeting in Washington, and, as president of 

 the academy, he invited me to present it. I assured him that Professor 

 Fontaine was the proper person to do this, and I made every effort to 

 induce him to do so, but he declined on several grounds, and the duty 

 devolved on me. He authorized me to make any use I thought best of 

 his manuscript, which was then in my hands, and it was from this that 

 most of the facts used by me were drawn. I could do this the better, 

 as, at Professor Fontaine's request, I had prepared the three tables of 

 distribution which form the concluding part of the published monograph. 

 From the data thus before me I prepared the paper which I read before 

 the National Academy of Sciences on April 20, 1888." 



Professor Marsh in describing the vertel^rate remains had expressed 

 himself so emphatically on the Jurassic affinities of the fauna that it 

 was natural that I should inquire particularly into the question whether 

 the flora could be regarded as confirming, or as not distinctly negativing, 

 that view. The dicotyledons presented the chief obstacle, this subclass 

 never having thus far been found below the Urgonian, and only one species 

 as early as this. But the Cretaceous dicotyledons thus far known are fully 

 developed, often belonging to genera still living, and it was clear to me 

 that this proved an extensive break in the record. It was this point 

 that I strove chiefly to bring out in this paper, and after fully discussing 

 it I gave my conclusions in the following form : 



On numerous occasions, dating as far back as 1878, 1 have expressed the opinion 

 that the dicotyledons could not have had their origin later than the middle Jura, 

 and it will not surprise me if the final verdict of science shall place the Potomac 

 formation, at least the lower member in which the plants occur, within that geologic 



« Evidence of the fossil plants as to the age of the Potomac formation : Am. .Journ. Sci., 3d ser., Vol. XXXVI, 

 August, 1888, pp. 119-131. 



