as to the Age of the Potomac Formation. 121 



ing the plants, the character of the specimens in place, and the 

 general nature and affinities of the flora of the Potomac for- 

 mation. 



In the course of this investigation the accuracy of my con- 

 jecture as to the true nature of the forms I have mentioned 

 has been abundantly verified, and we have now, described and 

 figured by Professor Fontaine, no less than seventy-five spe- 

 cies of undoubted Dicotyledons in this flora. These forms, 

 together with a number of others to which I shall presently 

 refer, have led Professor Fontaine to change his mind slightly 

 as to the age of this formation, and instead of Jurassic, as first 

 believed by Pogers, he now, as I understand, inclines to adopt 

 the later view of that authority that they are Jurasso-Creta- 

 ceous and will prove to form a group of passage beds between 

 the Jurassic and Cretaceous, similar to the Wealden of Europe. 

 Professor Fontaine, however, prefers to call the flora Neoco- 

 mian, which he gives a wide range as including all the older 

 Cretaceous from the Gault downward, and of which he re- 

 gards the Wealden as an equivalent fresh-water deposit. 



My object in the present paper is simply to show, in as clear 

 and direct a manner as possible, what the fossil plants of the 

 Potomac formation indicate as to its age, and I shall not advo- 

 cate any special view of the case. 



With the exception of five species described by Professor 

 F. H. Knowlton from a study of the internal structures of the 

 silicified wood and lignite, in a paper soon to be published,* it 

 is to Professor Fontaine that we are exclusively indebted for 

 the details which I have summarized in the present paper. 

 At the expense of vast labor he has studied, described and de- 

 lineated the extensive collections made almost entirely by him- 

 self, and has prepared an elaborate manuscript, with 174 plates, 

 which has not yet been published. He has very kindly placed 

 this unpublished work at my disposal, and it is from it that I 

 have chiefly compiled the data which I have to present. 

 Without such a compilation it would be impossible to deter- 

 mine the precise relations of this flora to those of other forma- 

 tions. Professor Fontaine seems not to have undertaken such 

 a compilation, and must have depended for his opinion of the 

 age to which the flora points upon the impression made on his 

 mind by each species separately and that produced in his 

 memory by the tout ensemble. Considering this, it is certainly 

 remarkable how accurate his judgment was, and I am not pre- 

 pared to affirm that the conclusions which flow mathematically 

 from the statistical view here presented, taken in themselves, 

 differ in any marked respect from Professor Fontaine's intui- 

 tions. 



* Bulletin U. S. Geological Survey, Xo. 53. 



