XXV111 INTRODUCTION. 



Wealden, through the Urgonian, and probably including some 

 Cenomanian forms. 



Lester "Ward has discussed the geologic age of the Potomac 

 flora in a paper published before the appearance of Fontaine's 

 Monograph ; in a table intended to show the floral elements of 

 this flora he demonstrates the predominance of the Wealden 

 facies. 1 The evidence of the plants is obviously in favour of 

 assigning these Eastern American strata to the Wealden period, 

 but Ward points to the vertebrate fossils as indicative of a 

 Jurassic age, thus furnishing another example of an apparent 

 discrepancy between plants and animals as indices of geological 

 position. He does not wish to argue for the Jurassic age of the 

 Potomac flora, but remarks that "the most it is intended to claim 

 is that, if stratigraphical relations and the animal remains shall 

 require its reference to the Jurassic, the plants do not present 

 any serious obstacle to such reference." 2 



Knowlton, 3 in his paper on the fossil wood and lignite of 

 the Potomac beds, has also pointed out this divergence of 

 opinion between palseobotanists and palseozoologists. 



dewberry, 4 in view of the large number of Angiosperms in this 

 flora, expressed himself in favour of a higher rather than a lower 

 horizon than the Wealden. The same author, in the paper 

 referred to, gives an account of the flora of the Great Falls Coal- 

 field, Montana ; this coal-basin lies on the northern slope of the 

 Belt and Highwood Mountains, subordinate folds of the Rocky 

 Mountain system. After speaking of Fontaine's determination of 

 the Great Falls plants, to whom they had been submitted for 

 examination, Newberry concludes that these identifications " prove 

 conclusively the general identity of the geological horizons of the 

 Potomac group, the Great Falls group, the Kootanie group of 

 Canada, and the Kome group of Greenland, and confirm the view 

 advocated by Prof. Fontaine and myself that the Potomac group 



1 Amer. Journ. ser. iii. vol. xxxvi. 1888, p. 126. 



2 Ibid. p. 131. 



3 Bull. U.S. Geol. Surv. No. 56, 1889, p. 38. 



4 Amer. Journ. vol. xli. 1891, p. 194. 



