XXVlll 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. 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.’ * 
Knowlton,’ in his paper on the fossil wood and lignite of 
the Potomac beds, has also pointed out this divergence of 
opinion between paleobotanists and paleeozoologists. 
Newberry,‘ 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 
Amer. Journ. ser. iii. vol. xxxvi. 1888, p. 126. 
Ibid. p. 181. 
Bull. U.S. Geol. Surv. No. 56, 1889, p. 38. 
Amer. Journ. vol. xli. 1891, p. 194. 
~ CO DO 
