126 L. F. Ward — Evidence of the Fossil Plants 



and Gard., and Rogersia longifolia with Glossochlamys trans- 

 Tnutans Ett. and Gard., from the London clays, are too remote 

 to possess any value for the present discussion. 



The relations of the Potomac flora to those of other for- 

 mations may now be summed up in tabular form. The 16 

 identical and 98 allied species are composed of 53 cryptogams, 

 all but 2 of which are ferns, 9 cycads, 35 conifers, and 17 dico- 

 tyledons, and as the lower types are somewhat more promi- 

 nently represented in the lower formations and the higher in 

 the higher, this relation may perhaps be best shown in the fol- 

 lowing form. 



Crypto- 

 gams. 



Geological Formations. 



|2 



Eocene 



Laramie Group j 



Senonian 



Dakota Group 1 



Cenomanian i 2 



Gault I 



Urgonian 1 



Neocomian I 2 



Wealden 4 



Kimmeridgian | 



Corallian ! 



Oolite 



Lias 



Rhetic 



9 

 1 



T 

 2 



11 

 1 

 1 



24 

 4 



Cycads. 



a .2 

 © 



2 < 



l 



2 

 1 

 2 1 



Conifers. 



Dicotyle- 

 dons. 



o .2 

 © ! p3 



2 < 



Total. 



9 

 11 

 23 



2 

 16 



1 

 13 



4 



6 

 31 



6 

 14 



From this exhibit it appears that no Jurassic species occurs 

 in the Potomac formation, although it contains a large number 

 of strongly Jurassic types. The Wealden furnishes the largest 

 number of identical species, the Cenomanian next, and the 

 Urgonian next. Of allied species, although the largest num- 

 ber occurs in the Oolite, the Cenomanian, Urgonian and Weal- 

 den, each furnish many. Taking the identical species and 

 considering the Wealden as Cretaceous, the flora would appear 

 to be decidedly Cretaceous, but if this showing is considered 

 in the light the Jurassic types cast upon it, it is difficult to be- 

 lieve it to be higher than Wealden or JSTeocomian. 



Another class of facts still further strengthen this view. 

 Leaving the allied species aside, and considering only the iden- 

 tical ones, we see that a large number of these recur in most of 

 the members of the Cretaceous. As already stated, there are 

 only 16 identical species, while the sum of those in all the 

 formations as here presented would be 35. This results from 

 their repetition. Three species are confined to the Wealdne, 



