GEOLOGICAL AFFINITIES OF THE POTOMAC PLANTS. 335 



The Kquisetii liave undergone a .significant cliang-e, assuming deeidodly 

 more recent features. The Rhjctic equiseta, although few in species, mam 

 to have attained a very hxrge size and to have lived in vast numljers in cer- 

 tain conditions of the marshes, hardly any other fossils being found with 

 them. They were also apparently unbranched. In tiie Potomac era the 

 species were small in size, copiously l)ranched, like some living kinds, and 

 they seemingh' no longer, as in the Rluctic, lived in such inunense colonies. 

 It can hardly be doubted that the mode of occurrence of the .specimens of 

 E. Vir(jhiicH))i, at the fishing hut above Dutch Gap Canal, fairlv' represents 

 its distribution in life. It seems to have grown sparingly, scattered here 

 and tliere in the midst of a vegetation consisting mainly of ferns, cycads, 

 and conifers, with now and then an angiosperm. At Baltimore /■>'. Mary- 

 Inmllcum grew rather abundantly with similar elements, but with a pre- 

 dominant angiospermous flora. These two, from their comparative abun- 

 dance, appear to be the most characteristic equiseta of the era. J^J. Lyelli 

 is almost wholly confined to Fredericksburg, and from its rarity there 

 seems to be a survivor from Jurassic times. This is indicated also by its 

 comparatively large size. While the general character and distribution of 

 the Potomac equiseta indicate an age decidedly younger than the Rluvtic 

 and even the Jurassic, the same result would l>e deduced from an examina- 

 tion of the two more characteristic species. 



JEqui^etiun Virfjinicum is so near to E. Burchardfi Dunk., that it is not 

 entirely clear that it should be made a distinct species. At least it may be 

 regarded as a representative of E. Burchardti in Amei'ica. E. Matijlandi- 

 cum belongs to the same type of small equiseta as E. Virginicum and E. 

 Burchardti, which, in its copious branching and small size, stands near to 

 the living species of the genus. 



So far, then, as the meager evidence of the Equiseta goes, we would 

 infer that the age of the Potomac beds is Wealden (i. e., older Neocomian) 

 or younger. 



FERNS. 



The evidence of the ferns, if we look to the species alone, is not clear 

 and positive for any conclusion. A typical Mesozoic or pre-Cretaceons 

 flora consists almost wholly of four elements. These are ferns, cycads, 



