POTOMAC FORMATION IN VIRGINIA AND MARYLAND. 595 



pahannock species that originated in them and persisted through the 

 Potomac period in Virginia, the new forms not yet found in the Virginia 

 Potomac ought to have a distinctly Jurassic facies and be comparable 

 to those of the other well-known Jurassic floras of the globe. The 

 following are the 12 new species occurring in the Maryland beds exclusive 

 of Rosiers Bluff: 



Abietites marylandicus, Vinegar Hill, 2 specimens. 

 Adiantijtes parvifolius. Federal Hill, 1 specimen. 

 Celastrophyllum ? mar_ylandicum, Federal Hill, 1 specimen. 

 Cladophlebis acuta angustifolia, Arlington, 11.5 specimens. 

 Ginkgo ? acetaria, Vinegar Hill, 1 specimen. 

 Pinites Leei, Union Tunnel, Baltimore, 1 specimen. 

 Plantaginopsis marylandica, Federal Hill, .5 specimens. 

 Proteffiphyllum Uhleri, Federal Hill, 1 specimen. 

 Rogersia angustifolia parva, Langdon, 5 specimens. 

 Selaginella marylandica. Vinegar Hill, 1 specimen. 

 Thinnfeldia marylandica, Arlington, 16 specimens. 

 Williamsonia ? Bibbinsi, Vinegar Hill, 1 specimen. 



It will be seen that, of these 12 new species, 4 are dicotyledons, and 

 1 of these, Rogersia angustifolia parva, comes from typical Arundel 

 strata. Eight of them are from Vinegar Hill and Federal Hill, which 

 are probably on a higher horizon and have not been regarded as probably 

 Jurassic. These, therefore, have really no proper bearing on the question 

 at issue. The Pinites Leei from Union tunnel may also be excluded 

 from the discussion; as I have stated, it has so modern an aspect as 

 to suggest that the clay in which it was found may be of Pleistocene 

 age. The only plants, therefore, which have any importance for the 

 claim that the iron-ore beds may 'be Jurassic are Cladophlebis acuta 

 angustifolia and Thinnfeldia marylandica. Both these are abundant 

 in the Arlington beds and are confined to them. The first is simply a 

 narrow-leaved form of the type Cladophlebis acuta, and might denote 

 a lower or a higher horizon according to whether the leaves were tend- 

 ing to become wider or narrower, which can no± be known. It can 

 not, therefore, be said to argue either way. As regards the second, there 

 are 3 other species of Thinnfeldia in the Older Potomac, one of which, 

 T. variabilis, runs up into the Brooke beds. The genus Thinnfeldia 

 is properly Older Mesozoic, but plants have been referred to it from the 



