POTOMAC FORMATION IX VIKGIMA AND MARYLAND. 595 



pahaniKX'k species lliat t)ri<!;iiiateil in them aiui persistetl throujili llie 

 Potomac period in Virginia, the ne\A- forms not yet found in the Virginia 

 Potomac ought to liave a distii\clly Jurassic facies and 1»e comparable 

 to those of the other well-known .lumssic floras of the glol)e. The 

 following are the 12 new species occurring in the ^laryl-md l)eds exclusive 

 of Rosiers Bluff: 



Abielito.s ]Miirvlaii(li(Mis, Yinop:ar Hill, "J s|)('cinions. 

 Adiantitos parxil'olius, I-'cdci'al Hill, 1 spiM-iincn, 

 Celastrc)j>li\ llilin f inalN lainlicnin. Federal ilill. 1 s|ieeiiiieii. 

 Cladoi>hleI)is acuta an.iiu.stil'olia. Arlinjrton, I 1") s|)eeiiiieii.s. 

 Ginkgo '. aectai'ia. Vinegar Hill, 1 s])ccinien. 

 Pinite.s Leei. I'liidii Tunnel, Baltimore, 1 speciineu. 

 Piantaginop.sis iiiarvlandica. Federal Ilill, ."> specinien.s. 

 Proteiv|)hvlluni I'hieri, Federal Hill, 1 specimen. 

 Rogersia august il'olia parva, Langdon, o speeimens. 

 Selaginella marvlandiea, \'inegar Hill, 1 specimen. 

 Thinnfeidia marvlandiea, 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, Rngcn^ia nngustifolia 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 fotind 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 Thmnfeldia marylandica. Both these are abundant 

 in the Arlington l)eds 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 not be known. It can 

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

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

 T. variahdis, runs up into the Brooke beds. The genus Thinnfeidia 

 is properly Older Alesozoic, btit plants have been referred to it from the 



