FLORA OF OLDER POTOMAC FORMATION. 363 



for publication. At my request and in compliance with the general wish, 

 Professor Fontaine prepared a summary of the most general conclusions 

 growing out of his prolonged studies and submitted it to the biological 

 section of the American Association for the Advancement of Science at its 

 New York meeting in 1887. Only a brief abstract of it was published." 

 As this paper accurately reflects the views that prevailed at that time 

 relative to the general character and probable age of the Potomac forma- 

 tion, I reproduce the parts bearing on these points : 



The name Potomac formation has been apphed to a series of newer Mesozoic 

 sands -ravels, and clays, sometimes cemented into sandstones and conglomerates, 

 which Appear along the inner margin of the coastal plain, forming the basal member 

 of the undisturbed Mesozoic and Cenozoic formations of the eastern United States, 

 in Viro-inia, Maryland, Delaware, and perhaps other States. It comprises two mem- 

 bers-an upper, consisting generally of variegated clays which are well exposed 

 about Baltimore, and a lower, consisting predominantly of sands and gravels, well 

 exposed in the bluffs of the Potomac River below Washington. The upper mem- 

 ber is known only north of Fredericksburg, and the lower is best developed from 

 Washington to Richmond (see p. 275). 



The ao-e of the formation, as indicated by its flora, appears to comcide approxi- 

 mately with that of the Lower and Middle Neocomian [misprinted Neuroman] of 

 Greenland and Europe (see p. 276) . 



It was in December, 1887, that Mr. J. B. Hatcher, under instructions 

 from Prof C Marsh, collected a considerable number of vertebrate 

 bones from an iron mine near Muirkirk, Md. He also found in the same 

 beds some small cones representing the genus Sequoia, and much sihcified 

 wood and lignite. The bones were described by Professor Marsh and the 

 results published at once.^ As to the geological significance of these 

 forms, Professor Marsh says : 



The fossils here described, and others from tne same horizon, seem to prove 

 conclusively that the Potomac formation in its typical localities in Maryland is of 

 Jurassic age, and lacustrine origin. There is evidence that some of the supposed 

 northern extensions of this formation, even if of the same age, are of marine or 

 estuary origin (see p. 94). 



At about this same time Prof. P. R. Uhler, who had long been actively 

 studying this formation in Maiyland^F^blis^^ e xtended 



a Proc Am Assn. Adv. Sci., 36th meeting, New York, 1887, Salem, 1888, pp. 275-276. 

 . Netted a new genus of Sauropoda and other dinosaurs from the Potomac formation, by O. C. Marsh. 

 • . Am. Journ. Sci., 3d ser., Vol. XXXV, January, 1888, pp. 89-94, 9 text figs. 



