FLORA OF OLDKR POTOMAC FORMATION. ^bb 



for pu))lieati<)ii. At my re(Hie4 and in coinpliancc wilh the jroneral wish. 

 Professor lontainc i)rei)are(l a suinniaiy of the most oeneral conchisions 

 oTowing out of his prolong(Ml studies and submitted it to the hiolosiical 

 section of the Anieriean Association for tlie Advancement of Science at its 

 Xew York meeting in 18S7. Only a brief abstract of it was pubHslied." 

 As this paper accurately reflects tli(> views that prevailed at that time 

 relative to the general character and probnble age of th(> Potomac forma- 

 tion, I rejiroduce the parts bearing on the e points: 



The iiiuuc Pounnar f<.iinati..n has hern a|)i)iii'd to a series of newer Mosozoic 

 sands, gravels, and clays, sometimes cemented into sandsfon(>s and cons;loinerates, 

 whicli' Tppoar alon^- the inn.>r niariiin of tiie coastal plain, formin- the basal nienil)er 

 of the undisturbed ^fesozoi(■ and Cenuzoi.' formations of the eastern Fnited States, 

 in Vii-inia, Marvland, Delaware, an.l perhaps other States. It .•omprises two mem- 

 bers "an upper', eonsistinc; uenerally of varie-ated clays whi<-h are well exi)o.sed 

 about Baltimore, and a lower, consisting predominanily of sands an.l -ravels, well 

 exposed in the blulVs of the Potomac Kiver below Washington. The ujiper mem- 

 ber is known only north of Fredericksburg, and the lower is best d.'veloped from 

 Washington to Richmond (see p. 27.i). 



The aue of the formation, as indicated by its flora. ai)pears to comcide ai)proxi- 

 inately widi that of the bower and Middle Neocomian [misprinted Xeuronianl of 

 Greenland and Euro|)(> (see p. 271)). 



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

 from Prof. 0. C. Marsh, collected a considerable numl)er of verteljrate 

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

 bed.s some small cones representing the genus Sequoia, and much silicified 

 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 formaticm 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 a-e, are of marme or 

 est miry origin (see p. 94). 



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

 studying this formation in Maryland, published a somewhat extended 



« Proc Vm Assn. Adv. Sei., 3(itl. moetinfc. New Yorlc, 1S87, Salem, 1S88, pp. 27.5-276. 

 /. Notice of a new gonvis of Saun.lioda and otlier dinosauis from ttie Potomac formation, l)y 0.( . Man^li: 

 Am. Journ. Sci., 3d ser., Vol. XXXV, .January. ISSS. pp. S!M)t, 9 text liRs. 



