Maryland Geological Survey 41 



strata younger than the Potomac. He enumerated the various types of 

 plant remains which he had collected from the different horizons. 



The same year McGee published his paper entitled " Three Formations 

 of the Middle Atlantic Slope," devoting much of his space to the Po- 

 tomac formation, erroneously referring the Bryn Mawr gravels, the 

 " yellow rocks," above Trenton, IsTew Jersey, and the " sand hills " east 

 of Princeton, New Jersey, to the Older Potomac. 



At the meeting of the National Academy of Sciences held in the 

 spring of 1888 Professor Ward prepared a paper on the " Evidence of 

 the fossil plants as to the age of the Potomac formation," which was 

 published in the August number of the American Journal of Science, 

 from which the following may be quoted : 



" On numerous occasions, dating as far back as 1878, I have expressed 

 the opinion that the dicotyledons could not have had their origin later 

 than the middle Jura, and it will not surprise me if the final verdict of 

 science shall place the Potomac formation, at least the lower member 

 in which the plants occur, within that geologic system. While the re- 

 maining types point strongly in this direction, I do not regard the 

 dicotyledons as at all negativing, but even more strongly suggesting, 

 this view. 



" Still, it may be admitted that, according to the ordinary modes of 

 arguing from similar statistics, the sum of all the facts here presented 

 would make the Potomac, considered from the view of the flora alone, 

 homotaxially equivalent to the Wealden of England and north Germany, 

 now usually included in the Cretaceous system. If the vertebrate re- 

 mains are Jurassic and the flora Cretaceous we only have here another 

 confirmation of a law exemplified in so many other American deposits, 

 that, taking European faunas and their correlated floras as the standard 

 of comparison, the plant life of this country is in advance of the animal 

 life. This law has been chiefly observed in our Laramie and Tertiary 

 deposits, but is now known to apply even to Carboniferous and De- 

 vonian floras. It is therefore to expected that we shall find it to prevail 

 during the Mesozoic era. If, therefore, it be really settled that the 

 fauna of the Potomac series is homotaxially Jurassic, and we take our 



