368 MESOZOIC FLORAS OF UNITED STATES. 



sj-stem. While the remaining types point stronglj- 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, considexed from the pouit of view of the ilora alone, homotaxially equiva- 

 lent to the Wealden of England and north Germany, now usually included in the 

 Cretaceous system. If the vertebrate remains are Jurassic and the flora Creta- 

 ceous we only have here another confu-mation 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 Devonian floras. It is there- 

 fore to be 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 starting point from the Old World geology, there will be 

 no more objection to regarding the Potomac flora as Jurassic than there is now in 

 contemplating the Laramie flora as Cretaceous. In fact, so far as the character 

 of the flora is concerned, there is much less difficulty in the case of the Potomac 

 than in that of the Laramie, since, as I have showTi, the Potomac flora, viewed in 

 all its bearings, can not be said positively to negative the reference of the forma- 

 tion to the Jurassic upon the evidence of the plants alone. 



I do not, however, desire to be understood as arguing for the Jurassic age of 

 the Potomac formation. The most that it is intended to claim is that, if the strati- 

 graphical relations and the animal remains shall finally require its reference to the 

 Jurassic, the plants do not present any serious obstacle to such reference (see pp. 

 130-131). 



As it has since been made clear that the vertebrate remains are not 

 conclusive as to the Jurassic age of the beds in Maryland and agree quite 

 as well with the assumption of a Lower Cretaceous, or at least a Wealden 

 age, there is even less difference between the evidence of the flora and 

 that of the fauna than was then supposed. Nevertheless I see no reason 

 to qualify the statements then made. There was some discussion of my 

 paper, Doctor Newberry denying the possibility of the formation being 

 Jurassic, and Professor Cope concurring in this view. I sent a copy of 

 my manuscript to Professor Fontaine, sajdng in my letter accompanying 

 it," dated May 21, 1888: 



I do not think that a proper understanding of my remarks commits me at all 

 to the Jurassic theory. It is true I say more about that than the other, but it is 

 because it had been assumed that a flora with so many dicotyledons must of necessity 

 be Cretaceous. All I aimed to prove was that tliis was not a necessary conclusion, 



