as to the A.ge of the Potomac Formation. 129 



Platypterigium has been heretofore found only in the Lias of 

 India. Its occurrence in the Potomac, therefore, in so far 

 favors the same conclusion. 



Brachyphyllum is a well known upper Jurassic type, but is 

 found as low as the Muschelkalk and as high as the Xeoeomian. 

 An occasional species in the Cretaceous would not be consid- 

 ered strange, but we find stems, leaves and fruits referable to 

 five species in the Potomac formation, all of which are com- 

 parable to those found in the Corallian and Kimmeridgian de- 

 posits of France and of Solenhofen in Bavaria. 



Other cases might be considered, but these will suffice to 

 show how strong a Jurassic facies the Potomac flora possesses. 



It remains to consider the second question, as to the true sig- 

 nificance of the Dicotyledons. Their presence is supposed to 

 indicate a later age than that denoted by the other groups of 

 plants. It was long supposed that the upper Quader beds of 

 Blankenburg contained the earliest remains of plants of this 

 class, but they were at length found in the considerably older 

 strata of Niederschona in Saxony. Since then their occur- 

 rence in strata of about the same age in Bohemia and Moravia 

 was made known, and it is the practice to treat the so-called 

 upper Cretaceous of Greenland, the Dakota Group of Kansas 

 and Nebraska, and the Earitan clays of New Jersey as homo- 

 taxially equivalent to these Continental deposits. 



With but a single exception no dicotyledonous plants had 

 been found lower than this horizon prior to the discovery of the 

 Fredericksburg bed. This exception was the occurrence among 

 the collections from Koine in Greenland, a deposit whose ani- 

 mal remains are said to fix its age as Urgonian, of a single 

 dicotyledonous species, the .Popidus jprimazvo, of Heer, all the 

 other plant remains from that deposit belonging to the lower 

 types. Although this discovery has not been confirmed by 

 subsequent research, and therefore remains subject to the doubt 

 whether the collections may not have become mixed with those 

 from higher beds made and sent at about the same time, still 

 there is nothing antecedently improbable in it, and it may pass 

 unquestioned. It does not, however, invalidate the general 

 proposition that up to this time the Cenomanian has furnished 

 the most ancient forms of dicotyledonous plants. 



This cannot now be said, and we have in the Potomac form- 

 ation a still earlier date at which to fix the observed origin of 

 the type of vegetable life which is now the predominant one 

 upon the globe. What then does this dicotyledonous element in 

 the Potomac flora prove as to the age of that formation ? Can 

 we argue from its analogy with other Cretaceous floras ? In doing 

 so great caution is required. As compared with these the Po- 

 tomac flora is wholly anomalous in this respect. In all the 



Am. Jour. Sci.— Third Series, Vol. XXXVI, No. 212.— August, 188S. 

 9 



