348 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



the Potomac decidedly point to the Neocomian as the age of the Potomac 

 beds. 



From this brief review of the flora, we see tliat there is in it a very 

 Large and important element that belongs to the Jurassic or typical Meso- 

 zoic flora; a less important, but still large element, that has near relations in 

 Cenomanian and even living forms; while the largest, most fully developed, 

 and characteristic element is most nearly allied to forms distinguishing the 

 Neocomian. 



All the important species common to the Potomac and the floras of 

 known formations are found in the Neocomian, including under this name 

 both the Wealden and Urgonian. 



If any additional evidence were needed of the Neocomian age of the 

 Potomac, it may be found in the peculiar union of old and new types, 

 whose evidence, if we consider them by themselves, is contradictory. 



Schenk, in Die Foss. Pflanz. der Werns. Schichten, page 29, in speak- 

 ing of the character of the Neocomian flora of the Wernsdorf beds, well 

 says that the flora of the older Cretaceous occupies in the development of 

 the plant kingdom a position similar to that of the Trias, for the forms 

 characteristic of two great periods of development meet in it ; that is, the 

 survivors of the past period (Mesozoic) and the new forms of the approach- 

 ing one (Tertiar}'). 



This being true, we should expect to find in any large collection of 

 Neocomian plants a great mingling of types. We would find the survivors 

 of the old floras and the newly arrived precursors of the more recent ones 

 mingled with a number that attain their development in and are peculiar 

 to the Neocomian. This is exactly what we find to be true of the Potomac 

 flora. That so many of these plants are new is perhaps to be explained, 

 in part at least, by the fact already mentioned, that the flora of this epoch 

 is very poorly represented and comparatively but little known. It is not 

 possible to say positively to what precise epoch of the Neocomian the 

 Potomac belongs. Its flora ranges from the Wealden through the Urgo- 

 nian, and probably includes some Cenomanian forms. 



