FLORA OF OLDER POTOMAC FORMATION. 371 



at Pi'ofessor Fontaine's request, from his manuscript, the first giving the 

 locahties in the formation from which each species was collected; the 

 second giving the distribution of all the species hitherto known from 

 other beds in all countries, and also of those species most closely allied 

 to Potomac forms; and the third showing the formations in their 

 ascending geological sequence from which Potomac species and species 

 allied to them have been obtained. Professor Fontaine discussed the 

 age and general character of the Potomac flora in the concluding portion 

 of the work. He thinks that much of the confusion that exists relative 

 to age arises from the use of terms in different senses, and especialh^ from 

 that of the term Wealden. On this point he says: 



Before the examination of the geological relations of the Potomac flora is under- 

 taken it will be well to define in what sense the names of certain geological forma- 

 tions, to which frequent reference must be made, will be taken in this memoir. 

 The two formations which are capable of misconception are the Wealden and 

 Neocomian. By some the Wealden formation is regarded as an independent group 

 forming the uppermost member of the Jurassic. Others regard it as a series of beds 

 contemporaneous with a portion of the lower Neocomian, formed in estuaries and 

 marshes at the time when a portion of the typical lower Neocomian, which is marine, 

 was being deposited in the sea. The latter view is the one assumed in this memoir. 



In this work the Neocomian period is taken as including the Urgonian and 

 Aptian of D'Orbigny, the former being, when present, the middle member and the 

 latter the upper member of the formation. When, then, reference is made to 

 Neocomian plants, fossils of the Wealden, Urgonian, and Aptian groups are included 

 and not distinguished (see pp. 331-.332). 



He then takes up the several classes, families, and genera, and con- 

 siders the bearing of each on the age of the beds. His final conclusion 

 is as follows: 



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

 plants a great mingling of types. We should 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 ti'ue of the Potomac flora. That so many of these plants are new 

 is perhaps to be explamed, 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 Urgonian, and 

 probably includes some Ccnomanian forms (see p. 348). 



Doctor Knowlton's bulletin, already mentioned, on the fossil wood 

 and lignites, appeared in advance of Professor Fontaine's work. Doctor 



