SIRATIGRAPHY OF THE POTOMAC GROUP 499 
up of a series of formations, containing clearly defined faunas 
and floras, and separated by marked structural breaks at various 
points. It is necessary, therefore, to correlate each division of 
the group upon its own merits, and not upon the floral or faunal 
relations of the underlying and overlying formations. 
The absence of knowledge regarding the character of the 
organic remains entombed in the Patuxent formation renders it 
impossible to speak with definiteness regarding the age of this 
division. Its position beneath the Arundel beds and the physi 
cal relations which it holds to this formation render it probable 
that they must both be assigned to approximately the same 
position in the geological time scale. The Arundel formation 
has afforded a number of vertebrate forms, largely Dinosauria, 
which Professor Marsh regards as indisputable proof of the 
Jurassic age of the deposits. The fossil plants which have been 
hitherto found in this formation are altogether in harmony with 
this view. No dicotyledonous types have been observed, while 
the ferns, conifers and monocotyledons could, so far as at present 
known, be as well referred to the Jurassic as to a later horizon. 
The evidence afforded by the vertebrate fossils is unfortunately 
incomplete as but few forms have as yet been figured and des- 
cribed, although Professor Marsh states that he possesses a large 
amount of unpublished material which fully corroborates the views 
which he has promulgated regarding the age of these beds. Pro- 
fessor Marsh admits the equivalence of these beds with portions 
at least of the Wealden formation of Europe which most authorities 
refer to the Cretaceous. The final interpretation of the verte- 
brate fauna evidently involves, therefore, the determination of 
the upper limits of the Jurassic itself rather than the correlation 
of the Potomac group simply, and this article is hardly the place 
to discuss the merits of so broad a question. It is not impossible, 
however, that portions of the Potomac as well as the Wealden 
may antedate the oldest known marine Cretaceous, and for that 
reason as well as on account of the distinguished authority of 
Professor Marsh the Patuxent and Arundel formations are pro- 
visionally referred by the authors to the Jurassic. The discov- 
