LOWER CRETACEOUS FORMATIONS AND FAUNAS 59! 
Raritan re 8 
Lower Cretaceous Patapsco * Potomac. 
é Arundel - Group” 
PY 
Upper Jurassic (?) ee a 
According to these authors all of the Potomac vertebrates 
that have been recorded have come from the Arundel formation, 
while practically the whole of the Potomac flora occurs in higher 
horizons above the principal unconformity which separates the 
Patapsco from the Arundel. All of Professor Ward’s plant- 
bearing ‘‘series” below the Albirupean are believed to be local 
subdivisions and variations of the Patapsco formation. The 
underlying beds are doubtfully assigned to the Upper Jurassic 
on the authority of Professor Marsh’s determination of the 
affinities of the vertebrates. It has been shown, however, that 
his comparisons are chiefly with the Wealden fauna, and if the 
difficult stratigraphy of the Potomac has now been correctly 
determined the evidence tends to prove the post-Wealden age of 
the principal plant-bearing horizons. 
There have long been differences of opinion as to the age of 
the Wealden, and it may well be that it is partly Jurassic, but its 
constantly close association with the Cretaceous, and the fact 
that where it is present the lowest marine beds of the Neocomian 
are always absent, are strong arguments for regarding it asa 
non-marine facies of the Neocomian. If all the Wealden 
deposits are transferred from the Cretaceous to the Jurassic 
because the dinosaurs are closely related to those of the Jurassic, 
then we know practically nothing of the land fauna of the Lower 
Cretaceous, and no one can say whether the dinosaurs that must 
have lived in early Cretaceous time, were very different from 
those of the Jurassic or not. Professor Marsh’s statements may 
be fairly interpreted to mean that the age of the Atlantosaurus 
beds is dependent on that of the Wealden, and if the latter is 
Cretaceous the former are also. There is nothing in the strati- 
graphic relations of the Atlantosaurus beds that would prevent 
their reference to the Lower Cretaceous, for they are every- 
where immediately overlain by Upper Cretaceous strata.° How- 
There is a possible exception to this in the Black Hills of South Dakota, where 
