AGE OF THE POTOMAC DEPOSITS 213 



of somewhat primitive type. At the same time a single dinosaurian 

 bone, somewhat waterworn, and possibly redeposited from the Arundel, 

 has been found in the Patapsco, although its fragmentary character 

 renders it impossible to determine its systematic relations. The results 

 of these observations, together with the discovery by the late Professor 

 Cope of a plesiosaur in the Raritan formation of New Jersey and of a 

 dinosaurian limb bone by Woolman in the Matawan formation of the 

 same state, although not definitely settling the age of the deposits, cast 

 further doubts on the Jurassic affinities of the Arundel and at the same 

 time of the underlying formation — the Patuxent. 



The question as to the age of the Potomac group is therefore narrowed 

 down to two propositions : 



First. Is the Arundel dinosaurian fauna conclusive evidence of the 

 Jurassic age of that formation, and therefore of the subjacent Patuxent ? 

 No less an authority than Professor Marsh, after a study of its dinosaurian 

 fauna, unquestionably refers the Potomac group to the Jurassic, although 

 at the time not cognizant of the complexity of its deposits. He regarded 

 the Potomac as a single formation, as has been the case with many other 

 geologists. In his view regarding the Jurassic age of the Potomac, Pro- 

 fessor Marsh has been supported by a few others, mostly among English 

 geologists, since the question here presented is recognized to involve the 

 age of the Wealden as well. Professor Marsh lays much stress on the 

 equivalence of the Potomac with deposits which he has regarded as 

 Jurassic in the Rocky Mountain district, but some doubts have been 

 expressed by others whether these deposits may not be younger. It 

 seems to the authors that further study by vertebrate paleontologists is 

 required before these questions can be settled and the Jurassic age even 

 of the two lower formations of the Potomac group can be accepted on 

 the evidence of the fossil vertebrates. 



Second. Are the floras of the Arundel and Patuxent formations, with 

 their primitive dicotyledonous types, of necessity Cretaceous ? There is 

 apparently no question regarding the Cretaceous age of the Raritan and 

 Patapsco formations, the uppermost beds of the Raritan even containing 

 floras that have been regarded by Professor Ward as middle Cretaceous. 

 The paleobotanists who have studied the floras of the earlier formations 

 admit that there are many forms which show Jurassic affinities. Pro- 

 fessor Fontaine, in his study of these floras, states that there was an 

 " overwhelming percentage of Jurassic types," but unhesitatingly refers 

 the Potomac flora as a whole to the Cretaceous, correlating the deposits 

 with the Cretaceous beds of England. This view is held by nearly all 

 paleobotanists who regard the presence of dicotyledons, although of 

 primitive types, as unquestioned evidence of the Cretaceous age of the 



