400 MESOZOIC FLORAS OF UNITED STATES. 



ing to Mr. Bibbins's statements in his contribution to the present paper, 

 quite as frequently in the Patapsco formation as in the Patuxent. 

 In discussing the age of the Potomac deposits the authors say: 



There has been much discussion as to the age of the Potomac group. Most 

 geologists, particularly those who have studied the floras, have believed the entire 

 group to be of Cretaceous age, while a few investigators, notably the late Professor 

 Marsh, of Yale Universitj^, have regarded it of Jurassic age. The authors of this 

 paper in an earlier publication pointed out this difference of view, and clearly showed 

 that the dicotyledonous floras were practically confined to the two upper formations, 

 while the dinosaurs on which Professor Marsh based the Jurassic age of the Potomac 

 group were found in the Arundel formation. As the result of these observations, and 

 without attempting to decide finally regarding the paleontologic evidence, they 

 placed the two lower foi-mations of the Potomac group questionably in the Jurassic. 

 Since the publication of the above paper the authors have made a very exhaustive 

 examination of the several formations and collected large numbers of anim.Tl and 

 plant remains. As the result of this work a considerable dicotyledonous flora has 

 been found to exist in the Arundel, although of somewhat primitive type. At the 

 same time a single dinosaurian bone, somewhat waterworn, and possibly redeposited 

 from the Ai'undel, has been found in the Patapsco, although its fragmentary char- 

 acter 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 underlj^ing formation — the Patuxent. 



The c^uestion 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 com- 

 plexity 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, Professor 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 verte- 

 brate 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 acepted 

 on the evidence of the fossil vertebrates. 



