438 Marsh — Jurassic formation on the Atlantic Coast. 



The horizons I especially refer to are in the Dakota, 

 Laramie, and Eocene, all essentially of lacustrine origin, 

 and now well known. Fossil plants in good preservation 

 have been collected in each of these in turn, and pro- 

 nounced by eminent botanists to be Miocene. Other paleo- 

 botanists of equal eminence have reviewed the evidence and 

 made the age somewhat older, but, as a rule, the conclusion 

 reached made the deposits in question at least one period later 

 than the animal remains indicated. To explain this discord- 

 ance, it was in one case gravely asserted that a Cretaceous verte- 

 brate fauna lived in the midst of a Tertiary flora. A larger 

 knowledge of the facts has since led to revision of the first 

 opinions on this point, and the Cretaceous age of both is now 

 admitted. 



It seems to me extremely probable that in the Potomac for- 

 mation we affain have an analoo^ous case. The botanists have 

 pronounced the plants Cretaceous, while the vertebrates are 

 certainly Jurassic. Change the botanical scale one notch, as 

 was done in the horizons above, and the flora and fauna agree^ 

 while the Jurassic formation, so long missing, is in its proper 

 place on the Atlantic coast as it is in the West. The North 

 American botanical timepiece was originally set by the Euro- 

 pean clock, which was one period too slow, as many facta 

 now indicate. Sooner or later, an adjustment must be made. 



Age of the Wealden. 



To illustrate this, I may mention, as the latest change in the 

 European time-standard, the Wealden formation, the Creta- 

 ceous age of which has long been considered a settled point. 

 I had studied this formation at many localities in England and 

 on the continent, as it contained a reptilian fauna similar to 

 one I had found in the Rocky Mountains, and regarded as 

 Jurassic. A further study of the Wealden reptiles caused me 

 to question their Cretaceous age, and a comparison of these 

 with allied forms from the Rocky Mountains led me to the 

 conclusion that both series were Jurassic. 



At the meeting of the British Association, at Ipswich, last 

 year, I read a paper on European Dinosaurs, including two from 

 the Wealden, and thus the question of their geological age 

 came up for determination. The facts I presented, based mainly 

 upon the reptilian fauna, strongly indicated the Jurassic age 

 of the Wealden, and I urged a re-examination of the question by 

 English geologists.* The subject has since been taken up by 

 Smith Woodward, with special reference to the fossil fishes, 

 on which he is high authority. In the Geological Magazine 

 for February, 1896, he gives the main results of his investiga- 

 tion, which prove that the fishes, also, of the Wealden are of 

 Jurassic types, thus placing the geological age of this forma- 

 tion beyond reasonable doubt. 



* Report, British Association for the Advancement of Science, p. 688, 1895 ; 

 and this Journal, vol. 1, p. 412, JSTovember, 1895. 



