Marsh — Jurassic formation on the Atlantic Coast. 447 



ITo geologist familiar with the facts will deny that the 

 variegated Potomac clays in Maryland are continuous with 

 those in Delaware, Pennsylvania, and i^Tew Jersey, and that 

 the similar basal clays on Long Island, and the other islands to 

 the eastward as far as Nantucket, are part and parcel of the 

 same series. There is now positive proof that the southern 

 end of this series is Jurassic, and it is certainly a fair conclusion 

 that the remainder is of the same age. The burden of proof 

 will rest upon those who hold to the contrary. 



To place the strata in question in the Jurassic section of 

 the Atlantic coast at once removes many difficulties that have 

 hitherto perplexed students of the Mesozoic of this region. 

 It completes the series, and shows in part, at least, what was 

 done in deposition during that long interval between the end 

 of Triassic and the beginning of Cretaceous time, when the 

 great barrier was broken down, which, from the Devonian to 

 the Cretaceous, shut out the waters of the Atlantic. 



I must leave it to others with leisure at their command 

 to work out the details of this well-marked series, and its 

 relation to those above and below. I have no time to devote 

 to the surface geology of this belt or to the earlier deposits of 

 Tertiary time. Just now, the Mesozoic interests me most of 

 all, especially its middle section, the Jurassic, as I believe 

 great injustice has been done, since this has been denied its 

 rightful place, and a name not its own stamped upon it. 



In a later communication, I hope to discuss this question 

 further, and especially the Jurassic beds south of the Potomac 

 Piver. 



Yale University, New Haven, Conn., November 16, 1896. 



Am. Jour. Sci.— FouRTa Series, Yol. II, No. 12. — December, 1896. 

 32 



