106 Marsh — Jurassic Formation on the Atlantic Coast. 



during that season, in following essentially the same strata to 

 the eastward through Delaware and New Jersey, and likewise 

 presented evidence showing that apparently the same Jurassic 

 beds were to be found in position beneath Long Island, Block 

 Island, and Martha's Vineyard, represented by the variegated 

 basal clays of these islands, which had previously been sup- 

 posed to be of much later age. The evidence seemed con- 

 clusive that in this series we had remnants of an extensive 

 formation of fresh-water origin, the strata consisting mainly of 

 soft sandstones and plastic clays of great thickness. In their 

 physical characters, and especially in their variegated brilliant 

 colors, these deposits differed widely from any others known 

 on the Atlantic border, and were only equalled in this respect 

 by the Jurassic beds of the Rocky Mountain region. The 

 presence on the Atlantic coast of such an extensive formation, 

 with its massive beds of plastic clay, all of fresh- water origin, 

 clearly proved the former existence of a great barrier between 

 the basin in which these clays were deposited and the Atlantic 

 Ocean, a barrier that has long since disappeared through sub- 

 sidence, or was broken down by the waves of the Atlantic, 

 which are still rapidly removing the remnants of the formation 

 along its eastern exposure, as may be seen on Block Island, 

 and at Gay Head on Martha's Vineyard. 



In discussing the age of this formation, its position above 

 the Triassic and below the marine Cretaceous, its characteristic 

 physical characters, distinct from those above and below, and 

 its western extension into the strata of undoubted Jurassic age 

 in the Potomac beds of Maryland, all pointed to the conclusion 

 that its members belong to the same general epoch, and were 

 deposited during Jurassic time. 



In the paper thus cited, I confined myself strictly to the 

 Potomac formation north of the Potomac Piver, and what I 

 believed to be its eastern extension as far as Martha's Vine- 

 yard, all of which I had personally explored. I particularly 

 avoided any discussion of the so-called Potomac beds south of 

 the Potomac Piver, although I had been over these deposits at 

 various points along the Atlantic border and around the Gulf 

 as far as the Mississippi River. I closed the paper with the 

 promise of taking up that part of the subject later. 



As the question was a difficult one and still under investiga- 

 tion, I likewise guarded myself against expressing the opinion 

 that all the so-called Potomac deposits were Jurassic. My 

 words on this point were as follows : 



" It cannot, of course, be positively asserted at present that the 

 entire series now known as Potomac is all Jurassic, or represents 

 the whole Jurassic. The Lias appears to be wanting, and some 

 of the upper strata may possibly prove to belong to the Dakota."* 



* This Journal, vol. ii, p. 436, 1896. 



