Marsh — Jurassic formation on the Atlantic Coast, 437 



Block Island, evidently once a part of Long Island, I have 

 also examined. Its basal clays agree in most respects with the 

 above representatives of the same horizon, as I have shown 

 elsewhere.* " 



Gay Head. 



By far the finest exhibition of the great formation in ques- 

 tion may be seen on Martha's Vineyard, especially at Gay 

 Head, which for a centnry has attracted the attention of geol- 

 ogists, who have tried in vain to solve its mysteries. My first 

 visit to this classic region was in September last, and I know 

 of no point on the Atlantic coast, from Nova Scotia to Florida, 

 of more interest to geologists. The striking resemblance be- 

 tween the variegated clilfs at Gay Head, the Potomac hills in 

 Maryland, and Como bluffs in Wyoming, will impress every- 

 one who has seen them. That all three are of essentially the 

 same geological age, I have good reason to believe. Two of 

 them are certainly Jurassic, as demonstrated by typical verte- 

 brate fossils, and I hope soon to prove that Gay Head, so simi- 

 lar in all other respects, also contains the same characteristic 

 vertebrate fauna that marks the Jurassic, — the long missing 

 formation on the Atlantic coast. 



It has already been shown that the vertebrate fossils of the 

 Potomac in Maryland prove its age there to be Jurassic, espe- 

 cially when taken in connection with the rich fauna of the 

 Atlantosaurus beds of the West. In deterniining the age of the 

 whole series, every aid that paleontology can render should be 

 brought to bear upon the question, but a discrimination greater 

 than has hitherto been shown is necessary to secure the best 

 results. 



In addition, then, to the evidence of vertebrate fossils as to 

 the age of this eastern formation, the testimony of the inverte- 

 brates and plants should also be considered. The invertebrates 

 known from these strata are few in number, but some of the 

 mollusks among them point to the Jurassic age, as Whitfield 

 has shown. f JSTearly all, however, were estuary or fresh-water 

 forms, which are now generally admitted to be of slight value as 

 witnesses of geological changes. 



Evidence of Fossil Plants. 



Pemains of plants are numerous, but usually fragmentary, 

 and these have been collected at many localities, and studied 

 by botanists of much experience in such investigations. The 

 verdict they have rendered has not been a unanimous one, but 

 is especially interesting, as it coincides at one point with the 

 decisions some of their predecessors have rendered as to the 

 age of other geological horizons in the succeeding forma- 

 tions of the West. 



• This Journal, vol. ii, p. 295, October, and p. 375, Novenaber, 1896. In the. 

 second paper will be found an abstract of the more important literature. 

 t Monograph IX, U. S. Geol. Survey, p. 23, 1885. 



