296: 0. C. Marsh -Geology of Block Island. 



Bluffs, on the southeast shore of the island, afford a fine exam- 

 ple of this, as here the clays show a vertical thickness of about 

 a hundred and fifty feet. Judging by the inclination of the 

 beds, which are highly inclined, a great thickness of strata is 

 here represented. This fact, together with the disturbance they 

 have undergone, is an important element in the problem of 

 their age, and indicates for them a much greater antiquity than 

 would otherwise be supposed from the nature of the deposits 

 themselves. 



These foundation clays 6i Block Island were evidently much 

 eroded before the glacial drift was spread over them. They 

 still constitute the bulk of the island, and their depressions 

 form an impervious stratum for the bottom of the numerous 

 ponds for which the island is renowned. These clays all 

 appear to be fresh-water deposits, and should certainly contain 

 vertebrate fossils. I found none in the limited time at my 

 command, but more careful exploration would undoubtedly 

 bring them to light, and thus determine the geological age of 

 these interesting beds. In the mean time, a comparison of 

 various deposits at some other points on the coast may, perhaps, 

 suggest a solution of this problem. 



In exploring the Tertiary and Cretaceous strata of the 

 Atlantic coast south of New York, I have had much expe- 

 rience, but know of nothing in these two formations there 

 exposed that offers a parallel to the deposits now under discus- 

 sion. The still older Potomac beds, however, have many sim- 

 ilar features, and to this horizon I should be inclined, on 

 present evidence, to refer the greater portion, at least, of the 

 Block Island clays. From the Potomac formation in Mary- 

 land, I have secured a large collection of fossil vertebrates that 

 indicate a period corresponding nearly to the Wealden of 

 England, which is now regarded by the best authorities as late 

 Jurassic, and to this age the vertebrate fossils of the Potomac 

 may likewise be assigned.* The special horizon in which 

 these vertebrates are most abundant in Maryland may be called 

 the Pleurocoelus beds, from a genus of Dinosaurs found there, 

 especially in the iron-ore clays, which are similar in physical 

 characters to some on Block Island. 



The Raritan clays of New Jersey, I regard as belonging to 

 the same series as the Potomac beds, since they hold the same 

 relative position stratigraphically, while the only vertebrate 

 fossils thus far reported from them (remains of a Dinosaur and a 

 Plesiosaur, both preserved in the Yale Museum) also tend, in 

 part, to confirm this view. The latter is probably from a 

 higher horizon. 



*This Journal, vol. xxxv, p. 90, 1888. See, also, Sixteenth Annual Report 

 U. S. G-eol. Survey, Part I, p. 183, 1896. 



