0. C. Marsh — Geology of Block Island. 297 



The well known clay deposits of Long Island I have not 

 carefully examined in place. There is much in the published 

 descriptions of them, however, to indicate that thej may 

 represent some of the same Jurassic beds. Xear the eastern 

 end of the island, there are beds of clay closely resembling 

 some on Block Island. 



The clay bluffs at Gay Head, on Martha's Yineyard, have 

 many characteristics of the same series, but the presence of 

 Cetacean remains in one portion of them indicates that this is 

 Tertiary. There are, however, some reasons for supposing 

 that the most of the clays are much older, and I believe 

 they contain representatives of the same great Jurassic forma- 

 tion. Some of the vertebrate remains from Gay Head, 

 described as Eeptilian {Graphiodon)^ I found on examination 

 to be Cetacean, and it is possible that others, now regarded as 

 Cetacean, may prove to be Dinosaurian, as was the case with 

 some Potomac fossils. 



The massive clay beds of Block Island were derived 

 from the decomposition of the granitic rocks to the north, 

 and were deposited in quiet waters. The iron ore now in 

 them came also from the northern crystalline rocks mainly 

 as magnetite, which may still be seen in the sands of all the 

 beaches of the island, and on one of them this mineral sand 

 was for a while used in making metallic iron. 



There appear to be no crystalline rocks in place on the 

 island, although some of the large imbedded boulders might 

 readily suggest such outcrops. The glacial drift covers most 

 of the surface, and the hills and shores are strewn with 

 boulders of crystalline rocks, — granite, gneiss, quartz, etc., 

 that came over the ice from the main land on the north. 

 Large masses of both the porphyritic and the garnet-bearing 

 gneiss, waifs from the Rhode Island shore, may be easily 

 recognized, and in the beach sand resulting from the attrition 

 of the latter, the separate garnets may be found. 



On some of the glacial hills near the shore, or around the 

 Great Pond, shell heaps of considerable antiquity may be 

 observed, but so far as I could ascertain, none of them have 

 been explored. One may be seen on the south side of the 

 road recently cut through a low hill, near the new steam- 

 boat landing on Great Pond. The deposits are several feet in 

 thickness, indicating a long occupancy of the place by some 

 of the early inhabitants of the island. The short examination 

 I was able to give this '' kitchen midden" disclosed many 

 marine shells, mainly species now living in the adjacent 

 waters, the most abundant of which were those of oysters, 

 clams, and scollops. Mingled with these were a few bones of 

 fishes, birds, and small mammals. 



