184 



Scientific Geology. 



their low level, and vicinity to the ocean, lead to the supposition that 

 they were deposited from salt water, by a process similar to that 

 which is now going on in the salt marshes in their vicinity. 



Plastic Clay. 



A cursory examination of the strata of Martha's Vineyard, ten 

 years ago, led me to adopt the opinion that they correspond essen- 

 tially with the Plastic Clay Formation of Europe: and I gave an 

 account of them under that name in the American Journal of Science. 

 Every subsequent examination has confirmed the opinion then formed. 

 And I have ascertained, also, that the same formation exists in seve- 

 ral places on the continent, in the southeastern part of the State ; and 

 likewise on Nantucket. Wherever in that region sands prevail ex- 

 tensively, I have reason to suppose this formation exists ; though I 

 am by no means sure but some of these sands may belong to a newer 

 formation. At some places, however, variegated clays underlie the 

 sand, and fossils are found corresponding with those on the Vine- 

 yard ; and such strata I cannot but regard as identical with the plas- 

 tic clay of that island. It is on the Vineyard, however, that this for- 

 mation is most fully exhibited ; and I shall first describe the strata 

 which exist there, and compare them with the plastic clay in Europe. 



There are but two places on the Vineyard where I have found the 

 strata laid bare to any considerable extent and depth. The one is a 

 cliff in the southeast part of Chilmark, laid bare by the action of the 

 ocean, and slowly wearing away from year to year. The other is 

 Gay Head; which is an elevated promontory from 150 to 200 feet 

 high, forming the western point of the island, still owned and inhab- 

 ited by the mixed descendants of the aboriginees. This spot is the 

 most interesting and instructive. The very heart of the formation 

 appears to be here laid bare. It consists in general of interstratified 

 inclined layers of gravel, conglomerates, sands, variegated clays, iron 

 ore, and lignite. 



Mineralogical Characters. 



1 . Clays. These greatly predominate in the clifTs at Gay Head ; 

 and by the vividness of their colors, attract the attention of the most 

 careless observer. Their variety in this respect is very great ; but 

 the following predominate. 1. White Pipe Clay. This generally 

 contains small plates of silvery mica, and obviously results from the 

 decomposition of granite. 2. Blood Red Clay. This is doubtless 



