244 Geology of Martha's Vineyard, 



grounds are, geologists will place that degree of confidenffe in 

 the opinions advanced, which they consider them as de- 

 serving. Besides it must be recollected, that this paper 

 does not profess to give a finished sketch of the regions it 

 embraces, but only to furnish hints towards their geology. 



The diluvial bowlders, occurring along the south-east- 

 ern coast of Massachusetts, are often in immense quanti- 

 ties ; and of a character, very similar to those on the above 

 named islands. 



I was told, by unquestionable authority, that a rocking 

 stone exists in Chilmark, a mile or two south-west of the 

 congregational meeting-house. But I could not visit it. 



3. Plastic Clay Formation. 



The Plastic Clay Formation in England, is composed of 

 an indefinite number of sand, clay, and pebble beds irregu- 

 larly alternating. It contains also, lignite, imperfect coal, 

 amber, organic remains, &c. Taking these characters as 

 the criterion of that formation, we must conclude, that it 

 underlies all the diluvial of Martha's Vineyard. Where- 

 ever the shores of that island are elevated, and the ocean 

 has encroached upon them so as to present perpendicular 

 cliffs, a series of strata answering to the above description, 

 are exhibited usually in great perfection and beauty. Gay 

 Head is well known for the bright and variegated colours 

 of its clay, sand and pebble strata ; which present a naked 

 front, of 200 feet in heighc. I was so unfortunate, howev- 

 er, through circumstances beyond my controul, as not to 

 be able to visit those cliffs ; although I passed within two 

 or three milea of them. But I felt my disappointment 

 somewhat mitigated, by having an opportunity to examine, 

 what I suppose to be a continuation of these cliffs, in 

 Chilmark, five or six miles from Gay Head, and probably 

 near their noith-eastern termination. Immediately beneath 

 a thin stratum of diluvial soil, lies a bed of shells, only a 

 few inches thick, and mostly in fragments. Below this is a 

 stratum of white sand, with some pebbles, often several 

 feet thick. Next occur irregularly alternating beds of va- 

 riously coloured clays, sand, ferruginous sand, pebbles, 

 clay and pebbles, and clay and sand intermixed. The clay 

 beds are white, brown, blackish, red, light and deep yel- 

 low, and finely variegated with sp'ots of white, red, and yel- 



