Section in Plastic Clay. 



197 



At the cliff in the east part of Chilmark, some of the clay is 

 schistose, and there we find the following section. A considerable 

 part of the cliff there, from 20 to 30 feet high, is covered by dilu- 

 vium, which has slidden down from the upper part. But where the 

 strata are exposed to view, they present the curved, contorted, and 

 inclined appearance exhibited below. The general dip, it will be 

 seen, is towards the south, as the left hand side of the sketch is the 

 southern extremity of the cliff The sketch embraces a horizontal 

 distance of about five rods. The geologist sees at once, that such a 

 cliff needs only to be consolidated, in order to constitute a graywacke 

 formation ; as I shall show by presenting a similar sketch from a 

 ledge of graywacke in Newport, R. Island, when I come to treat of 

 that rock. 



Section in the Plastic Clay : Chilmark. 

 Inference. 



After the preceding description, it seems to me that no geologist 

 will hesitate to refer this formation to the Plastic Clay of European 

 authors. The variegated clays, the interstratified lignites with am- 

 ber, and the intermixture of marine animals with terrestrial vegeta- 

 ble remains, must settle the question. Some peculiarities, have, in- 

 deed, been pointed out in the formation at the Vineyard. But they 

 are fewer than we might expect in depositions so widely separated 

 from one another as this is from those in Europe. 



Other places covered with the Plastic Clay. 



Nantucket. 



I colored this island as underlaid with plastic clay, not without 

 strong suspicions that its surface may be composed of a formation 

 higher up in the series. Its surface is entirely spread over with 

 sand, whose upper part has been disturbed by diluvial action, and in 



