198 Scientific Geology. 



perhaps no place do its cliffs rise more than 100 feet above the ocean. 

 But clay exists beneath this stratum of sand, and is somewhat varie- 

 gated, though generally bluish. In this clay I found a single fossil 

 much disintegrated : but it was a native of the formation, and was con- 

 tained in ferruginous sand interstratified with the clay. It resembles the 

 genus found in the newest tertiary, as mentioned in the proper place. 

 Twenty or thirty feet higher, in this same cliff, I found worn specimens 

 of Natica heros, (Say) and Pyrula carica, (Lamark): but I am inclined 

 to regard them as either diluvial or alluvial. Yet Lt. Prescott informs 

 me that similar shells are found all over the island, from 1 to 60 feet 

 below the surface: and he presented me with an uninjured specimen 

 of the Pyrula carica dug up by a well digger, several years ago, 30 

 or 40 feet below the surface, along with many other shells ; among 

 which, were Mactra solidissima, Venus mercenaria, and castanea, 

 Crepidula fornicata, Solen ensis, Pecten, Area, &c. : all of which 

 now inhabit the neighboring ocean. Can it be that the diluvium is 

 so thick as 30 or 40 feet ? Or must we suppose that the spot where 

 the town of Nantucket now stands, where these shells were dug up, 

 is alluvial ? Or finally, can we admit that the upper stratum of sand 

 on this island is one of the most recent of the tertiary formations ? 

 The second supposition may probably prove the true one. 



I was not able satisfactorily to ascertain the dip of the clay beds 

 on Nantucket. But Lt. Prescott, on whose accuracy of observation 

 I place great reliance, is confident that they dip a few degrees to the 

 south : and my own examination of some quite limited clay pits, 

 rather confirms this opinion. 



What is the actual dip of the Plastic Clay ? 



In no other place, except those above described, have I observed 

 the dip of this formation. But it will be seen that there is great dis- 

 crepancy in the dip as observed at two places on the Vineyard, and 

 on Nantucket : and I confess my inability to determine whether there 

 is any prevailing dip in these strata. It is interesting to ascertain 

 that it has a dip at all ; since it seems thus more nearly identified 

 with the well known plastic clay of the Isle of Wight, so well des- 

 cribed by Mr. Webster; 'and since also this fact has such an impor- 

 tant bearing upon the theory of the formation. But it is doubtful 

 whether its predominant dip can be determined from patches so lim- 

 ited as those which are found in Massachusetts. 



Elie de Beaumont represents " the extensive deposites, in part ter- 



