196 



Scientific Geology. 



much so as the crabs. Plate. XII. Figs. 26 and 27 represent two 

 of these relics. Both of them resemble species of the genus Manon 

 figured in Goldfuss' Petrifacta. Fig. 27 looks like his Manon peziza. 



On the surface of some of these specimens small pebbles are some- 

 times seen : and I have occasionally found them disseminated through 

 the specimen. In such case it may reasonably be doubted whether 

 an animal once occupied the place of the nodule. Yet if the prin- 

 cipal part of it be hydrate of iron, I can conceive how a few peb- 

 bles might be introduced along with the iron as it gradually took the 

 place of the animal. Still, I am not in a little doubt as to the origin 

 of these nodules. 



Testacea. I found in the green and ferruginous sand at Gay 

 Head, the casts of three genera, and as many species of shells. As 

 the shell itself is wanting, the characters are indistinct: and there is 

 evidence that the specimens have been more or less rolled : but the 

 two bivalves are probably referable to the genera Venus and Tellina, 

 and the univalve to the genus Turbo. The mineralizer is the same 

 ferruginous clay, or perhaps argillaceous oxide of iron, which has 

 petrified the crabs and zoophytes. Hence they all probably had the 

 same origin. Plate XI, Fig. 3, is a sketch of the Venus : Fig. 5, of 

 the Tellina: Fig. 4, of the Turbo. The latter is obviously some- 

 what broken. I could find only two specimens of it. Of the Venus, 

 I found only three or four : but I obtained 20 or 30 of the Tellina. 



Dip, Direction, Thickness, and Superposition of the Strata,. 



In giving the character of the varieties of this formation, I have 

 anticipated much that properly belongs to this place. 



One peculiarity in the clay of this formation, both on the Vineyard 

 and on Cape Cod, is the very general want of a slaty structure. The 

 strata seams are also often indistinct ; so that it needs careful exam- 

 ination to ascertain the dip or direction of the beds. Sometimes, 

 however, the schistose layers are obvious, as in some part of the cliff 

 in the southeast part of Chilmark. 



I have already remarked, that the strata at Gay Head dip from 

 20° to 45° a little to the east of north; and of course they run nearly 

 east and west. Without careful examination, however, that cliff 

 would be regarded as an unstratified mass of clay, sand, and lig- 

 nite. And in respect to some part of it, particularly its southern 

 part, I am still in doubt whether the dip and direction of the beds 

 correspond to the above statement. 



