146 N. S. SHALER — DEPOSITS OF EASTERN MASSACHUSETTS. 



portion which was laid down before, and a portion formed after, the greater 

 part of the disturbance had been effected. 



'flic geological age of the several members of the Vineyard series must 

 >till lie regarded as somewhat doubtful. The fossils found in Tisbury, near 

 Indian hill, and described in a bulletin of the Museum of Comparative 

 Zoology,* show the presence of distinct Cretaceous beds, probably belonging 

 to the middle or lower member of that series, lying apparently at the base 

 of the deposits found in place tin this island. The Lower portion of the sec- 

 tion at Gay Head is likely also to prove of Cretaceous age. The middle 

 portion of the Gay Head series is presumably of Tertiary age. Although a 

 good many fossils have been obtained from it, there are none of them of 

 sufficient determinative value to establish anything more than the general 

 relations of the deposit. The presence of the cetacean bones and the type 

 of form of the large shark teeth, as well as the general character of the 

 molluscan remains, pretty clearly establish the fact that the beds are above 

 the base of the Eocene and below the summit of the Miocene. On the whole, 

 the aspect of the fossils is most reconcilable with the supposition that the 

 beds are mainly, if not altogether, of Miocene age. The uppermost sands 

 contain no fossils, and their age is therefore undeterminable. Their general 

 aspect is that of rather recent accumulations, and if we consider the middle 

 portion of the section as of Miocene age they may perhaps be referred to 

 the Pliocene section. At any rate, J do not think it probable that they be- 

 long to the level of the Upper Miocene. 



( )n the basis of this determination as to the age of the Vineyard rocks, 

 we may seek to determine the time when the dislocations exhibited by 

 this Beries occurred. It is, in the first place, clear that these disturbances, 



which folded and faulted the beds, Continued down to the time when the 

 newest division of the section exhibited at Gay Head was deposited. If 

 these deposits be of Pliocene age we are compelled to suppose that the 

 orogenic movements were maintained down to that time. The question 

 whether the whole of the dislocation took place at this late age is not -" 

 readily determinable. It i- evident that after the time when the osseous 



conglomerate was deposited, w Inch presumably occupies a portion of the Mio- 

 cene division, the beds were subjected to considerable erosion, which broke 

 up tli.- depu-it ami delivered pebbles of the material.- to later Btrata. It is 

 possible, however, that this exposure of the osseous conglomerate to erosive 

 action was due not to orogenic dislocation but to the laying bare of the beds 



while in a horizontal position, in the form of an escarpment, which was 



attacked by streams or the sea. So far as is yet determinable, we may assume 

 either that the dislocation of the strata occurred in one period in the later 

 Tertiaries or thai it may have happened at various times between the depo* 



Op. olt 



