PERIOD OF DISLOCATION NOT CERTAINLY FIXED. 447 



sition of the Cretaceous and the formation of the last beds exhibited in the 

 section. There are unconformities observed by my assistant, Mr. Wood- 

 worth, apparently indicating a period of tilting coming immediately before 

 the deposition of the upper bowlder bed. It will require, however, more 

 detailed study to determine this point in a satisfactory manner. The rela- 

 tively slight disturbances of the later sands in the Weyquosque cliffs, if they 

 be orogenic, as it seems to me likely, would indicate a period of disturbances 

 coming after the lower members of the Vineyard series had been subjected to 

 considerable erosion. So, too, the disposition of the later sands in the Gay 

 Head section also indicate in a tolerably satisfactory way the existence of a 

 measure of disturbance after a considerable erosion of this series. 



It is as yet impossible to determine the area affected by the dislocatory 

 forces which have operated on Martha's Vineyard. One of the neighboring 

 localities of apparently the same age as the Vineyard series is that long 

 ago made known by Dr. Hitchcock, in his Geology of Massachusetts, as oc- 

 curring in Marshfield, Mass. With the help of my assistant, Mr. C. P. Siu- 

 nott, I have recently made a considerable study of this deposit. Several 

 excavations have shown that the area it occupies covers rather more than a 

 square mile in surface. The whole of the material appears to consist of 

 layers of greensand, in appearance substantially like those which occur at 

 Gay Head. It seems, however, from the fossils obtained that the identity 

 in physical character of the material does not afford legitimate presumption 

 as to their likeness in age. The few molluscan remains obtained appear to 

 be of an earlier time than those occurring in the greensands of Gay Head. 

 They are on the whole reconcilable with the supposition that the Marshfield 

 series is of Cretaceous age, probably belonging somewhere near the middle 

 of the series. It is a noticeable fact that these Marshfield beds appear 

 to retain their original, nearly horizontal, attitudes ; although the bedding is 

 not very distinct it is sufficiently clear that it is prevailingly horizontal, and 

 thus shows that orogenic disturbances have not operated in this field since 

 the layers were accumulated. 



My assistant, Mr. Aug. F. Foerste, has observed on Block island beds 

 which he considers as probably identical in age with those which are pre- 

 sumed to be Tertiary in the Gay Head series. These deposits of Block island, 

 according to Mr. Foerste's observations, lie at such angles as to make their 

 dislocation by mountain-building forces almost certain. I have not myself 

 had an opportunity of examining these Block island deposits ; but, accepting 

 the above-indicated observations of Mr. Foerste. it seems clear that we have 

 a prolongation of the mountain-building disturbances which have affected 

 this shore to the westward as far as that island. 



It will be interesting to determine whether these mountain-building dis- 

 turbances of late Tertiary age had any part in producing the very extensive 



