212 J. B. WOODWORTH — UNCONFORMITIES OF MARTHAS VINEYARD. 



interval on the island, and indicates here, as on Marthas Vineyard, that 

 there was a long interval between the last and earlier Pleistocene forma- 

 tions. 



Summary. 



It remains to attempt the correlation of the local geologic features 

 here described with the sequence of events made out for this region west 

 and south of the New England islands. The work of David White, 

 Hollick, Dall, and others leaves little doubt regarding the equivalence 

 of the pre-Pleistocene members of the section. The earlier Pleistocene 

 beds referred to in this paper, denominated Weyquesque by Shaler, 

 have been referred to as Columbia by McGee. With the beds bearing 

 this latter name south of the glacial region, the lower Boulder bed, the 

 Sankaty and Tisbury beds appear to be equivalent ; but in the district 

 of the New England islands this formation is divisible into lower and 

 upper members, separated from each other by a very marked uncon- 

 formity, accompanied by the profound disturbance of the lower beds ; 

 hence the need of distinct local names such as Sankaty and Tisbury 

 beds for the lower and upper divisions of the Columbia. 



Furthermore, those who hold that the dislocation of strata along the 

 line of these islands is due to the forward thrust of an ice-sheet must, 

 it seems to me, admit that at the beginning and again before the close of 

 this earlier series of Pleistocene deposits of Columbian or Weyquesque 

 age, glacial thrust was exerted upon the Atlantic coast in this latitude, 

 and this at times separated from the epoch of the superficial moraines 

 of the islands by a very long interglacial epoch. 



