494 Crosby and Ballard — Distribution and Age of 



Both specimens are distinctly glaciated, showing longitudinal 

 scratches over almost their entire surfaces. 



Concerning the origin and history of these specimens two 

 views are suggested : (I) They are fragments of consolidated 

 till, the product of an earlier ice-sheet, which was cemented by 

 the segregation of carbonate of lime, perhaps locally where 

 the enclosed shells were most abundant, during an interglacial 

 period. (2) They represent a wholly preglacial and probably 

 late Tertiary deposit, an uneroded remnant of which may even 

 now underlie the water and drift deposits of Boston Harbor. 

 This view is not, a priori, improbable, when we consider that 

 Cretaceous and Tertiary strata fringe the southern coast of 

 New England, appearing again in Marshfield, Mass., only 

 twenty-five miles from Boston, and undoubtedly underlying a 

 part, at least, of Massachusetts Bay, as indicated by the frag- 

 ments of rock carrying characteristic Eocene fossils found by 

 Upham in the drift of Cape Cod and described by the senior 

 writer.* The Peddock's Island fragments include both Gas- 

 teropods and Larnellibranchs ; but owing to the fragmentary 

 character of the shells and the firmness of the matrix we have 

 been unable to satisfactorily identify any of the species. We 

 feel no hesitation, however, in saying that the general aspect 

 of the shell fragments is strikingly similar to those found in 

 the till and that they quite certainly embrace Venus, Cyclo- 

 cardia, Scapharca, Watica and other species of the till. But 

 the chief facts telling against the Tertiary age of these speci- 

 mens are the comminution of the shells and the similarity of 

 the calcareo-argillaceous matrix to the till, and especially to the 

 cemented or concretionary masses of till. In view of all these 

 considerations, we believe that these fossiliferous erratics, 

 which agree, in color, with the gray rather than the buff till, 

 should be correlated with the cemented portions of the till 

 previously described. Among the various forms which the 

 cemented till assumes, the most important distinction is be- 

 tween the globular, distinctly concretionary and uneroded 

 forms and the worn and glaciated forms, the two specimens 

 from the west end of Peddock's Island belonging to the second 

 class. The uneroded globular forms are clearly of postglacial 

 origin and may be regarded as still forming ; but that the 

 eroded forms, including the glaciated masses form Peddock's 

 Island, are not truly preglacial is indicated, as already noted, 

 by the comminution of the shells and the decidedly till-like 

 character of the matrix. The only alternative, therefore, is to 

 regard them as, in some sense, interglacial. When the ice- 

 sheet first invaded this area, the shells on the bottom of Boston 

 Harbor were reduced to fragments and became a part of the 



*Proc. Boston Soc. Nat. Hist., vol. xx, pp. 136-140. 



