Fossil Shells in the Drumlins of the Boston Basin. 495 



ground moraine or till. During a recession of the ice-sheet, 

 the till was locally cemented or lithified by carbonate of litne 

 derived from the shells through the solvent action of meteoric 

 waters, the still undissolved shells thus becoming, in some 

 cases, completely enclosed in a firm matrix. During the 

 second advance of the ice-sheet the shells suffered further 

 comminution, and the cemented or lithified portions of the 

 till were more or less broken up and abraded or glaciated. 

 The arenaceous character of the till must favor the solution 

 and segregation of the carbonate of lime of the shells; and it 

 does not appear probable that the very limited amount of 

 lithified till antedating the close of the ice age of which we 

 have evidence required for its formation a prolonged recession 

 of the ice-sheet, or a period amounting to a true interglacial 

 epoch. Our studies may be regarded as lending some support 

 to the duality of the ice age ; but it does not appear that 

 during the vigorous discussion of this question now in progress 

 any agreement has been reached as to how extensive or pro- 

 longed the recession of the ice-sheet must be to constitute an 

 interglacial epoch in any given latitude. Important and 

 repeated recessions of the ice-sheet are conceded by all gla- 

 cialists, and the glaciation must have been intermittent in 

 lower latitudes while continuous in higher latitudes. Thus 

 the ice-cap which now covers a large part of Greenland is 

 probably a remnant of the great sheet whose southern margin 

 was a few thousands of years ago in the vicinity of New York 

 and Cincinnati ; and the age in which we live may eventually 

 prove to be interglacial. It does not appear to us, however, 

 that the facts presented in this paper indicate more than a 

 brief or temporary recession of the ice-sheet from Boston 

 Harbor ; and certainly we have no evidence that preglacial 

 conditions were reestablished here or that any of the shells 

 now found in the till date from an interglacial period. 



A nearly complete series of the drift shells, representing 

 all the known sections except the wells, and including the 

 glaciated rock fragments from the west end of Peddock's 

 Island, has been added to the collection illustrating the geology 

 of the Boston Basin in the Museum of the Boston Society of 

 Natural History. Mr. W. H. Dall of the XL S. Geological 

 Survey, and Professor A. E. Verrill, of New Haven, have 

 kindly identified some of the more doubtful species ; but we 

 feel that the drift fauna of the Boston Basin is still very 

 incomplete, and especially that we have missed several species 

 on account of the hopelessly fragmentary and abraded char- 

 acter of the material. In the collection of the shells we have 

 received important assistance from Mr. T. A. Watson, and 

 from Mr. H. D. Card, Mr. G. W. Stose, Miss Elvira Wood, 



