of Shells in the Till near Boston. 367 



some of these also range downward to considerable depths. 

 Three, of which two are doubtfully determined, are probably 

 restricted to comparatively deep water ; but even these are 

 often cast ashore in severe storms. Considering the outlines 

 of our eastern coast and the direction of the motion of the 

 ice-sheet, it seems probable that these fossils were living along 

 the shore and in the shallow edge of the sea on the area be- 

 tween the mouths of the Charles and Saugus rivers. In that 

 interglacial epoch the drumlins of this district had not been 

 accumulated, and the greater part of Chelsea, Revere, and 

 Winthrop, formed of these and other deposits of glacial and 

 modified drift, may then have been sea of similar depth with 

 the present harbor of Boston or the part of Massachusetts bay 

 between Winthrop and ISTahant. From this tract the south- 

 easterly moving ice-sheet, plowing up the marine beds and 

 their inclosed shells, with those then tenanting the sea, carried 

 them forward to form a portion of the till of the drumlins. 

 That the sea-bottom from which these shells were derived had 

 been shallow is evident from the predominance of the round 

 clam, which, according to Professor Yerrill, is seldom found 

 in any abundance below five fathoms. 



Glacially transported shells and fragments of shells have 

 been previously observed in till at Brooklyn, ]S\ Y., where E. 

 Desor and W. C. Redfield gathered fragments of the round 

 and long clams, oyster, and other species, " imbedded in a 

 reddish loam intermixed with pebbles and bowlders, many of 

 which are distinctly scratched ;"* and in till, or at least de- 

 posits of clay enclosing numerous stones and bowlders, on the 

 lower part of the St. Lawrence river, from the vicinity of 

 Quebec northeastward more than a hundred miles, chiefly on 

 the southeastern shore, to opposite the mouth of the Saguenay.f 

 But the descriptions of these beds containing shells and bowl- 

 ders on the St. Lawrence indicates that they were mostly, if 

 not altogether, deposited by water with floating ice during the 

 recession of the ice-sheet, while these marine shells lived where 

 they are now found, being thus comparable with the fossilifer- 

 ous, bowlder-bearing brick-clay of Paisley, Scotland.:}: In the 

 modified drift forming Cape Cod, derived from the melting 

 ice-sheet in which it had been contained, I collected ten years 

 ago fragments representing sixteen species of shells, all now 

 living, eight of which appear also in the foregoing list.§ 



* Bulletin de la Societe Geologique de France, second series, vol. v, 1847, pp. 

 89, 90 ; Quart. Journ. Geol. Soc, vol. v, p. 343 ; Am. Jour. Sci., II, vol. xiv, 1852, 

 p. 51. 



f J. W. Dawson's Notes on the Post-pliocene Geology of Canada, 1872, pp. 7, 

 45, and 50-53. 



\ T. F. Jamiesonin Quart. Journ. Geol. Soc, vol. xxi, 1865, pp. 175-177. 



§ American Naturalist, vol. xiii, 1879, p. 560. 



