Fossil Shells in the Drumlins of the Boston Basin. 493 



answer to the view, which might otherwise appear tenable, that 

 we have in these segregations remnants of a preglacial lithified 

 matrix of the shells. In fact, the form and mode of occur- 

 rence are, in many cases, strongly suggestive of such an 

 explanation ; and it is noteworthy that, although this adherent 

 material is always of the same general nature and composition 

 as the enclosing normal drift, it is also often much darker 

 colored, varying from the usual gray tint of the till through 

 dark brown to nearly black. Furthermore, the color is, in the 

 thicker segregations, often noticeably darker in the portions 

 lying nearest or in close contact with the shells. The blackness 

 largely disappears on calcination, indicating that it is due to 

 carbonaceous matter, the source of which, it appears probable, 

 may be in the shell itself. This is one fact pointing to the 

 conclusion that the shells, which are now white, opaque, more 

 or less chalky, and as truly fossilized as many Tertiary shells, 

 were recent in the fullest sense of the word at the time they 

 were incorporated with the drift through the action of the 

 ice-sheet; and it is in harmony with the fact that as yet no 

 extinct species have been found in the drift. 



Although the lithified matrix adhering to a small proportion 

 of the drift shells must, in view of the foregoing observations, 

 be regarded as of postglacial origin ; and the shells themselves 

 are probably immediately preglacial inhabitants of Boston 

 Harbor; one of the sections has afforded shells in an appar- 

 ently preglacial matrix. While searching for shells on the 

 west end of Peddock's Island, we found a rounded and dis- 

 tinctly glaciated fragment of a highly fossiliferous rock. It is 

 a gray, seemingly unstratified, argillaceous and distinctly cal- 

 careous mass, 6-J" long and 2" in greatest diameter, enclosing 

 grains and more or less angular pebbles up to an inch in length 

 of slate and other rocks, such as are common in the local drift. 

 In fact, it looks exceedingly like a lithified portion of the 

 drift, except that it is literally crowded with comminuted 

 shells — the shell fragments ranging in size from mere specks 

 to nearly an inch in diameter. When first found, the specimen 

 was completely imbedded in the solid and entirely undisturbed 

 gray till, one end projecting slightly from the side of a rain- 

 washed gully about 15 feet above the beach and 25 feet below 

 the top of the cliff. In digging it out, it was broken in two, 

 and the aspect of the section is similar to that of the surface. 

 That it is a true glacial erratic and reached the position where 

 it was found through the agency of the ice-sheet we can not 

 doubt. On a later visit to the section in company with Mr. 

 T. A. Watson, he found another fragment of nearly the same 

 size and shape and of very similar character save that the 

 enclosed pebbles are larger and the shell fragments fewer. 



