490 Crosby and Ballard — Distribution and Age of 



boulders" and 20 feet in "blue clay" (blue till). In the blue 

 c\&y he found a shell. Similar facts in many other wells along 

 the South Shore have, doubtless, escaped intelligent observation 

 and record. 



The collections made by Mr. Herman and ourselves show- 

 that several localities, notably Grover's Cliff and the east and 

 west ends of Peddock's Island, rival Great Head in the abund- 

 ance and variety of the fossils. The nine most prolific localities 

 are embraced in the accompanying table. The remaining 

 localities have afforded the species indicated below : West end 

 of Long Island, 33, 38, 43, 55 ; Long Island Head, 2, 38, 43, 

 47, 55 ; LovelPs Island and Nut Island, 38, 43, 55 ; George's 

 Island, 11, 19, 38, 43, 55; Quincy Great Hill, 33, 38, 43,^44, 

 55 ; Princess Head, 33, 38, 55 ; Peddock's Island, 2d drumlin, 

 33, 38, 43, 53, 55; Peddock's Island, 3d drumlin, 38, 55; 

 Strawberry Hill, 38, 43, 45, 47, 55. 



Previous writers, including Upham and Dodge, have cited 

 the occurrence of similar fossils in the drift, either unmodified 

 or modified, at other points along this Coast from Brooklyn, 

 INT. Y., to the mouth of the Saguenay, and have noted that 

 not a single certainly extinct species has been found ; and it 

 remains now to add that our studies in the Boston Basin have 

 not changed this important generalization. Upham has also 

 considered the climatic changes indicated by the general facies 

 of our drift fauna. Southern forms prevail, including the 

 round clam ( Venus mereenaria) which is enormously more 

 abundant than all the other forms taken together, although 

 now of rare occurrence north of Cape Cod. A few forms, 

 however, have a northward range far beyond this latitude ; 

 and, as LTpham states, the intermingling of characteristic 

 southern and northern forms in this assemblage of fossils from 

 the till seems to be readily accounted for by the gradual refrig- 

 eration of the climate which culminated in the formation of 

 the ice-sheet. The distinctly postglacial fossils dredged in the 

 vicinity of Boston, of which Upham has reported fifty-one 

 species,* are also mainly southern forms. The evidence thus 

 appears to be fairly conclusive that in preglacial and again in 

 immediately postglacial times the climate in this latitude was 

 milder than at present. 



The solution of the shells by meteoric waters is plainly indi- 

 cated by the facts that they are generally wanting in the super- 

 ficial, oxidized zone, and that such as are found in this zone 

 are commonly more fragile and have an etched or half-dissolved 

 aspect. Where the shells have wholly disappeared, their former 

 presence may in some cases be safely inferred from the lumps 

 and concretionary masses of till which are found chiefly in the 



*Proc. Boston Soc. Nat. Hist, vol. xxy, pp. 305-316. 



