of Shells in the Till near Boston. 363 



resulting from the enormous pressure of the overlying ice, and 

 also indicates that the accumulation of the drumlins took place 

 by gradual addition of till over their surface. Only a thin 

 layer of englacial till, with its numerous large bowlders, con- 

 tained within the ice-sheet and allowed to fall loosely from it 

 during its final melting, is observable upon these drumlins, 

 its probable thickness in this vicinity being not usually more 

 than one or two feet. 



Plentiful fragments of shells, up to one or two inches in 

 length and rarely of larger size, are imbedded, like the small 

 fragments of rock, in the dark lower portion of the till. They 

 were found most abundant in Grover's cliff, "Great Head, Ped- 

 dock's island, and the northern cliff of Hull, one or several 

 shell fragments being usually seen on each square yard of the 

 exposed surface, so that hundreds may be gathered in an hour. 

 In all the localities a single species, the round clam or quohog, 

 Venus mercenaries L., makes up probably ninety -nine per 

 cent, of the specimens found ; but no entire valve of this shell 

 was obtained among the thousands of its fragments. The spe- 

 cies next in numbers is Cyclocardia oorealis Conrad, which, 

 like the foregoing, is thicker and stronger than most species 

 and therefore better fitted to resist the grinding action of the 

 ice. The smaller size of the latter has enabled some of its 

 specimens to escape almost unbroken and with only slight 

 abrasion of its margin. In no instance, however, have the two 

 valves of this or any other species been found united. Some 

 of the fragments show little wearing or none, their broken 

 edges being sharp and the markings of their surface perfectly 

 preserved ; but the majority are considerably worn, and pieces 

 perforated by burrows, like the dead shells cast up on a beach, 

 are frequently found. ISTo glacial striation has been detected 

 on any of these shell fragments, and indeed it is rarely observ- 

 able on pieces of stone of so small size. 



The cliff at the northeast end of Peddock's island, though 

 not showing more of the large fragments than the other locali- 

 ties specially mentioned for their abundance, yet far surpasses 

 these in its multitude of very small fragments and even minute 

 particles of shells, from a quarter and an eighth of an inch in 

 length down to the least speck visible to the eye. In one place, 

 by no means exceptional, near the base of this cliff, the number 

 of these particles and specks of shells, ground up in the process 

 of formation and deposition of the till, averaged not less than 

 forty to each square foot of the section. This locality, too, is 

 the only one where the shell fragments were observed in the 

 yellowish upper part of the till nearer to the original surface 

 than a depth of ten or fifteen feet. Here small fragments of 

 shells, an inch or less in length, were found in considerable 



