SIXTH REPORT OF TITE DIRECTOR I909 I75 



We may first note the exceeding thinness of some of the re- 

 tained siHcious laminae which now project in places as far 

 as three or more centimeters beyond the suirfaces of its cal- 

 careous layers. The edges of some of these sheetlike exten- 

 sions are so fragile that they were broken in several places dur- 

 ing the careful handling which the specimen received before 

 being photographed. The edge of one recent break may be 

 seen in the middle thin sheet of plate 15 and is shown in plate 

 16, figure 2. That these and other fragile edges had been pre- 

 served shows conclusively not only that the specimen had 

 not been rolled by wave action for ages but also that wave 

 action had not been able to roll or throw smaller pebbles against 

 these edges since they were formed. This witness against the 

 power of wave motion in this locality to use small pebbles as 

 tools of erosion would lead to the conclusion that this speci- 

 men, when found, w^as resting on the same base on which it 

 rested at the time of cessation of motion of the last ice sheet. 

 This may be true, but there are tv^o different times in which this 

 pebble may have been moved and it v^ill be well to note them. 

 The bare areas mentioned may have been first cleaned off by sub- 

 glacial drainage moving under pressure and the pebble may then 

 have been rem.oved from the till to find permanent lodgment in 

 the more sheltered position over the fissure. If not moved 

 before the ice front passed to the north it could not have been 

 moved until the v^^aters of Lake Champlain had arrived at ap- 

 proximately their present level. While Cystid point is some- 

 what sheltered from storm action yet it is no easy matter to 

 land here in a rowboat during strong north or south winds. 

 Waves of exceptional force may have swept the till from the 

 bare areas mentioned and moved the pebble to the deeper and 

 better protected position in w^hich it was found. Whether left 

 in this position by the ice sheet or moved thither during either 

 of the periods mentioned above, it could have been subjected 

 to solution or abrasion only since the deposits of the Hoche- 

 lagan sea were swept away. If for long ages wave motion had 

 been unable to use small pebbles against its thin edges it must 

 have been unable even once to turn this specimen over during 

 that same long period and the erosive features we are about to 

 notice will yield strong evidence in support of this conclusion. 



One face of the supposedly dissolved portions of this specimen 

 is unmistakably marked by typical and confluent dentpits with 



