5-i 0. P. Hay — Deposit of the Glacial Drift 



but that they were carried along in and under the glacier, and 

 finally deposited as a terminal moraine, which, by the slow 

 retreat of the glacier to the north, at length became a continu- 

 ous sheet. 



That wind, blowing for long ages over the glacier from the 

 region lying to the south and west, may have deposited on it 

 more or less dust; and that superficial melting may have 

 brought some coarser materials to the surface, are suppositions 

 that are entirely probable. The " shearing action " of the moving 

 glacier may also, as H. Carville Lewis has recently shown us, 

 have brought a certain amount of sand, gravel and bowldefs 

 from the lower portions of the glacier to the surface. But all 

 these agencies combined probably did not result in producing 

 any considerable superficial deposit. We cannot calculate on 

 the dust with any confidence. Moreover, when once a thin 

 covering of debris, from whatever source, had accumulated over 

 the ice, it would protect this from further melting, and thus cut 

 off one important source of addition. The intestinal motion of 

 the glacier, too, would in all probability contribute little mate- 

 rials, on account of the sluggish movement of the glacial mass 

 as a whole. As regards Capt. Dall's fossil ice-sheet, it cannot, in 

 all probability, have any motion now, or have had any since 

 the days of the mammoth; otherwise all those fossil bones 

 would long ago have been shot out into the Arctic Ocean. 



Against the theory that the bowlder-clay accumulated under 

 the glacial sheet, Prof. Newberry makes protest on the ground 

 that the underlying rocks show that the ice was in close con- 

 tact with them, being separated from them by, at most, a thin 

 stratum of sand and gravel. He says: 



" It did not accumulate beneath the glacier, because the rock 

 surface on which it rests, is planed down, grooved and carved, as 

 it could only be when the ice fitted closely to it ; and since two 

 solid bodies cannot occupy the same space at the same time, the 

 clay could only have accumulated in the places where it is found, 

 after, or as, they were abandoned by the ice." — Geol. But. Ohio, 

 vol. iii, 34." 



Again (Geol. Sur. Ohio, vol. ii, 29) : 



" That the bowlder clay was not deposited beneath the glacier, 

 as sometimes stated, is apparent from the fact that it covers the 

 glaciated surface on which the ice rested, in a sheet sometimes a 

 hundred feet in thickness. It must, therefore, have accumulated 

 at the margin of the glacier.'''' 



To these arguments it seems to me sufficient to reply that it 

 is not necessary for us to suppose that, in order to produce all 

 the observed effects on the underlying rocks, the ice-sheet con- 

 tinued to move over ffnd in immediate contact with them dur- 



