MifiSlDENTIAL ADDRESS. 543 



Coa.st-sectioiis were concerned, I mention tbis experience in order to show that my 

 present scepticism respecting the Helvetian luterglacial Epoch is based, not upon 

 any preconceived objection to the idea, but upon the failure of the hypothesis 

 when I have put it to the test in tbis and other districts ; and I find also that 

 my experience in this particular runs parallel with that of many other investigators 

 of the so-called ' middle glacial ' deposits of England. 



Mariiie Detritus in Glacial Gravels. — From certain characters of the moundy 

 gravels on Flamborough Head and in Holderness, such as their rudely linear 

 arrangement, their indifference to the contours, and their relation to the middle or 

 Purple boulder-clays, it appears most probable that they represent the material 

 deposited along the margin of the ice-sheet by the surface-waters flowing from it 

 and from the adjacent land.^ From the occurrence of more or less fragmentary 

 marine shells in them, the gravels were, howeTer, originally supposed to be of 

 marine origin, and this view is still upheld by some geologists. It is the same 

 question in which so many of the so-called ' middle glacial ' sands and gravels 

 of the British Islands are involved, and upon which there has been so much dis- 

 cussion. If it be permissible for me to reiterate the well-known argument by 

 which the presence of marine shells in gravels of glacial origin is explained, it 

 may be outlined as follows. 



Since the basins around our islands are known to have been occupied by the 

 sea at the beginning of the Glacial Period, and since these basins were afterwards 

 filled by ice-lobes, which, as we have seen, moved outward in many places upon 

 the land, dragging with them much of the material of the old sea-floor, it is 

 inevitable that a certain amount of marine detritus will occur in the deposits 

 formed by the ice or derived from its melting. Just as we find shells, and some- 

 times even transported masses of marine deposits, intact in the Basement Clay, so 

 we find marine relics likewise, though usually more scattered and less perfect, in 

 the gravels derived from the same ice-sheet. This deduction is consistent with 

 our knowledge of existing glaciers and ice-sheets ; thus. Sir Archibald Qeikie has 

 recorded the presence of sea-shells in the moraine of a Norwegian glacier*; 

 Professors E. J. Garwood and J. W. Gregory have found an excellent illustration 

 of the same phenomenon in one of the Spitzbergen glaciers ^ ; and Professor R. D. 

 Salisbury, in describing the characteristic upturning of the layers of ice at the end 

 of one of the glacial lobes which descends into a shallow bay in North Greenland, 

 gives the following instructive note on the conditions which ho observed : ' Here 

 the upturning of the layers brought up shells from the bottom of the bay, and left 

 them in marginal belts where the upturned layers outcropped. These shells were 

 mingled with other sorts of debris. In one case their quantity could have been 

 measured by some such unit as the wagon-load.' ^ 



In our islands, as Professor P. F. Kendall has clearly shown in discussing the 

 drifts of Western England,'^ it is only where the ice-lobes have passed over 

 portions of the pre-existing sea-floors that we find marine remains in the drift 

 deposits ; while in other places, at the same or lower elevations, where there is 

 proof that the ice-flow was from the land, such remains are invariably absent. 



The occurrence of these shells in a few places at high elevations, all explicable 

 by consideration of the geographical circumstances, gave rise to the idea of a 

 great mid-glacial submergence, and upon this idea the hypothesis of a mild 

 mterglacial epoch has mainly hinged. In Professor Geikie's latest scheme this 

 supposed submergence is, indeed, reduced to moderate limits, but it is still the 

 essential factor in the argument. 



' Lamplugh, ' Drifts of Flamborough Head.' Quart. Journ. Gcol. Soo., vol. xlvii. 

 (1891), pp. 384-431. 



* Geological Sketches at Home and A hroad (London, 18S2), pp. 145-146. 



' ' Contributions to the Glacial Geology of Spitzbergen.' Quart. Journ. Geol. SoC , 

 vol. liv. (1808), p. 210. 



* 'Glacial Geology of New Jersey.' Bcj). Geol. Survey of New Jersey, vol. v, 

 (1902), p. 81. (The quoted italics arc in the original.) 



' In the late Professor H, Carvill Lewis's Glacial Geology of Great Britain 

 <and Ireland (London, 1894), Appendix A, pp. 425-431, 



