170 MAN AND THE GLACIAL PERIOD. 



ders. But if this hypothesis were accepted, it would be 

 necessary to invest the flotation of ice with a constancy of 

 direction entirely at variance with observed facts, for the 

 phenomena of terminal curvature is shown" with perfect 

 persistence of direction wherever the boulder-clay rests 

 upon the rock ; and, further, there is the highly signifi- 

 cant fact, that neither the sands and gravels nor the rock 

 upon which they rest show any signs of disturbance or 

 contortion, such as must have been produced if floating 

 ice had been an operative agent. 



" The uplift of foreign rocks is equally significant ; and 

 when we take into account the great distances from which 

 they have been borne and the frequency with which such 

 an operation must have been repeated, the inadequacy be- 

 comes apparent of Darwin's ingenious suggestion, that it 

 might have been effected by a succession of uplifts by 

 shore-ice during a period of slow subsidence ; while the 

 character and abundance of the molluscan remains invest 

 with a species of irony the application of the term ' shell- 

 bed ' to the deposit. 



" I now turn to the alternative explanation (see ante, p. 

 145), viz., that the whole of the phenomena were produced 

 by a mass of land-ice which was forced in upon Moel Try- 

 f aen from the north or northwest, overpowering any Welsh 

 ice which obstructed its course. This view is in harmony 

 with the observations regarding the ' terminal curvature ' 

 of the slates, the occurrence of sharp angular chips of slate 

 in the boulder-clay, and the coincidence of direction of 

 these indications of movement with the carry of foreign 

 stones. The few shells and shell-crumbs in the sands and 

 gravels would, upon this hypothesis, be the infinitesimal 

 relics of huge shell-banks in the Irish Sea which were 

 destroyed by the glacier and in part incorporated in its 

 ground-moraine or involved in the ice itself. The sands 

 and gravels would represent the wash which would take 

 place wherever, by the occurrence of a ' nunatak ' or by 



