280 NEOZOIC TIME. [CHAP. XIII. 



the letting in of sea-water beneath it, and this would take 

 place at a considerable depth below the surface of the sea, 

 all wave-action being completely prevented by the mass of 

 ice above. The inland extension of the Boulder-clay which 

 was thus allowed to form beneath the ice and the land 

 would only be limited by the progress of the submergence 

 and the variations in the thickness of the ice. Dr. J. Young, 

 the Eev. Boog Watson, and others have felt the same diffi- 

 culty in understanding the formation of Boulder-clay 

 underneath land-ice, and have suggested an explanation 

 which is substantially the same as that above offered. 1 



If, for instance, this subsidence took place before the 

 climax of the Glacial period, and the ice continued to in- 

 crease in thickness, a pause in the downward movement 

 would enable it to exercise pressure and erosion on the 

 material which had accumulated beneath it. Nay, it is 

 conceivable that the partial flotation of the Scandinavian 

 ice- sheet might enable this to cross the great plain of the 

 North Sea and to impinge upon the edge of the Scottish 

 ice- sheet, as Dr. Croll has suggested that it did (without 

 any such assistance). The increased pressure so caused, 

 and the different direction given to the movement of the 

 ice-masses, would then explain the existence of two distinct 

 lines of glaciation in Aberdeenshire, a point to which Pro- 

 fessor J. Geikie gives far too little attention. Probably all 

 the phenomena of the Glacial deposits of Scotland could be 

 explained more satisfactorily on the hypothesis of one sub- 

 sidence, and a subsequent upheaval during the prevalence 

 of glacial conditions, than on that proposed by the author 

 of " The Great Ice Age," for he admits a submergence in 

 late Glacial times which depressed the country at least 500 

 feet, and probably from the evidence in England and 

 Wales to a much greater extent. 



1 "Geol. Mag.," 1878, p. 162, and "Trans. Roy. Sue. Edln.," vol. 

 .xxiii. p. 539. 



