president's address. 31 



elevation to the height of Moel Tryfaen,' yet there is a common-sense 

 limitation, even to a destructive sorites. The argument, in fact, is 

 more specious than valid, till we are told approximately how thick 

 the northern ice must be to produce the requisite pressure, and whether 

 such an accumulation would be possible. The advocates of land-ice 

 admit that, before it had covered more than a few leagues on its south- 

 ward journey its thickness was less than 2,000 feet, and we are not 

 entitled, as I have endeavoured to show, to pile up ice indefinitely 

 on either our British highlands or the adjacent sea-bed. The same 

 reason also forbids us largely to augment the thickness of the latter 

 by the snowfall on its surface, as happens to the Antarctic barrier-ice. 

 Even if the thickness of the ice-cap over the Dumfries and Kirkcudbright 

 hills had been about 2,500 feet, that, with every allowance for viscosity, 

 would hardly give us a head sufficient to force a layer of ice from the 

 level of the sea-bed to a height of nearly 1,400 feet above it and at 

 a distance of more than 100 miles. 



Neither can we obtain much support from the instance in Spits- 

 bergen, described by Professors Garwood and Gregory, where the Ivory 

 Glacier, after crossing the bed of a valley, had transported marine 

 shells and drift from the floor (little above sea-level) to a height of about 

 400 feet on the opposite slope. Here the valley was narrow, and the 

 glacier had descended from an inland ice-reservoir, much of which was 

 at least 2,800 feet above the sea, and rose occasionally more than a 

 thousand feet higher. 1 



But other difficulties are far more grave. The thickness of the 

 chalky boulder clay alone, as has been stated, not infrequently exceeds 

 100 feet, and, though often much less, may have been reduced by denuda- 

 tion. This is an enormous amount to have been transported and distributed 

 by floating ice. The materials also are not much more easily accounted 

 for by this than by the other hypothesis. A continuous supply of well- 

 worn chalk pebbles might indeed be kept up from a gradually rising 

 or sinking beach, but it is difficult to see how, until the land had sub- 

 sided for at least 200 feet, the chalky boulder clay could be deposited 

 in some of the East Anglian valleys or on the Leicestershire hills. That 

 depression, however, would seriously diminish the area of exposed 

 chalk in Lincolnshire and Yorkshire, and the double of it would almost 

 drown that rock. Again, the East Anglian boulder clay, as we have 

 said, frequently abounds in fragments and finer detritus from the 

 Kimmeridge and Oxford clays. But a large part of their outfcrop would 

 disappear before the former submergence was completed. Yet the 

 materials of the boulder clay, though changing as it is traced across 

 the country, more especially from east to west, seem to vary little in a 



1 Quart. Journ. Oeol. Soc., liv. (1898), p. 205. Earlier observations of some 

 upthrust of materials by a glacier are noted on p. 219. 



