September 16, 1910] 



SCIENCE 



361 



told approximately how thick the northern 

 ice must be to produce the requisite pres- 

 sure, 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 southward jour- 

 ney its thickness was less than 2,000 feet, 

 and we are not entitled, as I have endea- 

 vored to show, to pile up ice indefinitely 

 on either our British highlands or the ad- 

 jacent sea-bed. The same reason also for- 

 bids 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 Spitzbergen, 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 oppo- 

 site slope. Here the valley was narrow, 

 and the glacier had descended from an in- 

 land ice-reservoir, much of which was at 

 least 2.800 feet above the sea, and rose occa- 

 sionally more than a thousand feet higher.*** 



But other difficulties are far more grave. 

 The thickness of the chalky boulder clay 

 alone, as has been stated, not unfrequently 

 exceeds 100 feet, and, though often much 

 less, may have been reduced by denudation. 

 This is an enormous amount to have been 

 tran,sported and distributed by floating ice. 

 The materials also are not much more 

 easily accounted for by this than by the 



" Quart. Journ. Geol. Soc, LIV., 1898, p. 205. 

 Earlier observations of some upthrust of materials 

 by a glacier are noted on p. 219. 



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 

 laud had subsided 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, how- 

 ever, would seriously diminish the area of 

 exposed chalk in Lincolnshire and York- 

 shire, 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 detritu§ 

 from the Kimeridge and Oxford clays. 

 But a large part of their outcrop 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 

 vertical direction. The instances, also, of 

 the transportation of bf^uiders and smaller 

 stones to higher levels, sometimes large in 

 amount, as in the transference of "brock- 

 ram" from ouccrops near the bed of the 

 Eden valley to the level of Stainmoor Gap, 

 seem to be too numerous to be readily ex- 

 plained by the uplifting action of shore-ice 

 in a subsiding area. • Such a process is 

 possible, but we should anticipate it would 

 be rather exceptional. 



Submergence also readily accounts for 

 the above-named sands and gravels, but not 

 quite so easily for their occurrence at such 

 very different levels. On the eastern side 

 of England gravelly sands may be found 

 beneath the chalky boulder clay from well 

 . below sea-level to three or four hundred 

 feet above it. Again, since, on the sub- 

 mergence hypothesis, the lower boulder 

 claj' about the estuaries of the Dee and the 

 Mersey must represent a deposit from pied- 

 mont ice in a shallow sea, the mid-glacial 

 sand (sometimes not very clearly marked 

 in this part) ought not to be more than 



