iS93. SHELLY-SANDS AND GRAVELS. 429 



coming, but unless some more solid food is vouchsafed us, I fear many 

 will die unconvinced. 



With so much theory and more hypotheses we might reasonably 

 expect the theoretical physics of the ice-sheet to be quantitatively 

 worked out. But have they been ? Has anyone yet, in proof of the 

 view that the shell-beds of Tryfaen have been pushed up from 1,200 

 to 1,400 feet above the level of the sea by a glacier advancing from 

 the north over the Irish Sea bottom, calculated the thickness of the 

 ice required at the "area of greatest precipitation" to overcome the 

 glacier descending from Snowdonia. As concrete examples cannot 

 be quoted of such work being performed by land-ice either in 

 Greenland or elsewhere at the present moment, it becomes important 

 for the support of the hypothesis that its theoretical possibility should 

 be established. If this has been done the investigation has not come 

 under my notice, though I have been studying glacial geology wsll- 

 nigh a quarter of a century. 



Unknown agencies are very unsatisfactory things to deal with 

 scientifically. It is as easy to deny as to affirm, but as the affirmations 

 seem to bottom upon the alleged impossibility of any other agent 

 than an ice-sheet being able to produce all the complicated phenomena 

 of the Drift, it will be the most reasonable procedure to examine also 

 the validity and force of the various objections which have been urged 

 against the older theory of submergence, and which I have in the 

 preceding pages attempted to summarise. 



We will deal with the objections to the submergence theory 

 seriatim. 



I. — Boulders, shingle, and gravel are found above the level of the rocks from 

 which they were derived. 



This by some is considered conclusive proof that the moving 

 agent must have been land-ice. Here, again, we want satisfactory 

 examples of an analogous kind drawn from existing glaciers. It 

 seems to me much more probable that a glacier such as the Irish-Sea 

 glacier is supposed to have been, would shear in its own substance, 

 than that it would push sea-bottom before it uphill. The same 

 reasoning applies to the supposititious case of the sea-bottom getting 

 frozen into the sole of the glacier, for if this were to take place the 

 obstruction to the movement of the ice over the sea-bottom would be 

 increased. We may also ask where the shearing took place if not in 

 the ice itself ? It is difficult to conceive a sea-bottom frozen solid 

 shearing under such stresses. Alternate thawing and freezing of the 

 bottom may be suggested to meet the difficulty, but this would render 

 the carriage of shells in the sole of the glacier without breaking more 

 hard to conceive than ever. But may it not be justly asked whether 

 resort to all these ingenious suppositions is not a sign of weakness ? 



Let us now consider what effect submergence would have. A 

 study of any of our shores will yield ample proof that the sea works 



