1873.] CAMPBELL — GLACIATION OP IRELAND. 223 



out to sea by later glacial action, still it would only have been 

 transferred a little further on ; and had any such drift been depo- 

 sited generally over the north of Europe, traces of it ought to be 

 found along the south and east margins of the Scandinavian drift. 

 He appealed to the vast scale of the changes of level to which this 

 part of the earth's crust had been subjected, and especially to the 

 shell-beds of Moel Tryfaen and Macclesfield, to prove that changes 

 of level of at least 1400 feet had taken place since the Glacial 

 period, and inquired whether elevation on even that scale would not 

 recall glacial conditions over a large part of the area under notice. 

 He again proposed to the Society a question which he had asked 

 several years before : — what was the maximum pressure which ice 

 would bear without becoming water or being crushed ? and whether 

 the consideration of this and the other conditions involved would 

 lead us to assign a limit to the possible lateral extension and vertical 

 thickness of an ice-sheet moving on a plain or uphill which would 

 affect such speculations as that under discussion. 



Mr. Mallet said, in reply to a question from the President, that 

 experimental data were as yet wanting to enable a precise determi- 

 nation of the limit of distance to which an extraneous force could 

 be transmitted through a prismatic mass of ice. The fundamental 

 point of such an inquiry was — what is the modulus of cohesion of 

 the most solid ice? A few experiments had been made, which 

 showed that the height of this modulus could not exceed a few hun- 

 dred feet. Let it be assumed, however, that it was as great as 

 5000 feet, or a mile. It was then obvious that a mass of ice, no 

 matter how deep or wide, lying in a straight, smooth, frictionless 

 valley, could not be pushed along by any extraneous force in the 

 line of the valley through a distance of more than a single mile ; 

 for at that point the ice itself must crush, and the direct force 

 cease to be transmitted further. This, of course, was far from being 

 the whole of the question of the transmission of force through ice ; 

 for when and wherever crushing took place, a certain portion (though 

 a small one) of the direct pressure was transmitted laterally by the 

 crushed fragments, especially if mixed with water, simulating the 

 quaquaversal properties of an imperfect liquid. For this to take 

 place, however, in the direction of the length of the ice-filled valley 

 supposed, the ice must be considerably more than a mile in vertical 

 depth. These simple considerations were alone sufficient, he thought, 

 to overthrow the notions which had been advanced by Prof. Eamsay 

 and others as to the excavation of great valleys by the pushing of 

 large masses of ice in the direction of their length. Mr. Mallet had 

 had ample opportunities for several years as an engineer of observing 

 the surface-features of Ireland, and indorsed the fact that almost 

 everywhere the surfaces of the rocky skeleton, when hard enough 

 or freshly uncovered, were found to be scratched, as were most 

 of the boulders in the detritus above. But were these scratches 

 necessarily evidence of the action of ice at all ? he thought not. 

 The general trend of the valley- and hill-ranges of Ireland was, as 

 stated in the paper, N.E. and S.W. : but the production of those 



