Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/281

 out to sea by later glacial action, still it would only have been transferred a little further on ; and had any such drift been deposited 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 determination 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 hundred 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. Ramsay 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