Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/280

 Discussion.

Prof. Ramsay agreed in the main with the views of the author, and with the opinion of Agassiz as to the great extension of cold at a certain period both in the northern and southern hemispheres, though he could not carry the theory quite so far as to leave merely a narrow equatorial belt unaffected by ice. He had, however, never seen any mountain-region in the northern hemisphere on which there were no traces of glacial action. As to Ireland, he knew of no portion of its surface which had not been glaciated, and the great striations actually extended, as they do in Scotland, right over the watersheds, and were evidently unconnected with any merely local features. At the same time, even where the general current of the upper portion of the ice was constant, yet there might have been and probably were, undercurrents, the course of which was determined by the form of the country traversed by the ice. He was not certain that the present features, resulting from denudation, were rightly attributed to glacial agency alone, as other causes appear to have been at work. He instanced cases of enormous denudation at early geological periods which it was difficult to trace to any glacial action. He thought that during the Glacial period the main features of the country were to a great extent modified by the great ice-sheet which capped it, without its having had so extensive an effect as that sometimes attributed to it. Still sufficient changes had been made on the surface to cause the rivers which were resuscitated after the close of the Glacial period to take new courses. The existence of old river-valleys, partially obliterated by glacial debris, proved to his mind that hills and valleys, and a diversified surface, existed previously to the Glacial period to almost as marked an extent as they do at the present day.

Sir Henry James observed that, having at one time been in charge of the Geological Survey of Ireland, he could indorse the views of the author as to the glaciation of that country, though he agreed with Prof. Ramsay as to the probability of valleys in Ireland and in Scotland having existed before the Glacial period and guided the flow of the ice. These no doubt were intimately connected with the varying hardness of the rocks.

Mr. T. M'K. Hughes remarked that there was no necessity for a polar ice-cap from any secularly recurring cold — seeing that the difference of temperature, known as a matter of observation to be due to geographical causes, was so very much greater than any variation of temperature which had been shown to be possible owing to astronomical combinations, that the astronomical causes might be neglected. He showed that the glaciation which was relied on as a proof of the passage of large masses of ice from the north, did not appear to come from the north pole, but from local centres, such as Scandinavia, Scotland, and the mountains of Wales and the N.W. of England, from which the ice moved in all directions. He pointed out that the contents of the drift appeared to be ignored ; for although in the British sles the polar drift might have been pushed