Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/466

 to be solved with the means at present at our disposal ; my own belief, after many years of observation, is, that subterranean forces (whether of upheaval or depression, by folding and fracturing the strata), sea, air, frost, rain, streams, and glaciers have all played their part in this task of mountain-sculpture. Perhaps it may be possible some day to approximate to the work done by the first two ; but the more I examine the Alps the more convinced I become that the next four have been the chief agents in producing the present configuration of their valleys, and that the effect of the last in the list has been superficial. Earth-forces, like Titanic hands, have moulded and broken, the sea has planed, the heat and cold have split like wedges, rain and rivers have chiseled and sawn, while the glacier rasp has rounded edges and filed down protuberances, but has done comparatively little among the harder rocks to deepen or excavate.

[P.S. Since the above 'paper was read, Mr. E. Whymper's ' Scrambles amongst the Alps ' has been published, in which are some very important remarks upon the question of glacier-erosion. Professor Tyndall, in his recent work ' Hours of Exercise in the Alps,' appears to speak rather less confidently with regard to his theory than in the paper quoted above.]

Discussion.

Mr. Whitaker suggested an analogy between the cirques and the combes in our own limestone countries.

Mr. Geikie regarded the cirques as analogous to the cwms of Wales and the corries of Scotland. They were not, however, confined to limestone districts, but occurred also in gneiss and granite rocks. He thought that the shape was much influenced by the bedding and jointing of the rocks, as there was an evident connexion between these and the shape of the cwms. He could not, however, see his way to account for the vertical cliffs surrounding the cirques. The Rev. T. Gr. Bonnet, in reply, observed that though cirques were not confined to limestones, the finest instances occurred in such rocks. When cirques occurred in crystalline rocks, the talus was usually much larger than in limestone.