Sept. 14, 1876] 



NATURE 



437 



the ice-sheet. In the fourth place, all phenomena of glacia- 

 tion which are exhibited on the mountain ranges, including the 

 distribution of erratics, can be adequately accounted for by the 

 three conditions or forms of moving ice which have been above 

 enumerated, and all of which are now in actual operation on the 

 globe — namely, ice moving, not up, but down mountain slopes 

 by the force of gravitation, and ice floated by water and driven 

 by currents, as icebergs or as floes. In the fifth place, these 

 phenomena of glaciation are essentially different from those 

 which would result from the motion of a universal ice-sheet, 

 supposing it to have existed, and supposing it to have had the 

 (impossible) motion which has been ascribed to it. In the 

 sixth place, that, in particular, the mode in which erratics are 

 distributed and the peculiar position of perched blocks are de- 

 monstrative of the action, not of solid, but of floating ice ; whilst 

 the surfaces of rock which have escaped glaciation on one side 

 and retain the deepest marks of it upon another side, are equally 

 demonstrative of exposure to moving ice, under conditions which 

 did not enable it to fit into the inequalities of surface over which 

 it passed. In the seventh place, the phenomena seem to me to 

 prove that some of the very heaviest work done by ice has been 

 towards the close of the glacial epoch, when the land was 

 emerging again from out of a glacial sea, and when all the cur- 

 rents of that sea, loaded with bergs and floes, were determined 

 entirely by the outlines of the rising land. In regard to the 

 much- disputed question of the glacial origin of lake-basins, ihe 

 conclusion to which I have come is one which to some extent 

 reconciles antagonistic views. I do not indeed believe that glaciers 

 can ever dig holes deep under the average slope of the surface 

 down which they move, but on the other hand they are the most 

 powerful of all abrading agents in deepening their own bed and 

 cutting away the rocky surfaces which lie beneath them. If valleys 

 thus deepened by the long work of glaciers and glacier streams are 

 afterwards submerged along with the whole country in which they 

 lay, and if that submergence is accompanied by partial and un- 

 equal rates of subsidence, they would inevitably : become hol- 

 lows into which the sea would enter, or in which fresh waters 

 would accumulate. In this sense and in this way it can hardly 

 admit of a doubt that those lakes, which are nothing but sub- 

 merged valleys, are due in part to glacial action, although the 

 other half of the causation on which they depend is to be sought 

 in the subterranean action of subsidence. 



In conclusion, I would observe that although the fact of a 

 great subsidence and a re-elevation of the land during the glacial 

 epoch has been generally admitted to be one of the facts of which 

 there is the clearest evidence, it is nevertheless a fact of which all 

 the conditions and all the consequences have been most imper- 

 fectly recognised. Without venturing to go so far back as to ima- 

 gine the process of subsidence and submergence, let us only think 

 for a moment of that movement of re-elevation which has cer- 

 tainly been one of the very latest of the great movements of 

 geological change. If it took place very gradually and very 

 slowly, it necessitated the supposition that every inch of our 

 mountain surfaces, up to at least 2,cxx) feet, has been in succes- 

 sion exposed to the conditions of a sea beach. Yet where are 

 the marks upon them of such conditions ? We may suppose 

 such marks to have been generally obliterated by later sub-aerial 

 denudation ; but against this is to be set the fact that the position 

 and distribution of perched blocks and other erratics deposited 

 by floating ice demonstrates in my opinion that very little indeed 

 of such denudation has taken place since they were placed where 

 we now see them. 



I could take any of you who are interested in this question to 

 a precipitous hill near Inveraray, some 1, 200 feet above the level 

 of the sea, from the top of which you can look down on the masses 

 of transported rock stranded upon its sides and base, precisely as 

 one might look down from the top of some dangerous reef in the 

 present ocean upon the debris of a whole navy of ships shattered 

 upon it in some hurricane of yesterday. There they lie, some 

 more or less scattered, some heaped upon and jammed against 

 each other with sharp angles and outlines wholly unworn and, 

 moreover, so distributed that you see at a glance their strict 

 relation to the existing heights and hollows of the land, which 

 must then have been the shoals and channels of the sea. These 

 contours cannot have been materially changed since that sea 

 was there. It seems that it must have been, geologically 

 speaking, only a few days ago. And this conclusion would seem 

 to be confirmed when we observe the phenomena which are pre- 

 sented in certain cases, where the land has clearly rested for a 

 considerable time, and the ocean has left in raised beaches the 

 evidence of its work at certain levels. Such raised beaches are 



to be found at many points all round our western coasts. But 

 incomparably the finest and most instructive example of them is 

 to be seen on the west coast of the Island of Jura, near the mouth 

 of Loch Tarbert, Jura, and extending for several miles to the 

 north. These beaches are visible from a great distance, because 

 these rolled pebbles are composed entirely of hard white quart- 

 zite of the jura mountains, which resists disintegration, and Is 

 unfavourable to the successful establishment of vegetation. 



I visited these beaches a few weeks ago, and, measuring the 

 elevation roughly with a graduated aneroid, I found that they 

 represent three more or less distinct stages of subsidence. One 

 beach being about the level of 50 feet above the present sea-level ; 

 another about 75 feet, and a third at about 125 feet. Some 

 others which I saw only from a distance appeared to be higher, 

 and I believe, but am not quite sure, that farther to [the north 

 they have been traced to the level of 160 feet. But the feature 

 connected with these sea beaches, and especially with the lowest, 

 or the 50 feet beach, is the evidence it affords — first, of the length 

 of time during which the ocean stood at that level, and, secondly 

 and particularly, the evidence it affords of the very recent date at 

 which it must have stood there. As regards the length of time 

 during which the ocean must have stood there, it is sufficient to 

 observe the beautiful smoothness and rotmdness of the pebbles. 

 They have been more thoroughly rolled and polished than the 

 corresponding pebbles on the existing shores, equalling in this 

 respect the famous pebble beds of the beach at Portland. Then, 

 as regards the very recent date at which the ocean must 

 have stood there, it is difficult to give in words an adequate idea 

 of the impression which must be left on the mind of every one 

 who looks at them. You see the curves left by the sweep of the 

 surf, the summit level of its force, and the hollow behind that 

 summit, which is due to the exhausted crest, all as perfect as if it 

 had been the work of yesterday. 



It is difficult to conceive how ordinary atmospheric agencies, 

 and even the tread of sheep and cattle, should not have broken 

 such an arrangement of loose material, but there are exceptionally 

 favourable circumstances for the preservation of these beds from 

 absence of considerable streams and the protection of surround- 

 ing rocks. There is little or no evidence of glaciation anywhere 

 around, and, although it is certain that the sea which stood at 

 those beaches so recently was a sea subject to glacial conditions, 

 it is equally certain either that it continued to work there after 

 those conditions had passed away, or, what is more probable, 

 that that particular line of coast was protected from the drift of 

 surrounding ice floes. If, now, we compare the evidence of 

 recent action in these sea beaches with the similar evidence 

 connected with the position of erratics at higher levels, which 

 can only have been placed there by floating ice, I cannot help 

 coming to the fconclusion that the submergence and re-eleva- 

 tion of the land to the extent of more than 2,000 feet above the 

 level of the present ocean has been one of the very latest changes 

 in the history of this portion of the globe ; and, moreover, that 

 the re-elevation has been comparatively rapid, probably by lifts 

 or hitches of considerable extent, and that there were few, if 

 any, pauses or rests comparable in duration with those which are 

 recorded in the Jura beaches and in the cutting of the existing 

 coasts. In conclusion let me repeat that, whether this conclusion is 

 correct or not — and I am well aware of the many difficulties which 

 surround it — the general fact of submergence and re-elevation is, 

 perhaps, as certain as any conclusion of geological science, and 

 that the consequences of it in accounting for the distribution of 

 gravels and the most recent changes of denudation have never as 

 yet been worked out with anything approaching to consistency 

 or completeness. 



Professor Geikie, F.R.S., remarked that those who had 

 watched recent discussion in physiographical geology might 

 have been prepared, as he himself was, for a much greater 

 divergence of opinion between the views expressed by the Duke 

 of Argyll and those entertained by the younger school of 

 geologists. While expressing his gratification at this approach, 

 and at the very clear and eloquent views of the author of the 

 paper, he ventured to point out some points where the arguments 

 seemed faulty. He showed that the Highlands might have been 

 covered with Old Red Sandstone, and that the antiquity of the 

 present mountains could not be traced back to the primeval up- 

 heaval to which undoubtedly the general ma.ss of Highland land 

 was due. He pointed out that the absence of raised beaches 

 could not be regarded as a disproof of the former presence of the 

 sea, but indicated a period of pause during either a rapid or 

 protracted upheaval. 



Professor Harkness fully agreed in what Professor Geikie 



