TRANSACTIONS OF THE SECTIONS. ' 85 



sui'face ice, tlrivenbv currents which were determined indirection by the chang'ing' 

 contours of the land during the processes of submersion and reelevation. 



It would be impossible on this occasion to illustrate or support these various 

 propositions b}' going into the evidences on which they rest. But as those of them 

 which relate to the operations of the txlacial epoch express a decided opinion upon 

 questions now involving much dispute, I must say a few words in explanation and 

 defence of that opinion. 



It will be seen that I disbelieve altogether in the theory of what is called an 

 Ice-cap ; or, in other words, I hold that there is no evidence that there ever existed 

 any universal mantle of ice higher or deeper than all the existing mountains, cover- 

 ing them and moving over them from distant northern regions. 



In the tii-st place, this theory presupposes conditions of climate which must have 

 prevailed universally over the whole northern hemisphere ; whereas over a great 

 portion of that hemisphere west of a certain meridian on the American continent, 

 all traces of general glaciation and of any general distribution of erratics disappear. 



In the second place, the theory assumes that masses of ice lying upon the surface 

 of the earth, more than mountain-deep, would have a proper motion of their own, 

 capable of overcoming the friction not only of rough level surfaces, but even of the 

 steepest gradients, for which motion no adequate cause has been assigned, and 

 which has never been proved to be the natural consequence of any known force, 

 or to be consistent with the physical properties of the material on which it is 

 supposed to have acted. 



In the third place, as a matter of fact there do not now exist anywhere on the 

 globe masses of ice which can be proved to have any motion of this kind, or to be 

 subject to forces capable of driving and propelling it in this manner and with the 

 etlects which the theory assumes. The case of Greenland, which is often referred 

 to as an example, does not present phenomena at all similar to those attributed to 

 the ice-sheet. 



In the fourth place, all the phenomena of glaciation 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, nanielv : — 

 ice mo\'iug, not up, but down mountain-slopes by the force of gravitation, and ice 

 floated by water and dri^^'en by cui-rents as icebergs or as Hoes. 



In the fifth place, these phenomena of glaciation are essentially different from 

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

 it to have existed and supposing it to have had the (improbable) motion which ha« 

 been ascribed to it. 



In the sixth place, and in particular, the mode in which erratics are distributed 

 and the peculiar position of perched blocks are demonstrative 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, are 

 equally demonstrative of exposure to moving ice under conditions which did not 

 enable it to fit into the irregularities of surfaces 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 done towards the close of the Glacial 

 epoch — when the land was emerging again from out of a glacial sea, and when all 

 the currents of that sea, loaded with bergs and iloes, were determined entirely by 

 the outlines of the rising land. 



In regard to the much disputed question of the glacial origin of Lake-basins, 

 the 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-sti-eams are 

 afterwards submerged along with the whole country in which they lay, and if that 

 submergence is accompanied by partial and unequal rates of sub.sidence, they 

 would inevitably become hollows into which the sei would enter, or in which 

 fresh waters would accumulate. In this sense, and in this way, it can hai'dly 



