126 Sir H. H. Howorth — Erratic Boulders in Drift. 



once we leave the steep mountain valleys where glaciers are 

 generated. Of course, to the people who deal with the kind of ice 

 that Alice found in Wonderland everything is possible, and nothing 

 needs proving ; but to most of us it is a condition precedent to our 

 assent to the postulate that an ice-sheet several miles thick can be 

 moved en masse along a level surface, that some sufficiently potent 

 impetus should be pointed out. 



The second way in which ice can move is by virtue of its plastic 

 nature. This I have discussed at great length in my work already 

 cited. There can be no doubt, as is shown by evidence of all kinds, 

 that the principal movement of a glacier is of this kind. There can 

 be just as little doubt that this movement must be slow under any 

 circumstances, and in a body so little plastic as ice that it requires 

 a veiy considerable slope in the surface of the glacier to initiate it. 

 Thirdly — and this is most important — this kind of movement is 

 largely confined to the surface layers of the glacier, and, except on 

 very steep slopes, there is hardly any movement at all in the lower 

 layers. Ice, in fact, acts very much as water does in regard to this 

 mode of motion, and I believe myself that on a level bed, whatever 

 the slope of the upper surface of the ice, there is virtually no motion 

 at all in the lowest laj'ers. This view, I know, is shared by most 

 physicists, and it is quite preposterous in the great mass of Glacial 

 geologists to habitually ignore the stubborn fact in their iterated 

 statements. The burden of proof lies upon them. Before a postulate 

 is admissible in science at all, its i-easonableness and its accordance 

 with the ordinary laws of nature ought to be made out, and until 

 they be made out the theory can have no permanent place in science ; 

 and this is at present the position of the theory of ice-sheets moving 

 over hundreds of miles of level surface, as continually postulated. 

 It is, in effect, an appeal to the reversal of the laws of nature. 



Let us grant, however, that the orthodox geologists are right, 

 and that on a priori grounds even, they have a serious case ; 

 and let us go on to examine some of the concrete facts that 

 have to be explained under any circumstances. First, what is 

 the kind of treatment which stones entrapped between a glacier 

 and its bed are likely to receive ? There is a school of 

 geologists who argue as if it were possible, or even conceivable, 

 that a great mass of almost solid ice, pressing down with 

 the pressure of many tons to the square foot, can in some way 

 disintegrate its own bed and drag up fragments of stone out of it, 

 as a carpenter takes old nails out of a box. Then Mr. Carvill 

 Lewis speaks of "a deposit composed of fragments torn by the glacier 

 from the basement rock over which it passes." How this extra- 

 ordinary proceeding is to be done I know not, nor have I ever seen 

 any attempted explanation of it, and until some explanation of some 

 kind is forthcoming I shall continue to treat it as an impossibility. 

 Another set of geologists urge that, in some way or other, stones can 

 travel upwards through glacier ice, and so rise from its base to its 

 surface. Here, again, the whole proceeding seems incredible to 

 anyone who analyzes what a moving viscous body really does with 



