76 



NA rURE 



[November 23, 1893 



magazine in wliich he is writing, and where, like the sermon 

 from the pulpit, what is said cannot be answered. This appeal 

 is not to mv taste, for I agree with the late Lord Tweeddale, 

 that truth is never so free from difficulty as when the good grain 

 has been thra-hed out by the flails of controversy. 



The position we are fighting about is too important, hovvever, 

 to go by default, for upon it rests a vast deal of induction in 

 other fields besides geology. 



My contention is, an 1 I am speaking to every man of science, 

 geologist or otherwise, that before Dr. Wallace can appeal to 

 ice as the excavator of lake basins on level, or nearly level, 

 plams/rt;- away from the slopes where glaciers grow, he must 

 establish two postulates, (i) That ice can convey thrust for 

 more than a very moderate distance. (2) That glaciers such as 

 we can examine and report upon are anywhere at this moment 

 doing the excavating work which he postulates. Without these 

 postuktes, his appeal to ice seems to me absolutely outside 

 science altogether, and to be a mere resort to some Dens ex 

 viachina, sucb as the mediaeval schoolmen based their reasoning 

 upon. 



In regard to the first postulate the experimental evidence 

 seems to me to be conclusive, and I have quoted it in my work 

 on the glacial nightmare. Mallet, writing on the modulus of 

 ice, says : " A few experiments have been made which show tha' 

 the height of this modulus cannot exceed a few hundred feet." 

 " Let it be assumed, however, that it is as great as 5000 feet, or a 

 mile. It is then obvious that a mass of ice, no mattei how 

 deep or wide, lying in a straight, smooth, friclionless valley, 

 cannot be pushed along by any extraneous force, in the line of 

 the valley, through a distance of more than a single mile, for 

 at that point the ice itself must crush, and the direct force cease 

 to be transmitted further. This, of course, is far from being the 

 whole of the question of the transmission of force through ice, 

 for when and wherever crushing takes place, a certain portion 

 (though a small one) of the direct pressure is transmitted laterally 

 Ijy the crushed fragments, especially if mixed with water. For 

 this to take place however, in the direction of the length of the 

 ice-filled valley, supposes the ice must be considerably more 

 than a mile in vertical depth." Mr. Oldham has carried the 

 question further, and I have quoted his arguments and experi- 

 ments on pages 596-597 of my book. His conclusion, after 

 postulating a quite transcendant modulus, as tested by observa- 

 tion, is: "The greatest distance to which a glacier could be 

 forced en masse is about five miles, so that a glacier debouching 

 on a plain could not exert any erosive power on that plain for 

 more than five miles from the commencement of its level course, 

 and consequently could not scoop out a lake basin of more than 

 that length, whatever its depth might be." 



Not only does this conclusion involve the postulating of quite 

 an impossible modulus for ice, but it also supposes that the 

 whole thrust of the ice coming down a slope is available, which 

 it clearly is not. A great deal of this thrust, as Mr. Irving has 

 shown, is expended in overcoming cohesion, in causing the 

 differential motion of a glacier, in forming crevasses which 

 largely intercept the thrust, and in causing the well-known 

 Bergschrund. To quote my own words, "a considerable 

 amount of the force of the gravity contained in a glacier is 

 used up within the glacier itself, and is not available either to 

 give it a forward thrust along a horizontal surface, or for 

 eroding purposes." 



80 far as I know, this is a perfectly candid statement of the 

 available evidence. Regelation has nothing whatever to do 

 with it. Directly ice crushes, the thrust is dissipated, the greater 

 part of it passing off in the direction of least resistance. To 

 me the case seems conclusive, but, says Dr. Wallace: "All 

 this is beside the question from my point of view. The work 

 of the ice on the rocks is as clear as that of palceolithic man on 

 the flints . . . and there is clear evidence that ice did 

 march a hundred miles, mostly uphill, from the head of Lake 

 Geneva to Soleure, whatever transcendental qualities it must 

 have possessed to do so." 



This form of dogmatic argument is assuredly incomprehen- 

 sible. I wonder Dr. Wallace is not afraid of the ghosts of his 

 own recent emphatic pronouncement on the glaciation of Brazil, 

 which he has now entirely abandoned, namely : " If the whole 

 series of phenomena here alluded to have been produced without 

 the aid of ice, we must lose all confidence in the method of 

 reasoning from similar effects to similar causes which is the very 

 foundation of modern geology." 



No, true geology is not founded upon hypotheses outside 



NO. 1256, VOL. 49] 



the laws of nature ; its secrets, when properly read, must be 

 consistent with those laws. Nur can the geologist who hopes 

 to see his work live, base his reasoning upon a peculiar scheme 

 of mechanics which experiment refuses to verify. 



If glaciers travelled further in former days, it was doubtless 

 because glaciers were larger in former days, because they de- 

 scended longer slopes, and had larger gathering grounds ; that is 

 to say, because the country where they grew was more elevated. 

 All this I, of course, adaiit was the case. That ice could travel 

 then any more than it can travel now over a considerable 

 distance of level ground, or excavate hollows in its track, by 

 virtue of the vis a tergo given it in its sloping cradle, is, it seems 

 to me, a subjective dream, and not an empirical conclusion. 



So much for the first postulate necessary to e3ta')lish Dr. 

 Wallace's conclusion. In regard to the second, I have little to 

 say. Glaciers exist in many countries. In some they have 

 retreated in historical times ; in others, we can travel under- 

 neath them for some distance. I know of no case, under any 

 conditions, where it can be shown that they have excavated rock 

 basins, small or big. If Dr. Wallace can quote any, it would 

 be an important addition to the case he makes. I must there- 

 fore conclude that, so far as our evidence goes, ice cannot 

 excavate lake basins on level plains, and that it is contrary to 

 the laws of the mechanics that it should do so. 



Dr. Wallace says, "No glacialist of the extremest school 

 would claim the rock basins of Bahia as proofs of glaciation." 

 This is an extraordinary statement. Why, the report on these 

 basins made by Mr. Allen, and inc )rporated by Hartt, was among 

 the most powerful pieces of evidence adduced by the latter for 

 the former glaciation of Bazil, which evidence Dr. Wallace 

 urged upon us a short time ago was completely unanswerable. 

 Lastly, in regard to Tasmania I do not quite follow him. He 

 says, " No doubt the conclusions of the various writers will be 

 fully harmonised by a more complete study of the whole subject. " 

 They are harmonised already. Tliey all agree that on the pla- 

 teaus and in the central district of Tasmania, where the lakes 

 abound, there are no traces of glaciation. So far as I know, the 

 only person who disputes it is Dr. Wallace himself, who has 

 never been there. What needs to be harmonised is his theory 

 with the facts as observed by all observers. 



I have replied at some length to Dr. Wallace's letter, not 

 only because I consider the issue a most critical one, but also 

 because of the distinction of its writer, who on so many ques- 

 tions has taught us lasting lessons, but who on this one seems 

 determined to set himself against the general conclusions of 

 those geologists who have most closely and laboriously studied 

 ice at work. 



I must now turn to Mr. LiTouche, whose courteous criticism 

 of my views appeared in a previous number of Nature. I am 

 not quite sure liow far we differ, for he apparently repudiates the 

 theory favoured by Ramsay and by Dr. Wallace, that the great 

 Alpine and Scotch lakes were excavated by glaciers. He limits 

 himself to certain rock basins in highly glaciated regions. In 

 regard to these having been excavated by ice, Mr. LaTouche 

 reminds me that ice is a viscous body, and moves, as Principal 

 Forbes argued that it does, almost entirely as a viscous body. 

 If Mr. LaTouche had favoured me by looking into my last book, 

 he would have found a long and very laborious chapter devoted 

 to establishing this very conclusion, but I do n )t see how it 

 assists his position. A viscous body, unless the viscosity 

 approaches ihat of a liquid, cannot move by mere hydrostatic 

 pressure, since the internal friction and the resistance and muiunl 

 support of its particles prevent it. The viscosity of ice is very 

 slight indeed, hence we cannot postulate for the nether layers of 

 a glacier with an uneven surface the movements we should 

 postulate in a liquid under the same conditions. With the forces 

 known to be requisite to make it shear, it seems to me that ice 

 cannot be supposed to move by hydrostatic pressure. 



Its actual motion is due almost entirely to its layers rolling 

 over each other as they do in pilch and other viscous Vjodies. 

 Now this movement in thick ice we kn')W is appreciable at the 

 surface, but the same conditions of friction and of drag, already 

 quoted, retard each successive layer as we go down, until 

 when we reach the lowest layers the motion due to viscosity is 

 exceedingly slight if it is even appreciable. Hence I cannot see 

 where the mechanical agent is to come from to excavate basins, 

 and how it is to work. 



When ice is moving on a slope, and the viscous movement is 

 helped by gravity, then no doubt the ice-foot shod wiih 

 stones becomes a tolerable eroding agent ; but I cannot under- 



