yune2^, 1875] 



NATURE 



143 



increase in the obliquitj'of the ecliptic would cause a greater 

 . difference in the seasons, and this difference we have seen 

 to be the very basis of Mr. Croll's own theory; the 

 results must be the same (and they are rightly seen by 

 Mr, Belt), whatever may be the cause of the difference 

 between summer and winter temperature. If this theory 

 were the true one, it is plain that both hemispheres were 

 glaciated at the same time, so that both theories cannot 

 be true ; but the matter of fact as to the synchronism 

 or otherwise of the glaciation of two hemispheres can 

 never in the nature of things be determined. But we 

 have still left the question. Has there been or can there 

 be any great change in the obliquity ? Astronomers say 

 no. Mr. Belt, however, thinks that the distribution of 

 sea and land and similar causes may make it possible for 

 greater changes to occur — a gratuitous supposition that 

 Mr. CroU shows to be groundless. This cause, then, 

 though it may have the general effect of lowering the 

 temperature of temperate and Arctic regions, is not suf- 

 ficient to cause a glacial epoch. 



On the whole, then, there appear to be several indepen- 

 dent cosmical causes which affect climate in a greater or 

 less degree, and the probable truth is that a glacial epoch 

 occurs when they all conspire to bring about the same 

 result. 



So far, by going from chapter to chapter, we have 

 endeavoured to bring Mr. Croll's arguments into some- 

 thing like logical order. The remainder of the book 

 scarcely admits of this ; indeed, we think the author 

 might well have bestowed more care in arranging his 

 matter if it was intended to form a consecutive whole ; 

 as it stands, there is much that can only be called a mis- 

 cellaneous collection of essays without any obvious con- 

 nection. Among these are his accounts of observations 

 on the North of England ice- sheet, and his specula- 

 tions as to the direction of its motion. There are also 

 two theoretical questions of [great interest discussed — 

 "The physical cause of the submergence and emer- 

 gence of the land during the glacial epoch," and " The 

 physical cause of the motion of glaciers." With regard 

 to the first of these questions, there are undoubted proofs 

 that great oscillations of the relative level of land and sea 

 have taken place in recent geological times, and the ques- 

 tion arises, Was it the land which sank and rose, or the 

 sea which changed its level ? It was rightly considered 

 one of the grand discoveries of geology when it was first 

 taught that the changeable sea was that which retained its 

 constant level, and that the " eternal hills " had been but 

 as yesterday beneath the waters ; and this principle is 

 not likely to pass away. By it all alterations of level 

 have been ascribed to the motion of the land, and none to 

 the rising of the sea. While agreeing, however, to the 

 principle, we may doubt its universality, and may be pre- 

 pared to entertain the question whether causes of limited 

 extent may not operate to raise the level of the sea, and 

 thus enable us to account more naturally for such rapid 

 changes as are sometimes indicated. There can be no 

 question but that any considerable amount of water which 

 by the fact of freezing should be retained in either polar 

 region, and form an ice-cap there, would correspondingly 

 shift the earth's centre of gravity and draw the remaining 

 water more over to the side on which the ice- cap lay ; and the 

 amount of elevation of sea-level might easily be calculated 



for any latitude, if we knew the extent of the cap and its 

 manner of deposition, i.e. its shape ; and the amount 

 would be doubled if the ice-cap were first on one hemi- 

 sphere and then transferred to the other. This calcula- 

 tion Mr. CroU attempts to make on the very ingenious 

 method of approximation that supposes the ice-cap such 

 as shall make the earth with the cap on one side a per- 

 fect sphere. The question can be worked out more 

 directly, as has indeed been done, though with varying 

 results, the^mean of which indicates that the rise at one 

 pole due to this cause would be about one-fifteenth of the 

 thickness of the ice melted off the other. If, therefore, 

 we want to account for an alteration of level of 500 feet 

 in England, corresponding to about 600 feet at the pole, 

 we should require to have somewhat less than two miles' 

 thickness of ice on the antarctic regions now. While 

 these figures represent data too far removed from the 

 truth to be at all reliable, and there are, moreover, other 

 causes that may affect the result, they serve to show the 

 kind of thickness required— that it is not twenty miles, 

 for instance. Are we prepared, then, to admit that there 

 may be two or three miles of ice on the south pole? 

 This does not appear to us at all an extravagant assump- 

 tion, when icebergs have been met with 700 or 800 feet 

 out of water, and which must therefore have been con- 

 siderably more than a mile in total height. We do not 

 think it therefore unreasonable to suppose that during the 

 glacial epoch, or indeed at other times, when there was 

 less ice at the south pole than now, the sea in our latitudes 

 may have stood at a higher level, and that many of the 

 elevated marine deposits and raised sea beaches are due 

 to this cause, and not to depression of the land ; for the 

 latter we have no other evidence, and it would involve such 

 vast changes in so recent times that we can scarcely 

 believe would leave all the main valleys and hills as they 

 were before the glacial epoch, and afford no evidences of 

 post-glacial faults. This argument of course does not 

 deny that there have been land oscillations during the 

 period, but only that they are not the only ones. 



This leads us to the last of the theoretical questions 

 discussed by the author of this work — the physical cause 

 of the motion of glaciers, the answer to which appears to 

 depend upon what is the amount of the shearing force of 

 ice. The remarks which Mr. CroU makes on the theory 

 and experiments of Canon Moseley are very forcible. 

 There is no doubt that the element of time enters largely 

 into the amount 'of force required to shear ice, and that 

 during this time heat is acting on the ice also, and conse- 

 quently that satisfactory experiments can only be made 

 on a glacier itself; and also that the theory of the 

 dependence of glacier motion on change of temperature 

 wiU not account for the greater descent in summer than 

 in winter. But what is Mr. Croll's own theory 1 He, like 

 Canon Moseley, calls in the agency of heat, and indeed, 

 since heat obviously makes a difference in the amount of 

 motion, we have only to find out hoiv it makes this differ- 

 ence to determine the cause of the whole motion. He 

 considers the motion of a glacier molecular, that the heat 

 entering at one end melts the first molecule, which then 

 descends by its weight and leaves room for the molecule 

 above it to descend, when it melts. This may look very 

 pretty at first sight, but the first molecule would never 

 descend and leave a vacuum behind itj so the second 



