October 5, 1893] 



NATURE 



55^ 



surface was already cool, and its temperature fixed while the in- 

 terior was still hot and cooling ; and, therefore, that the exterior 

 must be thrust upon itself and be crushed. But the phenomena 

 are really far more complex than at first appears. It is necessary 

 to distinguish between two kinds of contraction to which the 

 interior layers are subjected, viz. radial and circumferential. If 

 there were radial contraction only, then undoubtedly every con- 

 centric shell as it descended into smaller space would be crushed 

 together laterally. But there is for all layers, except the surface, 

 also a circumferential contraction, and this would have just the 

 opposite effect, i.e. would tend to stretch instead of crush. 

 Therefore, wherever the decrease of space by descent is greater 

 than the circumferential contraction, there will be crush ; and 

 where the circumferential contraction is greater than the decrease 

 of space by descent, there will be tension and tendency to crack. 

 There would be no rca! cracking, only because incipient cracks 

 would be mashed out, or rather prevented by superincumbent 

 pressure. Where these two are equal to one another, there will 

 be no strain of any kind. There is a certain depth at which 

 this is the case; it is called the 'level of no strain.' To 

 Mellard Reade is due the credit of first calling attention to this 

 important principle." 



After a diagrammatic representation of this principle, the 

 president continued as follows : — 



"Now laborious calculations have been made by Davison, 

 Darwin, and Fisher to determine the depth of this level of no 

 strain. All make it very superficial. Davison, taking an initial 

 temperature of 7000° F. makes it five miles below the surface. 

 Fisher, on the same data, only two miles, and with an initial tem- 

 perature of 4000 only 07 of a mile. It is easy to see that if 

 this be true the amount of lateral thrust must be small indeed. 



" Now undoubtedly there is a true principle here which must 

 not hereafter be neglected, but it is almost needless to say that 

 these quantitative results are in the last degree uncertain. The 

 calculations are of course based on certain premises. These 

 are a uniform initial temperature of, say, 7000° F. , a time of cool- 

 ing, say, looor 200 millions of years, and a certain rate of cooling 

 under assumed conditions. The depth of the level of no strain 

 increases with the time, and is still going downward. In a word, 

 iaa question so complex, both mathematically and physically, 

 and in which the data are so very uncertain, every cautious 

 geologist, while freely admitting the soundness of the principle, 

 will withhold assent to the conclusions. Huxley has reminded 

 «s that the mathematical mill, though a very good mill, cannot 

 make wholesome flour without good wheat. It grinds indifler- 

 enlly whatever is fed to it. It has been known to grind peas 

 ere now. It may be doing so again in this case. Let us 

 wait. 



" But besides withholding assent, and waiting for more light, I 

 may add that these calculations, of course, go on the supposition 

 that the whole contraction of the earth is due to loss of heat ; 

 but, as we have already said, it may be due also to loss of con- 

 stituent water. This would put an entirely different aspect on 

 the subject." 



Alternative Physical Theories. 



I have given the objections to the contraction theory 

 frankly and, I think, fairly. They are undoubtedly serious. Let 

 «s see what has been offered in its place." 



►: 



/. Readc's Expansion Theories. 



This, the most prominent among alternative theories, was 

 first brought forward in Mr. Reade's book on "Origin of 

 Mountain Ranges." Although I have carefully read all that 

 Mr. Reade has written on this subject, I find it difficult to get a 

 clear idea of his views. But as I understand it, it is in outline as 

 follows: (i) Accumulation of sedimentsoffshore, and isostatic sub- 

 sidence of the same. (2) Rise of isogeotherms and heating of the 

 whole mass of sediments and of the underlying crust in propor- 

 tion tothe thickness of the sediments. (3) Expansion of the whole 

 mass in proportion to the rise of temperature. If there were no 

 resistance this expansion would be in all directions (cubic 

 expansion). (4) But since the containing earth will not yield 

 to expansion laterally, this lateral expansion is satisfied by 

 folding, and this in turn produces vertical upswelling. Thus 

 the whole cubic expansion is converted into vertical expansion, 

 which is therefore three times as great as the linear expansion 

 in any one direction. (5) Elevation would of course anyhow lie 

 greatest along the line of thickest sediment ; but this by itself 

 would not be sufficient to produce a mountain. (6) But farther 



NO. 1249, VOL. 48] 



— and here the theory is more obscure — there is a concentration 

 of the effects of expansion, along a comparatively narrow line 

 of thickest sediments, by a flow of the hydrothermally plastic or 

 even liquid mass beneath, toward this central line, and then up- 

 ward through the parted starta, folding these back on either 

 side, and appearing at the crest as the granitic or metamorphic 

 axis. (7) In his latest utterances he seems to adopt the view of 

 Reyer, viz. that the uplifted strata slide back down the slope, 

 producing the enormous crumpling so often found, and exposing 

 a wider area of granite axis. (8) Fiom the same liquid mass 

 which lifts the mountain come also the great fissure-eruptions 

 and the volcanoes. 



" Mr. Reade makes many experiments to determine the linear 

 expansion of rocks, and he thinks that these experiments show 

 that when cubic expansion is converted into vertical expansion, 

 and this again concentrated along a line of one-fourth to one- 

 fifth the whole breadth of the expanding mass, it would explain 

 the elevation of the highest mountains. But still he seems 

 uncertain if it be enough. In fact, he declares that if it were 

 not for another factor yet unmentioned, he probably would 

 never have brought forward the theory at all. 



(9) " This factor is recurrency of the cause and accumulation 

 of the effects. And here the previous obscurity becomes inten- 

 sified. I have read and re-read this part without being able 

 wholly to understand him. He seems to think that when 

 expansion had produced elevation, the mountain thus formed 

 would not come down again by cooling and contraction ; but, on 

 the contrary, would wedge up by normal faulting, and set in its 

 elevated position. Afterward, by new accumulation of heat, 

 another elevation and setting would take place, and the moun- 

 tain grow higher, and so on indefinitely or until the store of heat 

 is exhausted. Therefore, he characterises his theory as that of 

 'alternate expansion and contraction,' or, again, as that of 

 ' cumulative recurrent expansion.' Such is a very brief, per- 

 haps imperfect, but I hope fair outline of Reade's theory. It 

 seems to me that there are fatal objections to it. These I now 

 state." 



Objections to the Theory. 



(1) " The first objection is inadequacy to account for the 

 enormous foldings of mountains, especially when there is no 

 granite axis to fold back the strata. It is true that Mr. Reade 

 makes comparison between his own and the contraction theory 

 in this regard, and seems to show the much greater efre£tivenes.s 

 of his own. This may be true if we accept his premises, and 

 compare equal areas in the two cases. But the contraction 

 theory draws from the whole circumference of the earth, and 

 accumulates the iffects on one line, while in Reade's theory 

 the expansion is of course very local. 



(2) " But the fatal objection is that brought forward by 

 Davison. It is this : sedimentation cannot, of course, increase 

 the sum of heat in the earth. Therefore the increased heal 

 of the sediments by rise of isogeotherms must be taken from 

 somewhere else. Is it taken from below ? Then the radius 

 below must contract as much as the sediments expand, and 

 therefore there will be no elevation. Is it taken from the 

 containing sides ? Then the sides must lose as much as the 

 sediments gain, and therefore must contract and make room for 

 the lateral expansion, and therefore there would be no folding 

 and no elevation. I do not see any escape from this objection. 



" Thus it seems that Reade's theory cannot be accepted as a 

 substitute. Is there any other ? " 



//. Button's Isostatic Theory.'^ 



\ 



" Dutton's discussion of isostasy is admirable, but his appli- 

 cation of it to the origin of mountains is weak. The outline is 

 as follows ; — 



" Suppose a bold coast line, powerful erosion and abundant 

 sedimentation. The coast rises by unloading, and the marginal 

 sea-bottom sinks by loading. Now, if isostasy is perfect, there 

 will be no tendency to mountain formation. But suppose a 

 piling up of sediments — but on account of earth rigidity — with- 

 out immediate compensatory sinking, and a cutting down of 

 coast land without compensatory rising. Then there would be 

 an isostatic slope towards the land. And the accumulated and 

 softened sediments would slide landward, crumpling the strata 

 and swelling them up into a mountain range. 



"The fatal objection to this view is that complete isostasy is 

 necessary to renew the conditions of continued sedimentation, 



'1 PhJ. Soc. of Washington, Bui'. Vol. xi. pp. 51-64, 1889. 



