464 



NATURE 



[September 10, iyo8 



middle was 40,000 feet in depth. Tlie pile for the Wah- 

 satch Mountains was bo. 000 feet thiclc, according to King. 

 The beds for the Appalachians were not laid down in a 

 deep ocean, but in shallow waters, where a gradual sub- 

 sidence was in progress ; and they at last, when ready for 

 the genesis, lay in a trough 40,000 feet deep, filling the 

 trough to the brim. It thus appears that epochs of 

 mountain making have occurred only after long intervals 

 of quiet in the history of a continent." 



The generally observed fact that the deposition of sedi- 

 ments in some manner involves their ultimate upheaval 

 has at various times led to explanations being offered. I 

 think 1 am safe in saying that although the primary 

 factor, the compressive stress in a crust which has ceased 

 to fit the shrinking world within it, has probably been 

 correctly inferred, no satisfactory e.xplanation of the con- 

 nection between sedimentation and upheaval has been 

 advanced. The mere shifting upwards of the isogeo- 

 therms into the deposits, advanced as a source of local 

 loss of rigidity by Babbage and Herschel, need not involve 

 any such loss so long as the original distance of the 

 isogeotherms from the surface is preserved. 



We see in every case that only after great thiclcnesses 

 of sediments have accumulated is the upheaval brought 

 about. This is a feature which must enter as an essential 

 condition into whatever explanation we propose to offer. 



Following up the idea that the sought-for instability is 

 referable to radio-thermal actions, we will now endeavour 

 to form some approximate estimate of the rise of tempera- 

 ture which will be brought about at the base of such 

 great sedimentary accumulations as have gone towards 

 mountain building, due to the radium distributed through- 

 out the materials. 



The temperature at the base of a feebly radio-active 

 layer, such as an accumulation of sediments, is defined in 

 part by radio-active energy, in part by its position relative 

 to the normal isogeotherms, whether these latter are in 

 turn due to or influenced by radio-thermal supplies or not. 

 It is convenient, and I think allowable, to consider these 

 two effects separately, and deal with them as if they were 

 independent, the resultant state being obtained by their 

 summation. 



In dealing with the rise of temperature at the base of 

 a radio-active layer we arrive at an expression which 

 involves the square of the depth. This is a very important 

 feature in the investigation, and leads to the result that, 

 for a given amount of radium, diffuse distribution through 

 a great depth of deposit gives rise to a higher basal 

 temperature than a more concentrated distribution in a 

 shallower layer. 



But this will not give us the whole effect of such a 

 deposit. .Another and an important factor has to be taken 

 into account. We have seen that the immediate surface 

 rocks are of such richness in radium as to preclude the 

 idea that a similar richness can extend manv miles inward. 

 Now, it is upon this surface layer that the sediments 

 are piled, and as thev grow* in thickness this original 

 layer is depressed deeper and deeper, yielding under the 

 load until at length it is buried to the full depth of the 

 overlying deposit. This slow and measured process is 

 attended bv remarkable thermal effects. The law of the 

 increase of temperature with the square of the depth 

 comes in. and we have to consider the temperature effect 

 not merely at the base of the deposited layer, but that 

 due to the depression and covering over of the radium- 

 rich materials upon which the sediments were laid down. 



The table which follows embodies an approximate state- 

 ment of the thermal results of various depths of deposit 

 supnosed to collect under conditions of crustal temperature 

 such as prevail in this present epoch of geological 

 history : — 



We.ikenine of earth's crust 

 as defined by the rise cf the 

 geotherm at 40 kilometres 

 Kilometres 

 40 to 32'6 

 40 to 29 '8 

 40 to 267 

 40 to 23-; 

 40 to t9'6 



NO. 2028, VOL. 78] 



I have deferred to the conclusion of this address an 

 account of the steps followed in obtaining the above 

 results. It is clearly impossible, within the limited time 

 allotted to me, to make these quite clear. It must suffice 

 here merely to explain the significance of the figures. 



The first column gives the depth of sedimentary deposit 

 supposed to be laid down on the normal radio-active upper 

 crust of a certain assumed thickness and radio-activiiy. 

 From the rise of temperature which occurs at the base of 

 this crust (due to the radio-activity, not only of the crust, 

 but of the sediments) the results of the second column 

 are deduced, the gradient or slope of teinperature prevail- 

 ing beneath being derived from the existing surface 

 gradients corrected for the effects of the radio-thermal 

 layer. The third column is intended to exhibit the effect 

 of this shift of the geotherms in reducing the strength of 

 the crust. I assume that at a temperature of 800° the 

 deep-seated materials lose rigidity under long-continued 

 stress. The estimated depth of this geotherm is, on the 

 assumptions, about 40 kilometres. The upward shift of 

 this geotherm shows the loss of strength. Thus in tlfc 

 case of a sedimentary accumulation of 10 kilometres the 

 geotherm defining the base of the rigid crust shifts up- 

 w:irds by 13 kilometres, so that there is a loss of effective 

 section to the amount of 30 per cent.' 



.^s regards the claims which such figures have upon 

 our consideration, my assumptions as to thickness and 

 radio-activity of the specially rich surface layer are, doubt- 

 less, capable of considerable amendment. It will be found, 

 however, that the assumed factors may be supposed to 

 vary considerably, and yet the final results prove such as, 

 I believe, cannot be ignored. Indeed, those who are in 

 the way of making such calculations, and who enter into 

 the question, will find that my assumptions arc not 

 specially favourable, but are, in fact, made on quite in- 

 dependent grounds. Again, a certain class of effects has 

 been entirely left out of account, effects which will go 

 towards enhancing, and in some cases greatly enhancing, 

 the radio-thermal activity. I refer to the thickening of 

 the crust arising from tangential pressure, and, at a later 

 stage, the piling up and overthrusting of mountain build- 

 ing materials. In such cases the temperature of the 

 deeper parts of the thickened inass must still further rise 

 under the influence of the contained radium. These effects 

 only take place, indeed, after yielding has commenced, 

 but they add to the element of instability which the 

 presence of the accumulated radio-active deposits occasions, 

 and doubtless increase thermal melamorphic actions in the 

 deeper sediments, and result in the refusion of rocks in 

 the upper part of the crust. " 



The effect of accumulated sediment is thus necessarily 

 a reduction in the thickness of that part of the upper crust 

 which is capable of resisting a compressive stress. Over 

 the area of sedimentation, and more especially along the 

 deepest line of synclinal depression, the crust of the globe 

 for a period assumes the properties belonging to an earlier 

 .age, yielding up some of the rigidity which was the slow 

 inheritance of secular cooling. Along this area of weak- 

 ness — from its mode of formation generally much elongated 

 j in form — the stressed crust for many hundreds, perhaps 

 1 thousands, of miles finds relief, and flexure takes place 

 in the only possible direction ; that is, on the whole up- 

 wards. In this way the prolonged anticline bearing 

 upwards on its crest the whole mass of deposits is formed, 

 and so are born the mountain ranges in all their diversity 

 of form and structure. 



We have in these effects an intervention of radium in 

 the dynamics of the earth's crust, which must have in- 

 fluenced the entire history of our globe, and which, 1 

 believe, affords a key to the instability of the crust. For 

 after the events of mountain building are accomplished, 

 stability is not attained, but in oresence of the forces of 

 denudation the whole sequence of events has to commence 

 over again. Every fresh accession of snow to the firn. 

 every passing cloud contributing its small addition to the 

 t ^ee At^p^ndix P. 



- Prof. C. Schmidt (Basel) has recently given reasons for the view that t'^e 

 Mesozoic schists of the Simplon at th- period of their folding were probahlv 

 from 15,000 to 2o,oor> metres beneath the surface (" Ec. Geo!. Helvetiae,'" 

 vol. ix.. No. 4. p S90). As another instance co-isider the compression of the 

 Laramidc range (Dawson. Bull. Geo!. Soc. Am., xii., p. 87). 



