692 



TRANSACTIONS OF SECTION C. 



influenced by radio-thermal supplies or not. It is convenient, and I think allow- 

 able, 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 (o preclude the idea that a similar 

 richness can extend many miles inward. 



Now, it is upon this surface layer that the sediments are piled, and as they 

 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 by 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 whicli foUosvs embodies an approximate statement of the thermal 

 results of various depths of deposit supposed to collect under conditions of crustal 

 temperature such as prevail in this present epoch of geological history : — 



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-activity. 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 temperature 

 prevailing 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 assump- 

 tions, about 40 kilometres. The upward shift of this geotherm shows the loss of 

 strength. Thus in the case of a sedimentary accumulation of 10 kilometres the 

 geotherm defining the base of the rigid crust shifts upwards by 13 kilometres, so 

 that there is a loss of effective section to the amount of 30 per cent.' 



As regards the claims which such figures have upon our consideration, 

 my assumptions as to thickness and radio-activity of the specially rich sur- 

 face layer are, doubtless, capable of considerable amendment. It will be found, 



' See Appendix B. 



