PMESlDENTlAL ADbHfiSS. 08? 



of the older -vie\V that the cooling of the earth from a considenttor status ha^s been 

 mainl7 controlled by radiation, but, on the contrary, this new revelation of scieace 

 has come to smooth over what ditliculties attended the reconciliation of physical 

 and geological evidence on the Kelvin hypotliesis. It shows us how the advent 

 of the present thermal state might be delayed and geological time lengthened, so 

 that Kelvin's forty or Hfty million years migbt be reconciled with the hundred 

 million years which some of us hold to be the reading of the records of denudation. 



On this more pacidc view of the mission of radium to geology, what has been 

 the history of the earth ? In the earlier days of the earth's cooling the radiation 

 loss was tar in excess of the radio-thermal heating. From this slate by a con- 

 tinual convergence, the rate of radiation loss diminishing while the radio-thermal 

 output remained comparatively constant, the existir j distribution of temperature 

 near the surface has been attained when the radio-thermal supply may nearly or 

 quite balance the loss by radiation. The question of the possibility of tinal and 

 perfect equilibrium between the two seems to involve the interior conductivity 

 and in this way to evade analysis. 



It will be asked if ihe facts of mountain building and earth-shrinkage are 

 rendered less reconcilable by this interference of uranium in the earth's physical 

 history. I believe the answer will be in the negative. True, the greatest develop- 

 ment of crustal wrinkling must have occurred in earlier times. This must be 

 so, in some degree, on any hypothesis. The total slirinkage is, however, not the 

 less because delayed by radio-thermal actions, and it is not hard to point to factors 

 which will attend the more recent upraising of mountain chains tending to make 

 them excel in magnitude those arising from the stresses in an earlier and thinner 

 crust. 



Underground Temperature. 



It would be a matter of the highest interest if we could definitely connect 

 the rise of temperature which is observed in deep borings and tunnels with the 

 radio-activity of the rocks. We are confronted, however, by the difficulty that 

 our deepest borings and tunnels are still too near the surface to enable us to 

 pronounce with certainty on the influence of the radium met with in the rocks. 

 This will be understood when it is remembered that a merely local increase of 

 radio-activity must have but little etlect upon the temperature unless the increase 

 was of a very high order indeed. A. clear understanding of this point shows us 

 at once how improbable ir, is that volcanic temperatures can be brought within 

 a very few miles of the surface by local radio-activity of the rocks. To account on 

 such principles for an elevation of temperature of, say, 1200° at a depth of three or 

 four miles from the surface, a richness in radium must be assumed far transcending 

 anything yet met with in considerable rock masses ; and as volcanic materials 

 appear to show nothing of such exceptional richness in radium we can hardly 

 suppose local radio-activity of the upper crust responsible for volcanic phenomena. 

 When we come to apply calculation to results on the radio-activity of the 

 materials penetrated by tunnels and borings, we at once tind that we require to 

 know the extension downwards of the rocks we are dealing with before we can 

 be sure that radium will account for the thermal phenomena observed. At any 

 level between the surface and the base of a layer of radio-active materials- 

 suppose the level considered is that of a tunnel — -the temperature depends, so far 

 as it is due to local radium, on the total depth of the rock-mass having the 

 observed radio-activity. This is evident. It wdl be found that for ordinary 

 values of the radium content it is requisite to suppose the rocks extending down- 

 wards some few kilometres in order to account for a few degrees in tempera- 

 ture at the level under observation. There is, of course, every probability of such 

 a downward extension. Thus in the case of the Simplon massif the downward 

 continuance of the gneissic rocks to some few kilometres evokes no difficulties. 

 The same may be said of the granite of the Finsteraarhorn massif and the gneisses 

 of the St. Gothard massif, materials both of which are penetrated by the St. 

 Gothard tunnel, and which appear to possess a considerable diffijrence in radio- 

 activity. In dealing with tlua subject, comparison of the results obtained at one 



