September io, 1908] 



NA rURE 



461 



and assume a distribution by means of which the existing 

 thermal state of the crust may be maintained. A specially 

 rich surface layer we must recognise, but this need be no 

 jiiore than a very few miles deep ; after which the balance 

 of the radium may be supposed distributed to any depth 

 with which we are thermally connected. Below that our 

 knowledge is indefinite. The heat outflow at the surface 

 is in part from the surface radium, in part due to the 

 cooling arising from the diminishing amount of uranium, 

 in part from the deep-seated radium. In this manner the 

 isogeotherms are kept in their places, and a state is 

 maintained which is in equilibrium with the thermal factors 

 involved, but which cannot be considered steady, using the 

 word in a strictly accurate sense, in view of the decay 

 of the uranium. 



While the e.xisting thermal state may, I think, thus be 

 maintained by radio-active heating and radio-active decay, 

 wo find ourselves in considerable difficulties if we extend 

 this view into the past and assume that the same could 

 be said of any previous stage of the earth's history. If 

 the heat emitted by the earth, when the surface was at 

 molting temperature, was in a state of equilibrium with 

 ihe radio-active supplies, then, at that date, there must 

 have been n^any thousands of times the present amount 

 of uranium on the earth, and the period of the consistentior 

 stains must be put back by thousands of millions of years. 

 .\part froiTi hopeless contradiction with every geological 

 indication as to the age of the earth, difficulties in solar 

 physics arise. For the sun must be supposed of equal 

 duration, and we are required to assume impossible amounts 

 of uranium to maintain his heat all that great lapse of 

 time ; and again this uranium would perish at just the 

 same rate as that upon the earth, so that at the present 

 time the solar mass must be, for by far the greater part, 

 composed of inert materials of high atomic weight : the 

 products of the transformations of the uranium family. 

 The difficulty is best appreciated when we consider that 

 even to maintain his present rate of heat-loss by radium 

 supplies, some 60 per cent, of his mass must be composed 

 of uranium. But there are other troubles to face if we 

 adopt this view. The earth, or rather those parts of it 

 whicit are sufficientlv near the surface to lose heat at the 

 requisite rate, would have cooled but one per cent, in 

 10" years. Shrinkage of the outer parts and cruslal thick- 

 ness will be proportionately small, and we must put back 

 our epochs of mountain building to suit so slow a rate of 

 cooling and shrinkage and refer the earlier events of the 

 kind to a past of inconceivable remoteness. Otherwise 

 we must abandon the only tenable theory of mountain 

 formation w-th which we arc acquainted. On such a time- 

 srale tlie ocean would be supersaturated under the influence 

 of the prolonged denudation like the waters of certain salt 

 lakes, and the sediments would have accumulated a 

 hundredfold in thickness. 



Nor do the facts as we know them require from us such 

 sacrifices. We are not asked to raise these difficulties on 

 supposititious quantities of uranium for the existence of 

 which there is no evidence. Radium has occasioned no 

 uuestioning of the older view that the cooling of the earth 

 from a consistentior status has been mainly controlled by 

 radiation. But, on the contrary, this new revelation of 

 science has come to smooth over what difficulties attended 

 the reconciliation of physical and geological evidence on the 

 Kelvin hvpothesis. It shows us how the advent of the 

 present thermal state might be delayed and geological time 

 longthoncd. so that Kelvin's forty or fifty million years 

 might be reconciled with the hundred million years which 

 some of us hold to be the reading of the records of denuda- 

 tion. 



On this more pacific 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 far in excess of the radio-thermal heating. From 

 this state by a continual convergence, the rate of radia- 

 tion loss diminishing while the radio-thermal output re- 

 mained comparatively constant, the existing distribution 

 of temperature near the surface has been attained when 

 the radio-thermal supoly may nearly or quite balance the 

 loss by radiation. The question of the possibility of final 

 and perfect equilibrium between the two seems to involve 

 the interior conductivity and m this way to evade analysis. 



NO. 2028, VOL. 78] 



It will be asked if the facts, of mountain building and 

 earth-shrinkage are rendered less reconcilable by this 

 interference of uranium in the earth's physical history. 

 1 believe the answer will be in the negative. True, the 

 greatest development of crustal wrinkling must have 

 occurred in earlier times. This must be so, in some 

 degree, on any hypothesis. The total shrinkage is, how- 

 ever, 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 effect upon the temperature unless the increase 

 be of a very high order indeed. .'V clear understanding 

 of this point shows us at once how improbable it 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 tempera- 

 ture 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 find 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. ."Xt 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 will be found that for 

 ordinary values of the radium content it is requisite to 

 suppose the rocks extending downwards some few kilo- 

 metres in order to account for a few degrees in tempera- 

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

 every probabilitv of such a downward extension. Thus 

 in the case of the Simplon massif the downward continu- 

 ance 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 v^hich are penetrated 

 by the St. Gothard tunnel, and which appear to possess, 

 a considerable difference in radio-activity. In dealing with 

 this subject, comparison of the results obtained at one 

 locality with those obtained at another is the safest pro- 

 cedure. We must accordingly wait for an increased 

 number of results before much can be inferred. I will 

 now lav the cases of the two great tunnels as briefly as 

 possible before you. 



.And first as to the temperature efl'ects observed in the 

 two cases. 



The Simplon tunnel for a length of some seven or eight 

 kilometres lies at a mean distance of about 1700 metres 

 from the surface. At the northerly end of this stretch 

 the rock temperature attains 55°. and at the southern 

 extremitv has fallen to about 3";°. The temperature of 

 ;i;° is the highest encountered. The maximum predicted 

 hv Stapff, basing his estimates on his experience of the 

 St. Gothard tunnel, was 47°. Other authorities in every 

 case predicted considerably lower temperatures. Stock- 

 alper. who also had experience of the St. Gothard, pre- 

 dicted 36° at a depth of 2050 metres from the surface, 

 and Helm 38° to 39°.^ 



1 -See the account ffiven >>v Fchardt, VerhancfL Sc/nri'izt'rrsclwn Naltirf. 

 GcscUsch,, 19C4, I.xxxvii., " Jahresversammlung," p. 204 f/ Jr?. 



