370 ANNUAL KEPOKT SMITHSONIAN INSTITUTION, 1908. 



able theory of mountain formation with which we are acquainted. 

 On such a time scale the 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 questioning 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 hypothesis. It shows 

 us how the advent of the present thermal state might be delayed and 

 geological time lengthened, 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 denudation. 



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 radiation loss diminishing while the radio-thermal output 

 remained comparatively constant, the existing distribution of tem- 

 perature near the surface has been attained when the radio-thermal 

 supply may nearly or quite balance the loss by radiation. The ques- 

 tion of the possibility of final and perfect equilibrium between the 

 two seems to involve the interior conductivity and in this way to 

 evade analysis. 



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. I believe the answer w'ill 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, 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 



