202 G. F. BECKER ISOSTASY AND RADIOACTIVITY 



300 kilometers from the surface, isostatic compensation takes place with- 

 out fusion — indeed, without any known cause; and this is not the whole 

 mj^stery, for it would seem that a shell no less than 180 kilometers in 

 thickness bounded by tlie eutectic level and Hayford's level must have 

 cooled many hundreds of degrees below its melting point without dis- 

 turbance of its isostatic equilibrium. I can not grasp such a situation. 

 It does not appear certain that on a globe so old as 1,300,000,000 years 

 there could be much more geology than on the purely radioactive earth 

 first discussed. In a cooling sphere the temperature at the eutectic level 

 sinks with the progress of time more and more below the temperature of 

 fusion. In a 68 X 10® year earth a rise in temperature of about 150° 

 would cause fusion, while in the 1,314 X 10® year earth an additional 

 600° would be needed to melt the rock at the eutectic level. It would 

 seem to me that as such a globe gTew old fusion would be a more and 

 more infrequent phenomenon, and that fusion would be more and more 

 rarely accompanied by effusion. It is even questionable whether any 

 eruptions could occur on a globe in wliich the eutectic level is 300 kilo- 

 meters beneath the surface. 



Conclusions 



The geodetic evidence for isostasy is so manifold and so consistent as 

 to amount to proof. Observed anomalies appear due in large measure 

 to irreguar distributions of density, and I conclude that the variations 

 in the load per unit area at the level of compensation are very much 

 smaller than the surface anomalies, while beneath this level the strains 

 are probably small quantities of the second order. 



Considered physically, the only interpretation I can put upon the 

 level of compensation is that it is the level of easiest fusion or of eutexia; 

 and, if so, at that level the tangent of the curve showing the temperature 

 of the earth as a function of depth is parallel to the curve representing 

 the melting point of the rock as a function of depth. Local fusion would 

 bring about compensation. Where, then, should we look for compensa- 

 tion, if not at the eutectic level ? 



Independently of this physical interpretation, the two curves just 

 referred to can not be far apart at the compensation level, for otherwise 

 a thick shell underlying this level must have cooled through, a large 

 temperature interval and must have undergone strains inconsistent with 

 compensation. 



Epeirogeny and orogeny may be explained as due to the conversion into 

 mechanical work of a part of the heat received by the outer shell of the 



