THEORIES REGARDING THE EARTH S INTERIOR. 239 



From Joly's figures there can be no doubt left in one's mind 

 that radium has at any rate a very great influence on the tem- 

 peratures observed in rocks, if it is not entirely responsible for 

 all the heat in the earth's crust. But the work of Strutt, who 

 initiated this line of research, is still more positive. The fol- 

 lowing table is taken from his work on the distribution of 

 radium in the earth's crust and on the earth's internal heat: — 



Rock. j Locality. Radium in billionths of a gramme 



per gramme of rock substance. 



Granite . . . . Rhodesia . . . . 9'56 



Granite . . . . Cape of Good Hope . . 7-15 



Granite . . . . Shap Fell . . . . * 6-65 



Granite . . . . Isle of Rum . . 676 



Olivine basalt .. Skye .. .. i'32 



Basalt . . . . Victoria Falls . . i -26 



Basalt .. .. Ovifak, Greenland .. •613 



Dolerite .. .. Isle of Canna .. i"24 



Dunite . . . . Loch Scaivig . . "664 



Stony Meteorite .. Dhurmsala .. .. i-i2 



Iron meteorite . . Three specimens . . o 



Native iron . . Disco, Greenland . . -424 



The general result of the analysis of the 33 specimens tested 

 is that the radium content is higher in siliceous than in basic 

 rocks and that meteoric iron is free of radium. Assuming 

 that the rate of increase observed in the crust of the earth is 

 1° F. for every 42*4 feet, according" to Prestwich's mean, then 

 if all the heat were solely produced by radium contained in 

 the earth, the amount would be -175 billionths of a gramme per 

 gramme of earth substance, taking it all through. All igneous 

 rocks, however, contain far more than this; the poorest of all, 

 tne 'Greenland basalt, contains 10 times as much and an aver- 

 age rock 50 to 60 times as much. 



If then the earth cannot contain on an average more than 

 •175 billionths of a gramme per gramme of earth substance and 

 that 5 billionths is a representative value for rocks in the crust 

 of the earth, then not more than 1/30 of the earth's volume 

 ■can consist of material similar to that encountered on the sur- 

 face. This would give a depth of rock crust of about 45 miles, 

 assuming a total absence of radio-active material within. 



Suppose, further, that all the heat in the earth's crust is 

 produced by radium, then the temperature at the base of the 

 45 miles will be 1530° C. Radium has been tested up to a 

 temperature of I200°C., at which point no diminution of its 

 properties was observed, so that there is no reason to suppose 

 that the temperature at the base of the rocky crust would in- 

 terfere with the emanation of heat. Remembering, however, 

 that the fundamental rock of the earth's crust is granite, the 

 radium content 5 is probably far too low; considering" also the 

 ■contributory effects of the chemical reactions, the depth of the 

 radium-containing crust can be halved or at any rate reduced 

 to Milne's estimate of 30 miles. There is a further considera- 

 tion not contemplated by vStrutt, namely; the probability that 



