66 



Arthur Holmes — Radio-activity 



6 X 10 2 calories per gram per second ; thorium, implying the series 

 Thorium to Thorium E in complete radio-active equilibrium, represents 

 a generation of 75 X 10 9 calories per gram per second. 



Taking the average composition of the earth's crust 1 as equivalent 

 to equal quantities of acid (granitic) and basic (basaltic or gabbroid) 

 plutonic rock types, with specific gravities respectively of 2*66 and 

 2-95, we obtain the following results from the figures given in Table I. 



The volume of rock required to maintain the earth in a state of 

 thermal equilibrium is clearly given by Q/ A, and the depth D of the 

 layer in which the radio-activity is concentrated is given by 



D = Q/47rr 2 . A (2) 



Under the assumed conditions, the maximum temperature 6 possible 

 within the earth will be that at the bottom of the radio-active layer, 

 and can be calculated from the expression 2 



6 = AD 2 /2k (3) 



Since from equations 2 and 1 



A J) = Q/477T 2 = Hdldx 

 we may also write 



e _ i(de\dx? (4) 



which gives the basal temperature in terms of known quantities 

 without involving the composite factor D. 



Substituting the values of A given in Table II we arrive at the 

 following values of D and for the three types of rock material 

 chosen for discussion : — 



1 Considered not only superficially but in depth also. 



2 Joly, Radio-activity and Geology, p. 272, 1909. 



