64 Arthur Holmes — Radio-activity 



(4) The existence of surfaces of physical discontinuity within the 

 earth as revealed by seismology. 1 



The broad conception of the structure of our planet which is 

 suggested by these converging lines of evidence is that of a metallic 

 core (average specific gravity == 7 - 8) occupying approximately half of 

 the earth's total volume, surrounded by a thick peripheral shell 

 (average specific gravity = 3'4) in which the silicate rocks are 

 concentrated, becoming increasingly acid and alkaline as the crust 

 (average specific gravity = 2*8) is reached. We are thus prepared 

 for the conclusion that the radio-active elements are concentrated in 

 the earth's outer shell, for the acid and alkaline rocks are just those 

 which are richest in radium and thorium. It is an interesting fact 

 that volcanic rocks, which are uniformly more acid and alkaline than 

 their plutonic equivalents, are also richer in the radio-active elements. 

 Hooks of gabbroid composition, which presumably underlie tbe 

 immense granite and gneiss areas of the continents, contain less than 

 a third as much radium and thorium as the granites. Ultra-basic 

 rocks, which would seem to be comfortably at home only below the 

 gabbroid zone, contain only a sixth as much. That the actual falling 

 off in depth may be somewhat greater than these figures suggest, is 

 indicated by the difference in radio-activity between volcanic and 

 plutonic rocks. If the transfer of igneous magmas from levels 

 corresponding to those at which we find stocks and batholiths, to 

 those at which volcanic eruptions take place, is accompanied by so 

 marked an increase in their content of the radio-elements, it seems 

 probable that the whole upward journey may be marked by a pro- 

 gressive enrichment of a similar kind. Thus, even in rocks of the 

 same composition, a decrease of radio-activity with depth is to be 

 expected, but since the upward concentration must be mainly ascribed 

 to gravitational differentiation, 2 and to the concentrating action of 

 volatile fluxes, such an effect would certainly become less important 

 as the depth increased. 



So far, only the density stratification has been assumed, and the 

 falling off of the radio-active elements in depth has been deduced as 

 a natural consequence of that assumption. This was done to avoid 

 any difficulty over the debated question of the inhibition of radio- 

 activity by pressure. Conversely, if the independence of radio-activity 

 be assumed, an assumption which, as I said before, is thoroughly 

 justified, and indeed ceases to be an assumption, down to the limited 

 depths with which petrologists are concerned, then the density 

 stratification of the earth's crust follows as a natural consequence, 

 as shown in the following discussion. 



4. The Decrease op Radio-activity with Depth. 



Since the view that the earth is getting hotter cannot be accepted, 

 let us allow to radio-active heating all that can be theoretically 

 granted to it, and assume that the total amount of heat lost from the 



1 Milne, Proc. Eoy. Soc, A, vol. lxxvii, p. 365, 1906 ; E. D. Oldham, Q.J.G.S., 

 vol.lxii,p.456, 1906; vol. lxiii, p. 344,1907; Nature, August 21, p. 635, 1913. 



2 In spite of their very high atomic weights, the radio-elements tend to be 

 carried upwards on account of their association with the lighter components of 

 rock-magmas. 



