238 THEORIES REGARDING THE EARTH's INTERIOR. 



Turning now to the heating effect of radium, we have the 

 work of Joly in the Simplon and St. Gothard rocks to guide us. 



In the St. Gothard tunnel, Stapff observed in the central 

 portions temperatures which worked out at a gradient of 46-6 

 metres for i°C., with small irregularities, which he attributed 

 to cold springs and the decomposition of the rock. At the 

 north end, where the tunnel pierces the granite of the Fin- 

 steraarhorn massif, there is a rise of temperature sufficient to 

 make the gradient 20*9 metres for every 1° C. Stapff ex- 

 plained the last rapid increase by imagining that the granite 

 retained some of the original heat of its molten condition, but 

 Prestwich, on the other hand, preferred to look upon it as the 

 result of mechanical actions which had comparatively recently 

 been in progress, and to which the upheaval of the Alps was 

 due. In the more recently bored Simplon tunnel, Stapff. bas- 

 ing his estimates on the experience obtained in the St. Gothard, 

 predicted a maximum temperature of 47° C., wdiile others pre- 

 dicted much lower ones. The actual temperatures observed, 

 however, rose to the north end to 55° C., and caused immense 

 difficulties in ventilation and working. This unexpected high 

 temperature was believed by Fox to be due to proximity to 

 volcanic rocks, but nothing of the sort has been noticed on 

 the surface. This fact nevertheless shows how impossible it is 

 to estimate increase of temperature in the earth's crust; in the 

 Simplon case the excess of heat almost stopped the working, 

 whereas the deep levels of the Witwatersrand, which should 

 be almost unbearably hot if the established temperature gradi- 

 ents existed in Nature, are comparatively cool. 



Joly investigated the Simplon and St. Gothard rocks with 

 a view to ascertaining their radium contents, and found that 

 the amount of radium contained in the various types encoun- 

 tered corresponded in a remarkable way with the temperature 

 gradients, so that in the high gradient region of the St. Gothard 

 the amount was 14-1 billionths of a gramme per gramme of 

 rock, whereas in the low gradient region it fell to 3-3 billionths. 

 In regard to the temperatures of the two tunnels, the following- 

 tables show the same correspondence : — 



Simplon Tunnel, amount of radium per gramme of rock substance in billionths 

 of a gramme : 



Jurassic and Triassic altered sediments 



Crystalline schists, partly Jurassic and Triassic and partly 



Monte Leone gneiss and primitive gneiss . . 



Schistose gneiss 



Antigorio gneiss 



j\Iean for all rocks 



6-4 



Archaean 7-3 

 .. 6-3 

 .. 6-5 

 6-8 

 71 



St. Gothard Tunnel, amount of radium per gramme of rock substance in 

 billionths of a gramme : 



Granite of Finsteraarhorn . . . . . . . . . . . . 7 '7 



Maximum 



Usernmulde 



St. Gothard massif 



Tessin mulde 



Mean in Central* Section 



14-1 

 4-9 

 3-9 

 3-4 



