﻿the Rocks of the St. Gothard Tunnel. 203 



influences. Their age appears to be undetermined. The 

 last section of the tunnel is through the sediments of the 

 Tessinmulde, and extends a length of 3 kilometres. These 

 sediments are also Mesozoic and are similar in many respects 

 to the Usern rocks, of which they may be an extension. 



The temperature gradients in the northern end of the 

 tunnel were, as observed by Stapff *, of a very exceptional 

 character. It is known that the thermal gradients in moun- 

 tain regions are generally lower than is commonly observed 

 in plains. The latter average about 30 metres to one degree, 

 the former about 40 metres to one degree or even lower. 

 Stapff, however, found the gradient at the northern end of 

 the tunnel to be 20*9 metres per degree. The mean gradient 

 in the central division of the tunnel was, on the other hand, 

 only 46'6 metres. The rise in temperature at the northern 

 end was by Stapff ascribed to the comparatively recent age 

 of the granite : his view being that it was still parting with 

 some of its original heat. That the source of heat is in the 

 granite seems certain, for although the abnormal gradients 

 extend into, and are indeed most marked where the tunnel 

 traverses the Usern sediments, it is improbable that these 

 sediments are directly influential. They probably are under- 

 lain, at no great distance downwards, by the granite, the 

 heat from which escapes upwards through the infolded 

 sediments. 



The interest attached to the measurements of the radio- 

 activity of the rocks arose from the coincidence observed 

 between the temperature gradients and the radium content. 

 Such a coincidence appeared clearly shown in the experiments. 

 That the coincidence is of considerable suggestive interest 

 will, I think, be admitted. If other coincidences of the same 

 kind were discoverable there is no doubt that a strong case 

 for ascribing some local gradients to radioactive effects would 

 be established. Pending such discoveries 1 considered that 

 the original observations in the radium content might with 

 advantage be checked by further measurements. It was 

 also desirable to measure the thorium content. 



The thorium content of the original rock solutions was 

 accordingly determined by a method possessing the requisite 

 sensitiveness. The results were published in a paper in the 

 Phil. Mag. for July 1909. The original radium measure- 

 ments were also given in detail in this paper. It was found 

 that the quantities of thorium in the granite at the north end 

 of the tunnel were also in excess of those observed in the 



* Trans. North of England Mining and Mec. Engineers, \\.\iii. p. 25, 



P2 



