MEaiDiLNTIAL ADDRfiSS. 689 



repeated ou duplicate specimens. Including' some ver}' exceptional results, the 

 mean would rise to 91 x 10"^-' grams per gram. 



Of the St. Gothard rocks 1 have examined fifty-one specimens selected to be, 

 as far as attainable, representative.^ 



Of these, twenty-one are from the central region, and their mean radium content 

 i.-<just3'3. The portion of the tunnel from which these rocks come is closely 

 coincident with Staptf's thermal subdivision of regions of low temperature.'' 

 This portion of the mountain offers the most definite conditions for comparison 

 with the Simplon results. The region south of this is affected by water circu- 

 lation ; the regions to the north are affected by the high temperature of the 

 granite. 



We see, then, that the most definite data at our disposal in comparing the 

 conditions as regards temperature and radio-thermal actions in the two tunnels 

 appear to show that the steeper gradient is associated with the greater radium- 

 content. 



It is possible to arrive at an estimate of the downward extension of the two 

 rock masses (assumed to maintain to the same depth their observed radio-activity), 

 which would account for the difference in gradient. In making this estimate, 

 we do not assume that the entire heat-flow indicated by the gradients is due to 

 radium, but that the difference in radium-content is responsible for the difference 

 of heat-flow. If some of the heat is conducted from an interior source (of what- 

 ever origin), we assume that this is alike in both cases. We also assume the 

 conductivities alike. 



Calculating ou this basis, the depth required to establish on the radium 

 measurements the observed difference in gradients of the Central St. Gothard and 

 of the Simplon, we find the depth to be about 7 kilometres on the low mean of the 

 Simplon rocks, and 5 kilometres on the high mean. There is, as I have already said , 

 nothing improbable in such a downward extension of primitive rocks having the 

 radio-activities observed; but as a different distribution of radium may, of course, 

 obtain below our point of observation, the result can only claim to be suggestive. 



Turning specially to the St. Gothard, we find that a temperature problem of 

 much interest arises from the facts recorded. The north end of the tunnel for 

 a distance of 2 kilometres traverses the granite of the Finsteraarhorn massif. 

 It then enters the infolded syncline of the Usernmulde and traverses altered 

 sediments of Trias-Jura age for a distance of about 2 kilometres. After this 

 it enters the crushed and metamorphosed rocks of the St. Gothard massif, 

 and remains in these rocks for 7h kilometres. The last section is run through 

 the Tessinmulde for 3 kilometres. These rocks are highly altered Mesozoic 

 sediments. 



I have already quoted Stapff's observations as to the variations of gradient in 

 the northern, central, and southern parts of the tunnel. He writes : ' They (the 

 isotherms) show irregularities on the south side, which clearly depend on cold 

 springs, they bend down rapidly, and then run smoothly inclined beneath the water- 

 filled section of the mountain. Other local irregularities can be explained hy 

 the decomposition of the rock ; but there is no obvious explanation of the rapid 

 increase in the granite rocks at the northern end of the tunnel (2,000 metres), and 

 it is probably to be attributed to the influence of different thermal qualities of the 

 rock on the coefficient of increase. For the rest these 2,000 metres of granite 

 belong to the massif of the Finsteraarhorn, and, geologically speaking, they do 

 not share in the composition of the St. Gothard. Perhaps these two massifs 

 belong to different geological periods (as supposed for geological reasons long agoj. 

 What wonder, then, if one of them be cooler than the other.' (Loc cit., p. 30.) 



Commenting on the explanation here offered by Stapff, Prestwich^ states 



' I would like to express here my acknowledgments to the Trustees of the 

 British Museum for granting me permission to use chips of the rocks in their 

 possession ; and especially to Mr. Prior for his valuable assistance in selecting the 

 specimens. 



- Trans. North of Enqland Mining and Mec. Engineers, xxxiii. p. 25. 



■' Proc. R.S., xli. p. 44. 



1008. Y Y 



