462 



NA TURE 



[September 10, 1908 



When the unexpectedly high temperatures were met 

 with, various reasons were assigned. Mr. Fox has sug- 

 gested volcanic heat. Others point to the arrangement 

 of the schistosity and the dryness of the rocks, where the 

 highest temperatures were read. The latter is evidently 

 to be regarded more as explanation of the lower tempera- 

 tures at the south end of the tunnel, where the water 

 circulation was considerable, than of the high tempera- 

 tures of the northern end. The schistosity may have 

 some influence in bringing the isogeotherms nearer to the 

 surface ; however, not only are the rocks intensely com- 

 pact in every direction, but what schistosity there is by 

 no means inclines in the best directions for retention of 

 heat. From the sections the schistosity appears generally 

 to point upwards at a steep angle with the tunnel axis.' 



Where there is such variability in the temperatures, 

 irrespective of the depth of overlying rock, there is 

 difficulty in assigning any significant mean gradient. 

 The highest readings are obviously those least affected 

 by the remarkable water-circulation of the Italian side. 

 The higher temperatures afford such gradients as would 

 be met in borings made on the level — about 31 metres per 

 degree. 



The temperatures read in the St. Gothard rocks were 

 of a most remarkable character. For the central parts 

 of the tunnel the gradients come out as 46-6 metres per 

 degree. Stapff, who made these observations and con- 

 ducted the geological investigations, took particular pains 

 to ascertain the true surface temperatures of the rock 

 above the tunnel ; and from these ascertained temperatures, 

 the temperatures in the tunnel rock and the overlying 

 height of mountain, he calculated the gradients. 



But this low gradient is by no means the mean gradient. 

 At the north end, where the tunnel passes through the 

 granite of the Finsteraarhorn massif, there is a rise in 

 the temperature of the rock sufficient to steepen the 

 gradient to 209 metres per degree. Stapff regarded this 

 local rise of temperature as unaccountable save on the 

 view that the granite retained part of the original heat. 

 This matter I will presently return to. 



Now, it is a fact that the radium-content of the Simplon 

 rocks, after some allowance for what I have referred to 

 as sporadic radium, stands higher than is afforded bv the 

 rocks in the central section of the St. Gothard, where 

 the gradient is low. For the Simplon the general mean 

 is (on my experiments) 7-1 billionths of a gram per gram. 

 This mean is well distributed as follows : — 



Jurassic and Triassic altered sediments 6'4 



Crystalline schists, paitly Jurassic and Triassic, partly 



Archaean •j--i 



Monte Leone gneiss and primitive gneiss 6 '3 



Schistose gneiss (a fold from beneath) 6'5 



Antigorio gneiss 6-8 



The divisional arrangement is Prof. Schardt's. Forty- 

 nine typical rocks are used in obtaining these results, and 

 the experiments have been in many cases repeated on 

 duplicate specimens. Including some very exceptional 

 results, the mean would rise to 91x10-'- grams per 

 gram. 



Of the St. Gothard rocks I have examined fifty-one 

 specimens_^ selected to be, as far as attainable, repre- 

 sentative." 



Of these, twenty-one are from the central region, and 

 their mean radium content is just 33. The portion of 

 the tunnel from which these rocks come is closely coinci- 

 dent with Stapff'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 bv water 

 circulation ; the regions to the north are affected bv the 

 high temperature of the granite. 



We see, then, that the most definite data at our dis- 

 posal in comparing the conditions as regards tempera- 

 ture and radio-thermal actions in the two tunnels appear 



1 Schardt, he. cit. 



2 1 would like to express here my acknowledgments tn the Trustees of the 

 Kritish Museum for grantine 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 England Mining and Mec. Engineers, xx-xiii., p. 25. 



NO. 2028, VOL. 78] 



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 differ- 

 ence of heat-flow. If some of the heat is conducted from 

 an interior source (of whatever origin), we assume that 

 this is alike in both cases. We also assume the conduc- 

 tivities alike. 



Calculating on 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 

 iTiean. There is, as I have already said, nothing improb- 

 able 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 Usern- 

 mulde 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 7§ 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 by the decomposition of the roclj ; but there 

 is no obvious explanation of the. rapid increase in the 

 granite rocks at the northern end of the tunnel (2000 

 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 2000 metres of granite 

 belong to the massif of the Finsteraarhorn, and, geo- 

 logically 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 ago). What wonder, then, if one of them 

 be cooler than the other." [hoc. cit., p. 30.) 



Commenting on the explanation here offered by Stapff, 

 Prcstwich ' states his preference for the view that the 

 excess of temperature in the granite is due to mechanical 

 actions to which the granite was exposed during the up- 

 heaval of this region of the Alps. 



The accompanying diagram shows the distribution of 

 temperature as given by Stapff, and the distribution of 

 radium as found from typical specimens of the rocks. 

 There is a correspondence between the two which is 

 obvious, and when it is remembered that the increase in 

 radio-activitv shown at the south end would have been, 

 according to Stapff, masked by water circulation, the 

 correspondence becomes the more striking. The small 

 radium valups in the central parts of the tunnel are re- 

 markable. The rocks of the Central St. Gothard massif 

 are apparently exceptionally poor in radium. 



At the north end the excess of radium is almost con- 

 fined to the granite, the rock to which Stapff ascribed 

 the exceptional temperatures. The radium of the Usern- 

 mulde is probably not very important, seeing that these 

 sediments cannot extend far downwards. The principal 

 local source of heat appears located more especially 

 beneath the synclinal fold, for Stapff's table (loc. cit.. 

 p. 31) of the gradients beneath the plain of Andermatt 

 shows a rising gradient to a point about 2500 metres 

 1 Proc. R.S., xli., p. 44. 



