688 TRANSACflONS OF SECTION C* 



locality with tbose obtained at another is the safest procedure. We mtlst 

 accordingly wait for an increased number of results before much can be 

 inferred. I will now lay the cases of the two groat tunnels as briefly as posfsible 

 before you. 



And first as to the temperature effects observed in the two cases. 

 The Simplon tunnel for a length of some seven or eight kilometres lies at 

 a mean distance of about 1,700 metres from the surface. At the northerly end 

 of this stretch the rock temperature attains 55°, and at the southern extremity has 

 fallen to about 35°. The temperature of 55° is the highest encountered. The 

 maximum predicted by Stapff', basing his e.slimatea on his experience of the St. 

 Gothard tunnel, was 47°. Other authorities in every case predicted considerably 

 lower temperatures. Stockalper, who also had experience of the St. Gothard, 

 predicted 36° at a depth of 2,050 metres from the surface, and Ileim 38° to 39°.' 



When the unexpectedly high temperatures were met with, various reasons were 

 assigned. Mr. Fox has suggested volcanic heat. Others point to the arrange- 

 ment of the schistosity and the dryness of the rocks, where the highest tempera- 

 tures were read. The latter is evidently to be regarded more as explanation of 

 the lower temperatures at the south end of the tunnel, where the water circulation 

 was considerable, than of the high temperatures 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 compact 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 atfected by the remarkable water- 

 circulation of the Italian .side. The higher temperatures aff"ord 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 40-6 

 metres per degree. Stapff, who made these observations and conducted the 

 geological investigations, took particular pains to ascertain the true surface tem- 

 peratures of the rock above the tunnel ; and from these ascertained temperatures, 

 the temperatures in the tunnel rock and the overlying height of mountain, lie 

 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 i.s 

 a rise in the temperature of the rock sufficient to steepen the gradient to 'JO'.) 

 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 i.s 

 afforded by 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 bilUonths 

 of a gram per gram. This mean is well distributed as follows: — • 



Jurassic and Triassic altered .sediments Gl 



Crystalline schists, partly Jurassic and Triassic, partly Archiican . 7'.S 



Monte Leone gneiss and primitive gneiss G-.^ 



Schistose gneiss (a fold from beneath) 6-.') 



Antigorio gneiss 68 



The divisional arrangement is Professor Schardt's. Forty-nine typical rocks 

 areused in obtaining these results, and the experiments have been in many ca.ses 



' See the account given by Schardt, Verhandl. Schrt'eiaerisehen Anfurf, Geaelhch. 

 1904, 87, ' Jahresversammlung,' p. 201 et seq. 

 '^ Schardt, loc. cit. 



