372 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



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

 Gotharcl, predicted 36° at a depth of 2,050 meters from the surface, 

 and Heim 38° to 39°.« 



When the unexpectedly high temperatures were met with, various 

 reasons were assigned. Mr. Fox has suggested volcanic heat. Others 

 j)oint to the arrangement of the schistosity and the dryness of the 

 rocks, where the highest temperatures were read. The latter is evi- 

 dently^ to be regarded more as explanation of the lower temperatures 

 at the south end of the tunnel, where the water circulation was con- 

 siderable, 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 com- 

 pact in every direction, but what schistosity there is by no means 

 inclines in the best directions for retention of heat. From the sec- 

 tions the schistosity appears generally to point upward at a steep 

 angle with the tunnel axis.^ 



AA^iere there is such variability in the temperatures, irrespective of 

 the depth of overlying rock, there is difficulty in assigning any sig- 

 nificant 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 w^ould be met in 

 borings made on the level — about 31 meters per degree. 



The temperatures read in the St. Gothard rocks were of a most 

 remarkable character. For the central parts of the tunnel the gradi- 

 ents come out as 46.6 meters per degree. Stapff, wdio made these 

 observations anc> conducted the geological investigations, took par- 

 ticular pains to ascertain the true surface temperatures of the rock 

 above the tunnel, and from these ascertained temperatures, the tem- 

 peratures 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 Fin- 

 steraarhorn massif, there is a rise in the temperature of the rock suffi- 

 cient to -steepen the gradient to 20.9 meters per degree. Staj^ff re- 

 garded 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 by the rocks in the central section of 

 the St. Gothard, where the gradient is low. For the Simplon the 



°' See the account given by Scharclt, Yerhandl. Schweizerisclien Naturf. 

 Gesellsch. 1904, vol. 87, " Jabresversammluug," p. 204 et seq. 

 ^ Schardt, loc. cit. 



