UEANIUM AND GEOLOGY JOLY. 371 



still too near the surface to enable us to pronounce with certainty on 

 the influence of the radium met with in the rocks. This will be under- 

 stood when it is remembered that a merely local increase of radio- 

 activity must have but little effect upon the temperature unless the 

 increase was of a very high order indeed. A clear understanding of 

 this point shows us at once how improbable it is that volcanic tem- 

 peratures can be brought within a very few miles of the surface by 

 local radio-activity of the rocks. To account on such principles for 

 an elevation of temperature of, say, 1,200° at a depth of three or four 

 miles from the surface, a richness in radium must be assumed far 

 transcending anything yet met with in considerable rock masses; and, 

 therefore, we can hardly suppose local radio-activity of the upper 

 crust responsible for volcanic phenomena. 



'\Mien we come to apply calculation to results on the radio-activity 

 of the materials penetrated by tunnels and borings, we at once find 

 that we require to know the extension downward of the rocks we are 

 dealing with before we can be sure that radium will account for the 

 thermal phenomena observed. At any level between the surface and 

 the base of a layer of radio-active materials — suppose the level con- 

 sidered is that of a tunnel — the temperature depends, so far as it is 

 due to local radium, on the total depth of the rock mass having the 

 observed radio-activity. This is evident. It will be found that for 

 ordinary values of the radium content it is requisite to suppose the 

 rocks extending downward some few kilometers in order to account 

 for a few degrees in temj)erature at the level under observation. 

 There is, of course, every probability of such a downward extension. 

 Thus in the case of the Simplon massif the downward continuance of 

 the gneissic rocks to some few kilometers evokes no difficulties. The 

 same may be said of the granite of the Finsteraarhorn massif and the 

 gneisses of the St. Gothard massif, materials both of which are pene- 

 trated by the St. Gothard Tunnel, and which appear to possess a 

 considerable difference in radio-activity. In dealing with this sub- 

 ject, comparison of the results obtained at one locality with those ob- 

 tained at another is the safest procedure. We must accordingly wait 

 for an increased number of results before much can be inferred. I 

 Avill now lay the cases of the two great tunnels as briefly as possible 

 before you. 



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



The Simplon Tunnel for a length of some 7 or 8 kilometers lies at 

 a mean distance of about 1,700 meters 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 estimates on his experience of the St. Gothard Tunnel, was 

 47°. Other authorities in every case predicted considerably lower 



