of the Rocks oftlie Loetschberg Tunnel. 471 



A. L. Fletcher, also employing the fusion method (Phil. 

 Mag. Feb. 1912\ found a mean of 0*8 for 24 calcareous and 

 l'l for 9 schistose rocks, both considerably lower than the 

 corresponding means found in this series of experiments. 

 The mean value for the schists, however, is not so high as 

 that obtained by Dr. Joly (Phil. Mag. Feb. 1912), also by 

 the fusion method, for the schists of the St. Gothard Tunnel 

 — namely, 2'8 x 10 -12 grin, per grin. It is rather unexpected 

 that the schistose rocks should, on the whole, prove to be 

 apparently slightly more radio-active than the Gastern 

 Granite, but probably if more specimens from the centre 

 of the granite zone had been obtained, this result would not 

 have been confirmed. The mean obtained for the limestone 

 is undoubtedly rather high, most previous investigators 

 having obtained values ranging from about 0'5 to 1*0. 

 Buchner [Konink. Akad. van Wetensch. te Amsterdam, Proc. 

 Feb. 1911) obtained a mean for 11 calcareous rocks of 1*4, 

 which agrees with my value of 1*5 for the Loetschberg lime- 

 stone pretty closely. He employed the solution method of 

 determining the radium, which usually seems to give slightly 

 lower results than the fusion method; so there is re illy little 

 reason to anticipate that there is any serious error in the 

 value obtained for the limestones. 



The general mean for all the rocks dealt with is also given. 

 It is of interest to note that the value 2'2 obtained falls be- 

 tween the limits of 2*0 to 2*5, between which the mean 

 radium content of the surface materials probably lies (Joly, 

 Phil. Mag. Oct. 1912). "We cannot attach too much im- 

 portance to this point nevertheless, as the specimens used 

 certainly could not be considered as really representative of 

 the surface materials, containing, as they do, 53 schists, 

 7 granites, 16 limestones, and 6 miscellaneous rocks. 



In conclusion, I wish to express my best thanks to Dr. Joly, 

 under whose direction this research was carried out. 



Iveagh Geological Laboratory, 



Mav J 920. 



