﻿the Rocks of the St. Gothard Tunnel. 



209 



averages, it will be found that in the Finsteranr or northern 

 section of the tunnel there is but little difference between 

 the earlier and later radium measurements. The average of 

 the former is 7*2 x 10 -12 , of the latter 6*0 x 10~ 12 . In com- 

 puting the latter, two experiments, Nos. 20 and 24, on vein 

 substances are omitted. They are probably without bearing 

 on the radioactivity of the granite generally, as they are, 

 according to Stapff, " gangartig," consisting m the one case 

 of a eurite or highly siliceous differentiation product, in the 

 other of a white felspar from a " grobspathiger eurite.'"' 

 Both are shown in Stapff 's marvellously detailed drawings 

 as narrow white veins traversing the body of the granite. 

 The one high result (14* 1 x 10 ~ 12 ) of the earlier results is 

 largely influential in raising them above the later ones. 



In the other sections of the tunnel the new experiments 

 are distinctly lower than the older. Here, again, a very few 

 exceptional rocks appear largely responsible. Two of these 

 were zircon-bearing (as Stapff points out), i. e. No. 47 north 

 end, which is a cipolin, and No. 132 south end. It will be 

 noticed that the newer results in most cases conform in radium- 

 richness to the neighbouring specimens. While I do not 

 think the lower means point necessarily to errors of excess 

 in the earlier results, it is certain that the method by fusion 

 is safer than the method by solution ; and, indeed, the pro- 

 cess of extracting the emanation is almost of mechanical 

 simplicity. It is not easy to go wrong in this part of the 

 operation either by errors due to contamination or failure to 

 liberate the emanation. The errors arising- from light (or 

 other causes) affecting the insulation of the electroscope are 

 fully guarded against. I think, therefore, that the newer 

 results are more reliable than the old, although I know of 

 no source of error affecting the latter. 



The following table gives the means for the several sections 

 of the tunnel. The recent means are printed in italics : — 





Ka per gram 

 XlO' 2 . 



Th per gram 



X u> 5 . 



Granite and gneiss of the 1 



Finsteraarhorn Massif J 



1 J 



7-2 

 6-0 



1-8") 

 $-64 



\ Altered sediments of the "\ 

 Usernmulde J 



4-9 



2-6 



0-97 



1-70 



Schists, &c. of the St. 1 

 Gothard Massif } 



3-9 



2-S 



3-4 



2-0 



1-18 



1-10 



(V.M 



0-44 



Altered sediments of the 1 

 Tessinniulde J 



• 



