Radioactivity of the Leinster Granite. Ill 



examination of the published results that exceptional richness 

 in the uranium-radium elements is not attended by a corre- 

 sponding richness in the thorium series. This is more 

 especially evident in the radium-thorium ratio of Vesuvian 

 lavas, when contrasted with the ratio for other lavas (Joly, 

 Phil. Mag. Oct. 1909). 



It seems desirable that the investigation of the radium 

 content of rocks should be accompanied by an estimate of 

 the thorium content also; not only to elucidate the question 

 of a possible relationship between the radium and the thorium 

 content, but in the interests of the geological applications of 

 radioactivity. The same solution may be used in both deter- 

 minations, and the experimental observation is actually more 

 readily made and confirmed by repetition of the test in the 

 case of thorium than in the case of radium. 



The results, it may be seen, are obtained from materials 

 taken at points ranging from the most northerly exposure of 

 the granite, and along the western and eastern side of the 

 chain to nearly the southern limit. The specimens from 

 Glendalough and Glenmalure may be regarded as from the 

 central axis. 



The specimen from Dundrum — No. 7 in the table — comes 

 from a recent boring at that place, and was taken from a point 

 190 feet below the surface level. 



The experiments therefore probably yield an approximation 

 of say 1*7 x 10 -12 gr. per gr. to the mean radioactivity of the 

 mass, so far as it is capable of being determined by the method 

 adopted, and as carefully applied as was found possible. 



That there is radium throughout the entire great mass 

 seems quite certain ; and it may be safely inferred that in a 

 material so homogeneous a somewhat similar distribution of 

 radioactive materials prevails at all points near the surface ; 

 and indeed it is probably allowable to assume that the 

 vertical distribution of radioactive elements throughout the 

 mass is not very different. 



In conclusion I have to thank Prof. Joly both for his 

 initiation of the work, and for his continual assistance 

 throughout its performance. 



Geological Laboratory, 



Trinity College, Dublin. 



Nov. 24, 1910. 



