﻿290 



Mr. A. L. Fletehe 



r on the 



unaltered chalk from Co. Antrim, referred to in the table 

 on the limestones. Any increase is perhaps hardly to be 

 expected in view of the low radioactivity of the sill itself, 

 but there has been probably no loss. The contact zone of 

 limestone with granite was similarly examined in the case of 

 a somewhat weathered specimen from Switzerland, with 

 similar results. 



T) , Radium 

 Kock. 



in grs. per gr. 



Limestone at contact 0-3x10-12. 



Granite at contact 0-7x10— 12. 



A positive result was obtained, however, from examination 

 of the contact zone of the altered Silurian Slates with the 

 igneous intrusion in a hand-specimen from Glendalough, Co. 

 "Wicklow, when the following figures were obtained : — 



Rock. 



Radium 



in grs. per gr. 



Greisen abutting 



0-8 

 3-3 



Schist 





The mean radioactivity of the altered Silurian Schists of 

 Leinster, based on two experiments, is about 1*1 x 10~ 12 — 

 normal for its type. There is therefore a positive, if 

 moderately small augmentation in this case, the more 

 emphasized by the relative poorness of the Greisen in radio- 

 active materials. The Schist itself shows not only the high 

 thermal and dynamic metamorphic characters shown by all 

 these Schists, but the development of such secondary minerals 

 as tourmaline indicates a certain amount of pneumatolytic 

 action involving the removal of some bodies from the igneous 

 and their recrystallization in the sedimentary rock. 



The intrusive rock is not petrologically similar to the main 

 granite-mass, but rather a greisen, and shows in addition a 

 band of alteration, and the production of abundant white 

 mica. Biotite is absent. In view of its magmaticaliy 

 differentiated origin, a relatively high radium content might 

 reasonably have been presupposed, and its deficiency in 

 radium relative to the main granite-mass, whose mean by the 

 fusion method is about 2 x 10~ 12 gr. per gram, is noteworthy, 



