Radioactivity of the Leinster Granite, 103 



Throughout the biotite of the Leinster granite pleochroic 

 halos are abundant, and they are occasionally seen in the 

 white mica. In the latter case they are less conspicuous. 

 Primary muscovite often includes biotite flakes in which 

 halos are plentifully developed, while they are absent or only 

 faintly visible in the surrouuding muscovite. 



Professor Sollas considers the corroded edges of these 

 included plates as evidence for the derivation of the mus- 

 covite from originally large crystals of biotite, by a process 

 of magmatic corrosion. "In the Leinster granite, primary 

 muscovite may be regarded as to a great extent the product 

 of the action of the magma on the primarily formed biotite." 

 This view appears to involve an equal distribution of radio- 

 active primary minerals, e. g. zircon, allanite, uraninite (?), 

 in both primary muscovite and biotite. 



It is interesting to note that the recent advance in our 

 knowledge of the origin of halos bears upon this question of 

 the genesis of the muscovite from the biotite. Thus the fact 

 that a zircon may he found in the muscovite and the segment 

 of a corona, having the zircon for centre, in a closely 

 adjoining flake of biotite, is no proof that biotite once 

 extended to and surrounded the zircon, which would, in fact, 

 have affected the biotite by a rays sent across the intervening 

 muscovite. 



It has been found impossible in the experiments which are 

 cited below to connect definitely the radioactivity of the rock 

 directly with the visible abundance of dark mica present. 

 Indeed, the highest result obtained was upon a somewhat 

 gneissic granite from Grlendalough which contained much 

 silvery white mica. 



Prof, the Hon. H. J. Strutt has shown (Proc. R. S. 

 lxxviii. A. p. 150) that in a granite, the heavier minerals 

 include most of the uranium-radium elements, and that 

 zircon was the most radioactive of the constituents in the 

 granite examined by him. This result has been recently 

 confirmed by J. W. Waters (Phil. Mag. June 1910) working 

 in Prof. Stratt's laboratory. 



Biotite being a mineral of early consolidation, and forming 

 around the still earlier nuclei of the zircon and apatite, comes 

 to possess in this way a greater radioactivity than minerals 

 of later consolidation. From this point of view the radio- 

 activity of the biotite is dependent upon its place in the order 

 of consolidation, and its presence or absence does not affect 

 the radioactivity of the rock. The inferiority of some of the 

 muscovites is readily accounted for in the view advocated by 

 Prof. Sollas, that much of it is derived from the alteration 



