The Age of Pleocliroic Haloes. 645 



this an estimate of the quantity of radioactive substances 

 which may be concerned in generating the halo. 



It is evident that while we can obtain the numerical values 

 involved in (a) with a considerable degree of accuracy, 

 certain assumptions enter into the numerical values required 

 in (b), which render them uncertain within particular limits. 

 Thus, while the dimensions o£ the nucleus may be determined 

 with fair accuracy, and its mineral nature inferred with 

 considerable confidence, we have no means, at present, of 

 ascertaining the amount of radioactive material it contains. 

 This amount can only be stated on the basis of analyses made 

 on specimens of the mineral large enough to be subjected to 

 examination. Evidently, however, a probable minor limit to 

 the age is obtained by making our assumptions exceed by a 

 safe margin any of the ascertained results. 



The following experiments and observations have been 

 confined to the brown mica, Haughtonite, of Co. Carlow. 

 In this mica, haloes due to the uranium family of elements 

 are very beautifully defined, and are found in every stage of 

 development ; from the smallest, due to uranium or ionium, 

 to fully darkened haloes completed to the range of RaC. 

 The mica is of a rich clear brown colour, with high lustre 

 and perfect cleavage. Particulars with reference to this 

 mica and of its contained haloes have been given elsewhere *. 



The age of the containing granite is probably late Silurian 

 or early Devonian. The evidence for this is found in its 

 relations with the surrounding rocks. It has upheaved and 

 metamorphosed slates of Silurian age, and fragments of it 

 are contained in Old Red Sandstone sediments which rest 

 unconfonnably on the upturned edges of the Silurian slates, 

 and overlap directly on to the granite. Its upheaval is 

 generally referred to the Caledonian earth-movements f . 



Some of the haloes may have been formed somewhat 

 subsequent to the crystallization of the containing mica. This 

 is suggested (a) by the fact that some of the crystallized 

 nuclei giving rise to the haloes are oriented in the plane of 

 cleavage ; their greatest dimensions coinciding with that 

 plane as if they had been developed in situ ; and (b) by the 

 existence of veins traversing the biotite which evidently 

 contain radioactive substances, seeing that they are bordered 

 by the characteristic staining, and faithfully reproduce the 



* Phil. Mag. April 1910 ; Proc. Royal Dublin Soc. xiii. p. 73 ; ' Bed- 

 rock,' Jan. 1913. 



t Mineralogical and geological particulars respecting the granite are 

 given "by Sollas (Trans. Royal Irish Academy, xxix., Jan 1891). The 

 Geological Survey of Ireland Reports on Co. Dublin and Co. Carlow 

 &c, may also be consulted. 



