Pleochroic Halos. G37 



square in cross-section. Thorite is isomorphous with zircon, 

 and its presence in syenites has been demonstrated before 

 now. The inner radius is not sharply defined in this case ; 

 the measurement given is approximate only. Reference to 

 the table of calculated radii will show that the agreement 

 between observed and calculated values is in general very 

 striking. 



Stages in the Development of Pleochroic Halos. 



We have included in the preceding table of observations 

 only those h;ilos whose development appears to be nearly or 

 quite perfect. In some cases these halos may be described 

 as "over-exposed"; thaL is, the detail within the halo is 

 obliterated by excessive darkening of the medium. In others 

 we see the halo at an earlier stage; corona and pupil being 

 distinct, or the former even detached from the latter and 

 encircling it as a delicate ring. Plainly this last is an earlier 

 stage of development. But halos may be observed in still 

 earlier stages. We have been so fortunate as to find in a 

 single large crystal of biotite (var. haughtonite) from the 

 Leinster granite, uranium halos in every stage of develop- 

 ment. Those stages we shall presently describe ; but, first, 

 some account of the conditions attending the occurrence of 

 these halos will be of interest. 



The crystal of biotite, which measures about 2*5 cm. in 

 its greatest dimension, is enclosed within a fringe of silvery 

 white mica, a relation not uncommon in the Leinster granite. 

 This appears to be an original association, the muscovite 

 crystallizing after and in continuation of: the biotite. But 

 the same association is believed to occur as the result of meta- 

 morphic action in certain rocks. The boundary is abrupt 

 an I, even in the microscope, the demarcation is sharply de- 

 fined. Halos are developed only in the biotite, and principally 

 in the vicinity of the junction of the micas. The central 

 parts of the biotite crystal are halo-free and clear of all in- 

 clusions. The radioactive substance is often found extending 

 along cracks in the biotite, which then are intensely dark, 

 and may carry strings of halos, their nuclei contained in 

 the cracks, their spherical shells extending into the clear 

 biotite. In other places the halos are crowded in clusters or, 

 again, sc ittered without apparent connexion. Much radio- 

 active blackening has also occurred around elongated or 

 shapeless inclusions, or in stains and blotches. The radio- 

 active darkening never extends into the limpid muscovite; 

 a halo often appearing as bisected where it meets the colour- 



