PLEOCHROIC HALOES 139 



the circle is very perfect and the central mineral 

 clearly defined at its centre we find by measure- 

 ment that the radius of the darkened area is 

 generally 0*033 mm. It may sometimes be 0*040 

 mm. These are always the measurements in 

 biotite. In other minerals the radii are a little 

 different. 



We see in the photograph (Fig. 3), much magni- 

 fied, a halo contained in biotite. We are looking 

 at a region in a rock section, the rock being 

 ground down to such a thickness that light freely 

 passes through it. The biotite is in the centre of 

 the field. Quartz and felspar surround it. The 

 rock is a granite. The biotite is not all one 

 crystal. Two crystals, mutually inclined, are cut 

 across. The halo extends across both crystals, 

 but owing to the fact that polarised light is used 

 in taking the photograph it appears darker in one 

 crystal than in the other. We see the zircon 

 which composes the nucleus. The fine lineated 

 appearance of the biotite is due to the cleavage 

 of that mineral, which is cut across in the section. 



The question arises whether the darkened area 

 surrounding the zircon may not be due to the in- 

 fluence of the radioactive substances contained in 

 the zircon. The extraordinary uniformity of the 

 radial measurements of perfectly formed haloes 

 (to use the name by which they have long been 

 known) suggests that they may be the result of 

 alpha radiation. For in that case, as we have 

 seen, we can at once account for the definite 



