1 3 6 
effect of the active rays on the enclosing mineral can be clearly 
perceived under the microscope. For a distance of about one- 
twentyfifth of a millimetre in every direction from the surface of 
the active mineral the enclosing hornblende is discoloured, and 
this discoloured portion of the matrix is usually more strongly 
pleochroic than the unaffected portion. Such pleochroic halos, 
as they are called, are spheroidal or ellipsoidal, according to the 
shape of the crystal of zircon, etc., which gives rise to them. 
It is interesting to note that they were found to be abundant in 
the hornblende of the Wodgina schists. The phenomenon is of 
course known in many other parts of the globe, particularly, so 
far as this State is concerned, in rocks from Kalgoorlie, Ravens- 
thorpe, and the Darling Ranges. 
