PLEOCHROIC HALOES 145 



some scores of millions of years, for here is the 

 same effect in a biotite crystal (Fig. 9). Along 

 what are apparently tubular passages or cracks 

 in the mica, a solution, rich in radioactive sub- 

 stances, has moved ; probably during the final 

 consolidation of the granite in which the mica 

 occurs. A continuous and very regular halo has 

 developed along these conduits. A string of 

 halo-spheres may lie along such passages. We 

 must infer that solutions or gases able to establish 

 the radioactive nuclei moved along these conduits, 

 and we are entitled to ask if all the haloes in this 

 biotite are not, in this sense, of secondary origin. 

 There is, I may add, much to support such a con- 

 clusion. 



It must not be thought that the under-exposed 

 halo is a recent creation. By no means. All are 

 old, appallingly old; and in the same rock all 

 are, probably, of the same, or nearly the same, 

 age. The under-exposure is simply due to a 

 lesser quantity of the radioactive elements in the 

 nucleus. They are under-exposed, in short, not 

 because of lesser duration of exposure, but be- 

 cause of insufficient action; as when in taking a 

 photograph the stop is not open enough for the 

 time of the exposure. 



The halo has, so far, told us that the additive 

 law is obeyed in solid media and that the in- 

 creased ionisation attending the slowing down of 

 the ray obtaining in gases also obtains in solids ; 

 for, otherwise, the halo would not commence its 



