PLEOCHROIC HALOES 155 



capable of yielding both helium and lead to the 

 rocks. Such atoms might have been collateral in 

 origin with uranium from some antecedent 

 element. Like helium, lead may be a derivative 

 from more than one sequence of radioactive 

 changes. In the present state of our knowledge 

 the possibilities are many. The change rate is 

 known to be connected with the range of the alpha 

 ray expelled by the transforming element; and 

 the conformity of the halo with our existing know- 

 ledge of the ranges is reason for assuming that, 

 whatever the origin of the more active associate 

 of uranium, this passed through similar elemental 

 changes in the progress of its disintegration. 

 There may, however, have been differences in the 

 ranges which the halo would not reveal. It is 

 remarkable that uranium at the present time is 

 apparently responsible for two alpha rays of very 

 different ranges. If these proceed from different 

 elements, one should be faster in its change rate 

 than the other. Some guidance may yet be forth- 

 coming from the study of the more obscure 

 problems of radioactivity. 



Now it is not improbable that the halo may 

 contribute directly to this discussion. We can 

 evidently attack the biotite with a known number 

 of alpha rays and determine how many are re- 

 quired to produce a certain intensity of darken- 

 ing, corresponding to that of a halo with a nucleus 

 of measureable dimensions. On certain assump- 

 tions, which are correct within defined limits, we 



