PLEOCHROIC HALOES 153 



in some nuclei may amount to only a few atoms. 

 Even in the case of the larger nuclei and more 

 perfectly developed haloes the quantity of radium 

 involved is many millions of times less than the 

 least amount we can recognise by any other 

 means. But the delicacy of the observation is 

 not adequately set forth in this statement. We 

 can not only tell the nature of the radioactive 

 family with which we are dealing; but we can 

 recognise the presence of some of its constituent 

 members. I may say that it is not probable the 

 zircons are richer in radium than I have assumed. 

 My assumption involves about 3 per cent, of 

 uranium. I know of no analyses ascribing so great 

 an amount of uranium to zircon. The variety 

 cyrtolite has been found to contain half this 

 amount, about. But even if we doubled our 

 estimate of radium content, the remarkable nature 

 of our conclusions is hardly lessened. 



It may appear strange that the ever-interesting 

 question of the Earth's age should find elucida- 

 tion from the study of haloes. Nevertheless the 

 subjects are closely connected. The circum- 

 stances are as follows. Geologists have esti- 

 mated the age of the Earth since denudation 

 began, by measurements of the integral effects of 

 denudation. These methods agree in showing 

 an age of about io 8 years. On the other hand, 

 measurements have been made of the accumula- 

 tion in minerals of radioactive debris the helium 

 and lead and results obtained which, although 



