S o SCIENCE PROGRESS 



velocity into the surrounding mineral— the alpha rays — occasion 

 the colour changes referred to. These changes are limited to 

 the distance to which the alpha rays penetrate ; hence the halo 

 is a spherical volume surrounding the central substance. 1 



The time required to form a halo can be found if on the one 

 hand we could ascertain the number of alpha rays ejected in, 

 say, one year from the nucleus of the halo, and, on the other, if 

 we determined by experiment just how many alpha rays were 

 required to produce the same amount of colour alteration as we 

 perceive to extend around the nucleus. 



The latter estimate is fairly easily and surely made. But to 

 know the number of rays leaving the central particle in unit 

 time we require to know the quantity of radioactive material in 

 the nucleus. This cannot be directly determined. We can only, 

 from known results obtained with larger specimens of just such 

 a mineral substance as composes the nucleus, guess at the 

 amount of uranium, or it may be thorium, which may be present. 



This method has been applied to the uranium haloes of the 

 mica of County Carlow. 2 Results for the age of the halo of from 

 20 to 400 millions of years have been obtained. This mica was 

 probably formed in the granite of Leinster in late Silurian or in 

 Devonian times. 



The higher results are probably the least in error, upon the 

 data involved ; for the assumption made as to the amount of 

 uranium in the nuclei of the haloes was such as to render the 

 higher results the more reliable. 



This method is, of course, a radioactive method, and similar 

 to the method by helium storage, save that it is free of the risk 

 of error by escape of the helium, the effects of which are, as it 

 were, registered at the moment of its production, so that its 

 subsequent escape is of no moment. 



Review of the Results 



We shall now briefly review the results on the geological age 

 of the earth. 



By methods based on the approximate uniformity of denuda- 

 tive effects in the past, a period of the order of 100 millions of 

 years has been obtained as the duration of our geological age ; 

 and consistently whether we accept for measurement the sedi- 



1 Phil. Mag, y March 1907 and February 1910; also Bedrock, January 1913. 

 1 Joly and Rutherford, Phil. Mag., April 1913. 



