374 Prof. J. Joly on tie 



be shown that a special sort of: selective absorption for one 

 or more of the elements likely to bring in error is exercised 

 within the minerals dealt with, the error can be apprised at 

 its true worth by comparative observations npon associated 

 substances which do not contain appreciable amounts of the 

 parent radioactive bodies, and which have been exposed to 

 like vicissitudes of history. 



The earliest determination of age by the radioactive method 

 is, so far as I am aware, that made by Rutherford (Phil. Mag. 

 Oct. 1906, p. 368). The helium in a specimen of fergusonite 

 was determined by Ramsay and Travers as amounting to 

 1*81 c.c. per gram. The mineral contained about 7 percent, 

 of uranium. From this Rutherford deduced the age as about 

 240 millions of years. The geological position of this mineral 

 is not specified, nor is the possible influence of thorium taken 

 into consideration. 



The principal development of the method by helium ratio 

 is due to Strutt, whose work upon the subject has appeared 

 in five papers in the Proceedings of the Royal Society 

 (1908-1910). These experiments deal with phosphatized 

 fossil remains and nodules, haematite and other iron ores, 

 zircons, and sphenes. Some of these determinations are 

 evidently not available as an estimate of the time since their 

 formation, being plainly deficient in helium. Such results 

 of course strengthen the conviction that loss of helium must 

 occur in some cases. 



The results arrived at by Strutt are not always concordant. 

 Thus we find two sphenes of Archaean age and from the one 

 locality (Renfrew Co.) affording 222 and 715 millions of 

 years; and again two Archaean sphenes from the one locality 

 (Twederstrand, Norway) 213 and 449 millions of years. 

 Zircons show for Palaeozoic time 140*8 to 321 millions of 

 years. Here the lower figures are supported by results on 

 haematite. This one mineral gives for the time since the 

 Eocene age 30*8, since the Carboniferous 141, and since 

 the Devonian 145 millions of years. Limonite gives for 

 post-Carboniferous time 145 million years. These are 

 closely agreeing results. Other iron ores give, however, 

 inconsistent results. All are, of course, reconcilable if we 

 assume that the lower results are in every case due to loss of 

 helium. It is a little unfortunate in this connexion that the 

 minerals used for the greater ages are more retentive in their 

 nature (sphene and zircon) than the substances dealt with for 

 determination of the lesser periods of time. 



Strutt, in his final paper, selects from his results the 



