286 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1911. 



conditions. These sources of error would also tend to go on aug- 

 menting with the lapse of time. Unless, however, it can 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 compara- 

 tive observations upon 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. 1 The helium in a speci- 

 men of fergusonite was determined by Ramsay and Travers as 

 amounting to 1.81 cubic centimeters per gram. The mineral con- 

 tained about 7 per cent 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 experi- 

 ments deal with phosphatized fossil remains and nodules, haematite 

 and other iron ores, zircons, and sphenes. Some of these determina- 

 tions 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 (Ren- 

 frew County) 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, recon- 

 cilable if we assume that the lower results are in every case due to 

 loss of helium. It is a little unfortunate in this connection 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 following as 

 summarizing the data of his earlier papers: 



'■■ Phil. Mag., OCtt>b"eT ; S$Bjj p. &S. 



