AGE OF THE EARTH—HOLMES 235 
64 66 68 70 72 
74 76 78 80 82 
Ficure 3.—To illustrate the relationships between the ‘abundances of Pb208, Pb7, and Pb%8 
(relative to Pb20¢=1) in lead samples prepared,from common ores of lead. 
of the invaluable data provided by 
Nier. He and his coworkers have 
determined the relative abundances 
of the isotopes in 25 samples of lead 
from common lead minerals of various 
geological ages. The results, graphi- 
cally summarized in figure 3, reveal 
that the relationships between Pb™®, 
