THE AGE OF ROCKS 101 



than in any other way the age of the stratum in 

 which they occur. In a uranium mineral, for 

 example, each I per cent, of lead in terms of the 

 quantity of uranium signifies the lapse of a period of 

 80,000,000 years. Errors of course are possible, if 

 lead should have been an original constituent of the 

 mineral, but these are minimised by taking a large 

 number of different minerals. On the other hand 

 every cubic centimetre by volume of helium per gram 

 of uranium in a uranium mineral signifies 9,000,000 

 years, and as here helium, being a gas that forms 

 no compounds, cannot have been initially present, and 

 as, moreover, some will have escaped the age of the 

 mineral by this method is a minimum, whereas the 

 age by the lead content may be too high. The 

 carboniferous rocks tested by this new method appear 

 to have an age of some 350,000,000 and the oldest 

 Archean rocks of over 1,500,000,000 years. 



The actual production of lead has not yet been 

 proved directly in the same way as the production of 

 helium has, though, but for the war, in all proba- 

 bility this would now have been accomplished. But 

 even without the actual direct proof of this kind there 

 is practically no room for doubt on the point. Indeed 

 by a very important development, about which a few 

 words may be said in conclusion, we know that not 

 only uranium but also thorium both produce the 

 element lead as the final product, and though the 

 lead from uranium is absolutely identical chemically 

 and spectroscopically with the lead from thorium, 

 yet they are different. Stranger still, the lead which 

 chemists are familiar with as one of the elements is 

 probably a mixture of both kinds. 



We have seen that the expulsion of an a-particle 

 ought to lower the atomic weight of the element 

 expelling it by 4 units, 4 being the atomic weight of 

 helium. In its transformation into radium, uranium 



p 



