32 FORMATION OF THE EARTH 



It is only since the discovery of radium that we have been 

 able, by a simple calculation, to approach the question of the 

 age of the earth, or, speaking more accurately, the age of some 

 of the minerals constituting its solid crust. Strutt has drawn 

 attention to the fact that some of these minerals contain at 

 the same time uranium, itself radio-active, and a certain 

 proportion of a substance resulting from the decomposition of 

 uranium, viz. helium. He has calculated that n million 

 years are necessary for a gramme of uranium-oxide to produce 

 a cubic centimetre of helium. From the quantity of helium a 

 mineral contains we can thus arrive at an estimate of the 

 quantity of uranium it possessed at the time of its formation, 

 and the time necessary for the transformation of this uranium 

 into helium. The calculation yields 622 million years for the 

 zirconium in the archsean rocks of Ontario ; 145 million years 

 for certain Devonian hematites ; 400 million years for some 

 minerals, and only 40 million years for others. On the other 

 hand, certain Swedish and American rocks yield figures of 

 1,300 and 1,400 million years, and specimens from Colombo, in 

 Ceylon, reach 1,600 million years. These different figures enable 

 11s to determine the age of the terrainin which the buried minerals 

 analysed are found, and thus to calculate the time that has 

 •elapsed between the geological periods corresponding with the 

 laying down of the different strata. It may be, of course, that 

 the differences are in part due to the fact that the minerals 

 ■examined are not found in conditions equally favourable for 

 the preservation of helium. 



A further problem has been propounded, namely the date of 

 the appearance of life on the earth, which must necessarily have 

 been subsequent to that at which the temperature of the earth's 

 surface fell below 100 degrees. Lord Kelvin was the first to 

 interest himself in this problem, and arrived at an approximate 

 solution by treating the earth, for the purpose of calculation, 

 .as a homogeneous ball brought to a red heat and then allowed 

 to cool. According to this hypothesis, which is very far 

 removed, it is true, from the actual facts, Lord Kelvin, 

 according to the secondary hypothesis favoured, found the 

 answer to vary between 20 and 100 million years. But the 

 calculation can be made in another way by reckoning the time 

 necessary for the different geological layers to be formed. We 

 :are assuming, of course, that during the geological periods which 



