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FOSSIL FUXG1 



Several years ago a more exact method of estimating the earth's 

 age was provided from observations involving radioactive rocks, 

 which showed that atoms of uranium spontaneously decompose 

 into atoms of lead and helium, thus liberating radiations. By de- 



Fig. 81. Dial of geologic time clock. The proportion of time in each era 

 and of the most important divisions of each is indicated. 



termininp; the amount of uranium in uranium-containing rocks, to- 

 gether with the amount of lead associated, Professor A. Holmes 

 concluded that the Pre-Cambrian period began 1580 million years 

 ago. The primeval crust of the earth was formed earlier; hence 

 it may be concluded that 2000 million years constitutes a con- 

 servative estimate of the earth's antiquity. As shown by the 

 geological time clock, the age of the earth has been set at 500 

 million years as a minimum. Whether the earth's age is assumed 



