A DEFINITION OF EVOLUTION 



which the new method led indicated that the earth was vastly older than 

 had been generally believed, and the method was received with skepti- 

 cism. But it has since become the standard by which the accuracy of other 

 methods of dating is judged. The method is based upon the fact that 

 uranium 238 will slowly disintegrate to produce lead with an atomic 

 weight of 206 and helium. The rate at which this occurs is calculable. 

 With any definite amount of uranium, one half of the molecules will break 

 down, forming lead and helium, in the course of 4,510,000,000 years. As 

 this figure is independent of the actual quantity of uranium originally 

 present, it is called the "half-life" of the element. Now, if a uranium-bearing 

 rock is found, the ratio of uranium to lead 206 can be determined, and 

 from this, utilizing the half-life, the interval since the formation of the 

 rock can be calculated. 



Recently, additional geochemical methods have been developed. Potas- 

 sium 40 yields calcium 40 and argon 40; rubidium 87 yields strontium 87; 

 thorium 232 yields lead 208; and uranium 235 yields lead 207. Each of 

 these parent elements has its characteristic half-life, ranging from 126,- 

 000,000 years to as much as 60,000,000,000 years. 



While the "lead" method is now universally accepted, it has serious 

 limitations. Uranium is not a common element, and it is often found in 

 geological formations which are not readily fitted into the geological time 

 scale. The introduction of additional geochemical methods helps to over- 

 come these limitations, and so there are good grounds for hope that an 

 accurately dated scale of geological time may be available in the near 

 future. For the present, it may be said that the oldest dated rocks are 

 more than 3,000,000,000 years old, and the oldest which might have borne 

 life are about 2,000,000,000 years old. Around three quarters of that 2,000,- 

 000,000 years during which life might have existed passed before the 

 beginning of the Cambrian Period, with which the useful fossil record 

 begins, for a lead measurement of the age of a late Cambrian deposit gave 

 a figure of 440,000,000 years. The next exact determination is in the earlv 

 Permian Period, at an age of 230,000,000 years. Thus the entire Paleozoic 

 Era probably lasted about 300,000,000 years; the Mesozoic about 130,000,- 

 000 years; and the Cenozoic about 75,000,000 years up to the present. 

 There is an accurately dated deposit from the beginning of the Eocene 

 Epoch which places this at 58,000,000 years ago. The dates are few, but 

 fortunately they are widely scattered in geologic time. But as )'et, accurate 

 determinations are not available for the extent of any of the periods or 

 epochs. 



The Radio-Carbon Method. Another radioactive method has been de- 

 veloped which shows promise for shorter range determinations of age, 

 up to perhaps 40,000 years. It has been found that fixing organisms utilize 

 a small, but constant proportion of their organic carbon in the radioactive 

 form. The half-life of radioactive carbon is 5568 ± 30 years. Hence, rem- 

 nants of bone, wood, or other carbon-containing remains of dead or- 

 ganisms can be assayed for their radio-carbon content. The difference 

 between the average amount in fresh tissue and in the fossil may then be 

 treated as being due to radioactive disint(\<:;rati()n, and the age of the 



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