MEASURING TIME IN GEOLOGY 33 



and thorium respectively. Assuming that all lead found was formed 

 only by these two decay processes, the age of a sample could be 

 'established' by using the formula for the half-life of the two parent 

 minerals. 



This obviously made little sense, amongst other reasons, because 

 lead not only contains the radiogenic isotopes ^"^^Pb, -""Pb and 

 ^•^^Pb, which are the end-products of the decay of uranium and 

 thorium, but also the isotope ^""*Pb, which is not radiogenic at all. 



Of course, at that time one was aware of the fallibility of this 

 assumption. But little sense is better than no sense at all. The method 

 was used, simply because there was no better one. Later work has 

 shown that most of the early measurements were far off the mark; 

 but, at that time, they served a definite purpose. They gave the first 

 more or less trustworthy indication of the stupendous length of 

 geologic time. In the early days of absolute dating, this fact was 

 already of paramount importance. Geologists, physicists and astrono- 

 mers had stressed and re-stressed their conclusions that the age of 

 the earth and of the universe must be inconceivably long. Here were 

 the first direct measurements of the age of rocks, of matter that one 

 could touch, which actually were that old. 



Mass spectrometry 



Because of the importance absolute dating has for an evaluation 

 of the setting in which the origin of life on earth took place, I had 

 better explain the workings of mass spectrometry a little further. 

 Many readers will be interested to know just what is the basis of 

 these absolute datings, covering such tremendously long periods. The 

 reader who is not interested in this somewhat more technical aspect 

 can skip the following pages and resume the narrative in the final 

 remarks of this chapter. He must, in that case, accept these dates at 

 their face value, even if he is a little dubious about such statements. 



As mentioned above, the basic instrument now in use in absolute 

 dating is the mass spectrograph or the mass spectrometer. In it 

 individual atoms are separated according to their mass or atomic 

 weight. Of a given sample the relative amounts of atoms of various 



Fig. 6. The decay rates of the uranium, actinium and thorium series 

 of natural radioactive elements (from Holmes, 1937). 



