38 MEASURING TIME IN GEOLOGY 



actual elements, most rocks, even those containing very minor quanti- 

 ties of these elements, can still be dated by both the potassium — 

 argon and the rubidium — strontium methods, as well as by various 

 lead techniques. Consequently, it is common nowadays to see a single 

 age determination of a given rock sample reported upon by some 

 ten or more authors. Some of these are responsible for the geologic 

 mapping and sampling, others for the chemical preparation, whilst 

 the rest has made the requisite isotope measurements for the various 

 types of age determination. Moreover, there are frequent inter- 

 laboratory checks made on parts of a single rock sample to ensure 

 reproducibility of the measurements. 



If we now look at the results, it is quite natural for a person not 

 acquainted with absolute dating to become bewildered by the figures 

 produced. Ages of millions and billions of years are discussed, and 

 if it is natural to distrust such pronouncements and to question their 

 validity, this reserve may have grown because of the many times 

 absolute dates have proved to be erroneous in the past. Moreover, 

 not only have many absolute dates suffered quite considerable cor- 

 rections, but these have also tended mostly towards older and still 

 older ages. So one quite naturally gained the impression that for 

 every year that these nuclear scientists investigated the age of the 

 earth, this age did rise a couple of million years. 



In a sense this development sometimes gave the impression of 

 bluffing, in producing older and older dates. My aim is to show that 

 absolute dating in geology is not bluffing, nor even guesswork any 

 more. It is based not only on a number of sound physical laws, but 

 on several parallel techniques by which the results can be checked 

 independently. 



On the other hand, we must admit that these techniques are still 

 difficult to apply. Not only do they ask the utmost of present-day 

 electronic instrumentation, but also the rocks which now form the 

 crust of the earth, which contain the physical clocks used, have, of 

 course, been subjected to all kinds of vicissitudes during the long 

 eras of their history. 



However, let us skip these details. In broad outline we may con- 

 clude that the absolute dates so far are to be trusted as as near an 

 approximation of the real age of the rocks in question as we may 

 possibly obtain at this moment. 



