Chapter III 



MEASURING TIME IN GEOLOGY 



RELATIVE AND ABSOLUTE DATING 



Measuring time in geology is a young science — even for geology, 

 which is itself one of the youngest of natural sciences. Two decades 

 ago the first comprehensive volume on this subject appeared, the 

 highly original Dating the Past of Professor Zeuner of London. The 

 same title could well have served as a heading for this chapter. It is, 

 however, not fear of plagiarism which led me to choose another 

 heading. Instead, the difference in wording is meant to convey the 

 extremely rapid development over the last twenty years of the 

 science of measuring the time elapsed during the geological periods. 



Professor Zeuner gave full attention to various methods of meas- 

 uring time, as they then existed. But apart from the actual counting 

 of years, as applied in dendrochronology, where the yearly growth 

 rings of trees are examined, or in the so-called varved clays, which 

 also exhibit a clear yearly sequence of coarser and finer layers, the 

 methods now employed were still in their infancy. These newer 

 methods are all based on radioactive decay of various natural ele- 

 ments. Two decades ago, they were still in the process of develop- 

 ment. It was only by the postwar advance in electronic instrumenta- 

 tion that they gained a practical value for geology. 



Still, it is well to stress that even so short a time ago Zeuner's 

 book was a considerable step forward in that it focussed attention on 

 the possibilities existing for real dating, for measuring in years the 

 time elapsed since certain geological phenomena took place. Until 

 then, practically all 'dating' in geology was still done by the time- 

 honoured method in which only a relative age was established. 



This is, of course, common knowledge for a geologist. But just 

 because it is such common, everyday usage, this relativity of most 

 geologic dating is not always properly stated. As we are interested in 



