140 VERTEBRATE PALAEONTOLOGY 



Potassium method 



This is one of the most promising new methods for the dating 

 of rocks. Potassium is one of the most common elements in rock 

 and its isotope K 40 occurs as 0-0119% of the natural element. K 40 

 decays to form Ca 40 by beta emission, the decay having a half-life 

 period of 1-35 X 10 9 years. This would take us back 1,000 

 million years. There is a second path of decay open to K 40 . 

 It can capture an electron and turn into A 40 . This latter system 

 has not yet been fully worked out but it is probable that both 

 methods will prove of great use in dating rock strata (Ahrens 1956). 

 A paper by Mayne, Lambert and York (1959) shows that whereas 

 the previous methods estimated the Upper Cambrian to be 450 

 million years old, the K method gives a value of 650 million 

 years. This would mean that these rocks are some 200 million 

 years older than previously thought, a point of considerable 

 interest since it indicates the size of the errors to be expected in 

 estimates based on other methods. 



Subjective Methods 



We have, then, as yet, no accurate objective clock that will 

 allow us to determine the absolute age of the majority of the rocks 

 of the world. Instead we have to go mainly on stratigraphical 

 data and there too we find several problems. 



From a stratigraphical point of view one cannot state the 

 absolute age of a given piece of rock ; all that one can do is estimate 

 the relative age of the rock, and where this is based on the thick- 

 ness of the deposit and the rate of deposition, the results are bound 

 to be only approximate. If the various levels are complex and 

 stratified, then it is often possible to determine contemporaneity, 

 especially when the strata are close together. The situation is 

 much more complex when one has to decide if rocks in different 

 parts of the world are contemporaneous. Thus the great Caledonian 

 Oregony gave rise to a marked separation of the Devonian and 

 Silurian rocks in North- West Europe, but this separation is not 

 found in the Appalachian syncline. Similarly the rock strata in 

 Maryland, U.S.A., show an unbroken series of deposits from the 

 Late Silurian to Early Devonian without any sign of a boundary. 



To what extent can one place the fishes found in England, 

 Scandinavia, Germany and the United States in their correct 



