ABSOLUTE DATING 29 



rings with sliarp boundaries in many different minerals, we can feel 

 safe that the constants of radioactive decay have, indeed, not changed, 

 even over the extremely long period of geological history. 



Consequently, it is almost certain that the physical clocks operating 

 on the decay of natural radioactive elements really give absolute 

 dates. It is, however, somewhat presumptive to say that they date in 

 years. The clocks have been working at the same rate over the 

 billions of years, but our methods of reading them are still far from 

 the reliability achieved in laboratory physics. In rocks we are never 

 quite sure that we do really measure exactly the remaining amount 

 of the parent mineral, nor all the atoms of the ultimate stable ele- 

 ment actually produced by the radioactive decay. Consequently an 

 error of, say, 5 per cent may be expected in quite good rock age 

 measurements. This margin, in a rock one billion years old, would 

 still leave us with an uncertainty of fifty million years. 



So the normal geologic methods, which ascertain the stratigraphic 

 sequence of relatively older and younger rocks, cannot altogether be 

 replaced by absolute dating techniques. Instead, the latter only supply 

 us with a number of fixes in which our relative stratigraphical data 

 can be arranged. In modem geological dating both methods, absolute 

 and relative, must go hand in hand. 



Absolute age of igneous rocks and sediments 



Moreover, all absolute dating of older rocks is made on igneous 

 rocks and ore minerals, and not on sediments. With this method, it 

 is possible to establish the birthday of a given igneous rock or ore 

 mineral. This means that we take the age of an igneous rock to be 

 the time elapsed since it crystallized from a molten magma in the 

 earth's crust, and with an ore mineral the time elapsed since it was 



fossils, which have not been obliterated by metamorphosis accompanying 

 the orogeny. 



The relative age of intrusion A is : younger than the lower series of 

 sediments, and older than the upper series. The relative age of in- 

 trusion B is : younger than the upper sediments and a great deal 

 younger than the lower sediments. 



Absolute dating is possible for the two intrusions. The absolute age 

 of the sediments can, however, only be approximated as either older or 

 younger than the dated intrusions. 



