26 MEASURING TIME IN GEOLOGY 



large numbers of atoms of a radioactive isotope are present, will 

 statistical laws govern the overall decay process. The probability that 

 a given fraction will decay in a certain time is constant for each 

 radioactive isotope. In that case it is possible to state how the total 

 number of radioactive atoms of a given isotope will decrease with 

 time. As the disintegration of individual atoms is random, the rate 

 of decay is proportional only to the number of parent atoms. This 

 constant of proportionality is called the decay constant. The decay 

 rate of a radioactive isotope is conveniently expressed in terms of its 

 half-life. This is the time for any number of atoms of that isotope to 

 decay to one half its original value (Russel and Farquar, 1960). 



Constancy of radioactive decay 



In all methods using radioactive decay for the measurement of 

 time in geological periods, there is one basic assumption. This is that 

 the decay rate did not vary even over these very long periods. Now 

 theoretical physics tells us this is so. According to theoretical physics, 

 radioactive decay is a nuclear process, which is innate in a given 

 atomic nucleus. Unlike the configuration of the peripheral electrons, 

 which determine the chemical properties of an atom, the nuclear 

 constitution cannot be changed through external influences. Neither 

 heat, cold, pressure nor changes in the electrical or magnetic field, 

 all of which have changed within certain rather narrow limits during 

 the history of the earth, have any influence on a nuclear process such 

 as radioactive decay. Experimental physics corroborates this doctrine 

 of theoretical physics, within the limits of the experiments devised so 

 far. The limits in geology, however, are not the extremes of heat or 

 cold, of gravity or of earth magnetics, or even of extreme pressures. 

 But one essential factor can never be duplicated in the laboratory, 

 i.e. the extreme length of time connate to geological processes. 



Is it justified, a geologist will ask, to extrapolate the doctrine of 

 nuclear physics, a science barely twenty years old, to geologic eras 

 some billions of years in duration? 



Pleochroitic rings 



Geology itself supplies an answer to this question in the curious 

 phenomenon of the pleochroitic rings. Many minerals contain darkly 

 coloured spots, which under the microscope appear as a number of 



