368 THE DEEP SEA 



era, where they have been used to measure ages of from some tens 

 of millions of years up to some billions of years. In the chronology 

 of the oceanic sediments presently available, the ages to be 

 measured are much younger — from several thousands up to several 

 millions of years. For reasons that will be stated later, we do not 

 as yet possess as great a number of well-proved radioactive 

 methods to measure intervals of time of this order as for longer 

 ones. I should like to describe briefly the present state of research 

 in this field and the outlook for progress in the near future. 



Four names representing three important stages in our progress 

 in this field should be mentioned here: Joly, who as early as 1908 

 had discovered the high radium content of pelagic clays; Hans 

 Pettersson, who in 1937 suggested the precipitation of ionium in 

 the ocean and who instigated a fruitful European collaboration in 

 the field of oceanic radiochemistry; and Piggot and Urry, who 

 from 1940 to 1950 laid the foundations of radioactive methods for 

 the chronology of pelagic sediments. 



Principles of Radioactive Methods of Age Determination 



Let us briefly recall the well known principles for the radioactive 

 methods in geological chronology. A group of radioactive atoms 

 can serve as a geological chronometer because it undergoes an 

 irreversible evolution, an evolution which progresses at a constant 

 rate, independent of physical or chemical conditions (or at least 

 of terrestrial conditions). This evolution is their spontaneous 

 transformation into stable atoms. 



Let R be a radioactive nuclide with X the probability that this 

 nuclide will spontaneously disintegrate in unit time. This X is a 

 constant characteristic of every nuclear species and is independent 

 of the chemical or physical conditions of the medium. By dis- 

 integration, R will be transformed, either directly or indirectly, 

 into a stable nuclide S. Should the transformation be indirect, that 

 is, by way of other radioacti\-e nuclides, R forms a radioactive 

 family. 



Let iVo be the number of atoms of R present at time to. This 

 number will decrease exponentially with time owing to the dis- 



