120 THE OCEAN FLOOR 



the order of 1 to 2 millimeters of red clay per 1,000 

 years." 



How shall the irregularities found by KroU in the 

 vertical distribution of radium on cores from the 

 equatorial Pacific Ocean be explained? Variations in 

 the rate of total sedimentation or in the rate of ionium 

 precipitation is an explanation which meets with dif- 

 ficulties when the variations in radium content are very 

 abrupt and the character of the deposit is apparently 

 homogeneous. An alternative explanation — that the 

 variations may be due to a migration of radium to 

 lower levels, leaving its mother element ionium behind 

 — depends on present determinations of ionium by a 

 photographic method. This method has been evolved 

 at the Institut des Recherches Nucleaires in Brussels by 

 Picciotto and his co-workers, in collaboration with and 

 with some support from the Oceanographic Institute in 

 Goteborg.^ In nuclear photographic plates exposed to 

 the radiation from a sediment sample from which all 

 radioactive elements except thorium and ionium have 

 been eliminated, the short tracks produced by their 

 alpha particles are counted. The results of this impor- 

 tant work indicate that there exists radioactive equilib- 

 rium between radium and ionium in the maxima of the 

 radium curves, except in the uppermost surface layer, 

 where there is an excess of ionium. 



That the superficial radium maxima near the sedi- 

 ment surface cannot well be uranium supported seems 

 obvious, since a concentration of radium as high as 50 



