130 THE OCEAN FLOOR 



ganese nodules has been proved by Koczy and con- 

 firmed by J. Poole and Christine Mathews in Dublin 

 by a photographic method. 



Summing up, we may say that of the radioactive 

 elements present in sea water thorium and ionium are 

 particularly rare, owing to their precipitation. The 

 isotope ionium, constantly being produced from the 

 dissolved uranium, is also constantly being removed 

 by precipitation from sea water on the bottom, where 

 it gives rise to its descendant, radium. In the deposit its 

 concentration is decreasing with age, that is with in- 

 creasing depth below the sediment surface, although 

 this decrease is not as a rule so regular as it should be 

 under completely undisturbed conditions of sedimenta- 

 tion. 



The content of uranium in sea water appears to be 

 fairly constant at present but may have varied in earlier 

 epochs because of a changing rate of transportation to 

 the ocean and because of its rate of extraction, together 

 with the extraction of organic residue ("sapropel"). 

 With very rare exceptions the uranium present in the 

 sediments is limited to a few units of the sixth decimal 

 place of gram per gram. The equiUbrium value of 

 uranium-supported radium, therefore, is generally of 

 the order of one unit of the twelfth decimal place 

 gr Ra / gr, that is, a small fraction of the radium found 

 in the uppermost layers of the red clay and other abyssal 

 sediments. It is perhaps inevitable that press reports 

 of lectures on the radioactive elements in the ocean 



