DEEP-SEA RADIUM 129 



integration of such unsupported radium in relation to 

 the very slow rate of accumulation of the abyssal sedi- 

 ments, such adsorbed radium should have practically 

 disappeared from the upper sediment layer below a 

 depth of 10 to 20 millimeters and cannot, therefore, 

 have contributed to the radium contained in shghtly 

 lower layers. 



Finally, as regards the long-lived radioactive element 

 thorium, it follows from geochemical considerations 

 that most of the thorium transported to the ocean by 

 rivers becomes precipitated over the shelf and the 

 uppermost parts of the continental slope, which ex- 

 plains the relatively high concentration of thorium in 

 shallow-water sediments. Earlier attempts to measure 

 the amount of thorium contained in sea water gave only 

 upper limits for its concentration.^^ Koczy, working 

 in Goteberg, was the first to compute the thorium 

 content in sea water at between 10"^ and 10'^ 

 gr Th / liter, that is, less than 1 % of the uranium 

 present. ^^ Whereas in continental rocks and sediments 

 there is an excess of thorium over uranium in the 

 proportion of two to three over one, the reverse is the 

 case in ocean water. Thanks to Picciotto's photographic 

 method of determining the ionium present in sediment 

 samples, their content of thorium has also been deter- 

 mined, giving values of about 6 X 10 '^^ gr Th/gr.^- 

 This implies that in the deep-sea sediments also there 

 is an excess of thorium over uranium in the proportion 

 of three to one. That thorium is also present in man- 



