128 THE OCEAN FLOOR 



centimeter. If the radium in the nodules is unsupported 

 by its predecessors in the radioactive element series, it 

 should decrease to 50% of its surface value after 1,600 

 years, to 25% in another 1,600 years, etc. In this way 

 it becomes possible to measure the rate of radial growth 

 of a nodule, the results showing a one-millimeter incre- 

 ment in the course of 1,000 years. This is the first 



Zone C 



i 



I Zone b 



' Zone CI ( nucleus) 



-ProfUesC- 



Fig. 42. Radium determination in a manganese nodule 



known case where the disintegration of radium itself 

 has been used for chronological purposes.^ 



It is naturally possible to ascribe part of the high 

 radium content in the uppermost layers of red clay to 

 an adsorption from sea water of unsupported radium 

 by the dispersed manganese peroxide present in the 

 sediment. However, considering the rapid rate of dis- 



