SECT. 3] 



PELAGIC SEDIMENTS 



701 



(a) 



(b) 



Fig. 28. (a) Aggregates of microcrystalline harmotome and nontronite. The aggregates 

 contain Ba2+ and Fe2+ as principal cations, and often considerable amounts of thormm 

 (in one instance 2200 ppm). The density varies owing to substitution of lighter cations 

 for barium; the highest density measured is 3.2. The aggregates have a high magnetic 

 susceptibility. The microcrystallites rarely exceed a few microns in length, (b) Radio- 

 autograph of harmotome-nontronite aggregate showing emerging alpha particle 

 tracks, [(b) from Picciotto and Arrhenius, unpublished.] 



