SECT. 3] 



PELAGIC SEDIMENTS 



675 



studied the metallic spherules and have investigated the nature of the silicate 

 chondrules. The size distributions both of the nickel-iron spherules and of the 

 olivine-pyroxene chondrules found on the deep ocean floor (Fig. 12) are in 

 remarkable disagreement with the tentative size distribution of micrometeorites 

 derived by impact, counting from the satellites 1958 Alpha and 1959 Eta 

 (LaGow and Alexander, ojp. cit.). An exponential increase in number of particles 

 with decreasing size below 10-20 [j. is indicated by the satellite observations, 

 although cosmic particles from the ocean floor show a marked decrease in 



No. of spherules 

 in deep sea 

 sediment 



40 



30 



20 



IO- 



C- 



No of satellite 

 impacts of 

 micrometeorites 

 per m^sec 



10' 



10" 



5 10 20 40 



10- 



Fig. 12. Size distribution of cosmic spherules from pelagic sediments, and from micro- 

 meteorites in oviter space. Shaded histogram = iron spherules; Ime-boiuided histogram 

 = silicate spherules from pelagic sediments (data from Hunter and Parkin, 1960); 

 filled circles = impact observations from satellites 1958 Alpha and 1959 Eta (computed 

 from data in LaGow and Alexander, 1960, assuming an average particle density of 3.0). 



frequency below 25 y. and a comparatively large number of big (90 to 300 [i) 

 silicate chondrules. This corroborates the idea that the cosmic material found 

 on the ocean floor represents debris of meteorites, as suggested by Fredriksson 

 (1959), rather than the original cosmic dust. 



Large numbers of magnetic spherules of unknown origin and composition, 

 ranging in size from less than a micron to several microns, are frequently 

 observed in sediments (see, for example, Crozier, 1960). Analyses of such bodies 

 indicate a heterogeneous origin. Many of the magnetic spherules previously 

 assumed to have been derived from outer space have been shown by Fredriks- 

 son (1961) to consist of volcanic glass with inclusions of magnetite and metallic 

 iron. Others, in size-range of 0.5-5 y.. appear to consist of goethite and might 

 have been accreted by marine bacteria. It therefore appears necessary to 

 define individual cosmic spherules not only on the basis of shape and mag- 

 netic properties but also on chemical composition. Considering the difficulties in 

 23 — s. Ill 



