298 REX AND GOLDBERG [CHAP. 5 



mid-latitudes over the break in the tropopause (Feely, 1960). This occurs over 

 the meander path of the jet stream making it somewhat unhkely that we shall 

 be able to see a mineralogical contribution of stratospheric fall-out among the 

 much more abundant accumulation of purely tropospheric fall-out in pelagic 

 sediments, 



2. Eolian Materials in Marine Sediments 



Darwin (1846) was aware of the atmosphere as a path of transport of dust 

 from the continents to the sea and may well have been correct when he stated 

 that "a widely extended deposit may be in the process of formation" by such 

 mechanisms. Over the past 100 years or so, a modest literature has evolved 

 emphasizing the importance of wind-transported materials in marine sedi- 

 mentation. 



Three genetic classes of eolian transported materials are evident : 



(1) Extra-terrestrial materials 



(2) Solids of biological origins from the continents 



(3) Solids of inorganic origin from the lithosphere, 

 including debris from volcanic activity. 



A. Extra-terrestrial Components 



In 1876 Murray directed attention to some rather curious spherical particles 

 in deep-sea sediments which had a highly magnetic character. Their sizes were 

 seldom greater than 0.2 mm and more commonly in the 30-60 micron range. 

 These spherules normally have an inner nucleus of iron, surrounded by a mag- 

 netic crust. The metallic phase suggested a similarity to iron meteorites and led 

 Murray to postulate a cosmic origin for the spheres, a hypothesis later confirmed 

 by chemical analyses (Smales, Mapper and Wood, 1958 ; Castaing and Fredriks- 

 son, 1958; and Hecht and Patzaic, 1957). Murray attributed the oxidized 

 coating to a combustion of the iron nucleus while passing through the earth's 

 atmosphere ; this conclusion was experimentally tested by Castaing and 

 Fredricksson (1958) who found that the contents of nickel, cobalt and iron 

 were in like concentration in both the shell and metallic core. 



No geographic distributions of such spherules have as yet been established, but, 

 on the basis of their concentration in deep-sea sediments as a function of depth 

 below the sediment-water interface, Pettersson and Fredriksson (1958) suggest 

 that the frequency of such spherules may have been higher in recent times than 

 in the past. They further compute the yearly accruement of the spherules to 

 the earth's surface to be of the order of 2.5 to 5 x 10^ grams annually. 



B. Biological Components 



The pioneering studies of Ehrenberg (1847) on Darwin's samples of Sahara 

 dust collected over the Atlantic and other dust samples showed the abundance 

 of diatom fragments in materials collected from the atmosphere. The occurrence 

 of freshwater diatoms in marine sediments (Lohman, 1941 ; Kolbe, 1955, 1957) 



