CHAP. 33] THE PRESERVED RECORD: PALEONTOLOGY OF PELAGIC SEDIMENTS 881 



discussion is limited to those small skeletal fragments and placoid scales which 

 can be obtained in considerable numbers by sieving a few grams of any sample 

 of pelagic clay. It is not at present possible to use these for stratigraphic 

 correlations, but further investigation to determine their applicability seems 

 desirable because they are practically the only microfossils preserved in many 

 clays lacking siliceous and calcareous remains. 



G. Radiolaria 



Attempts (summarized by Kobayashi and Kimura, 1944) have been made to 

 assign ages to radiolarian assemblages on the basis of the proportion of cyrtoi- 

 deans to sphaeroideans (two great subdivisions of the Radiolaria). Although it 

 is indeed true that cyrtoideans seem to have developed later than sphaeroideans 

 in geological time, the proportion of species belonging to the two groups in any 

 assemblage can be expected to depend largely on ecological conditions as well 

 as on age. The only reliable method of radiolarian age determination is through 

 comparison of assemblages of species from all available parts of the geological 

 column. This approach has recently resulted in correlations which agree with 

 those based on other microfossil groups (Riedel, 1957, 1959a). Relatively few 

 workers have investigated fossil Radiolaria, and there is no doubt that future 

 descriptions of assemblages of known age will enhance their value for strati- 

 graphic interpretations. 



H. Reworking of Older Microfossils into Younger Sediments 



In the absence of supporting evidence, it is not safe to assume that the micro- 

 fossil assemblage in any given sample from the surface of the deep-sea floor 

 includes only Recent species. In addition to turbidity currents transporting 

 and mixing sediments in and near areas of relatively high relief, there seem to 

 be abyssal currents capable of eroding and transporting sediments over some 

 areas of low relief on the ocean floor. Evidence of deep-sea erosion and transpor- 

 tation is summarized by Heezen (1959). In view of the fact that late Tertiary 

 sediments may be covered by Quaternary deposits only a meter thick in areas 

 of slow sediment accumulation, it is not surprising that relatively minor 

 agencies of disturbance are able to expose Tertiary sediments on the deep-sea 

 floor where the general relief is of the order of 100 m. 



Recognition of reworked Tertiary elements in Recent sediments is hampered 

 by the incompleteness of our knowledge of the Tertiary representatives of 

 pelagic microfossil groups. Where reworked Tertiary microfossils can be re- 

 cognized in Quaternary sediments, it may also be concluded that inorganic 

 Tertiary sediment components have accompanied the microfossils, and this has 

 considerable bearing on the interpretation of the chemical or mineralogical 

 character of the sediment as a whole. If a sufficient number of closely spaced 

 sediment samples could be collected from around an isolated Tertiary outcrop, 

 it might be possible to determine the strength and direction of the water 



