878 RiEDEL [chap. 33 



(1958) in the Mediterranean, and by Stubbings (1939) in the Arabian Sea. 

 Similar alternations of diatom assemblages have been described from northwest 

 Pacific sediment cores by Jouse (1960). Such sequences can be related to 

 Quaternary climatic changes, particularly when account is taken also of litho- 

 logic changes throughout the cores, as has been done, for example, by Bramlette 

 and Bradley (1940) in the North Atlantic, by Arrhenius (1952) in the Pacific, 

 and by Wiseman (1954) in the equatorial Atlantic. By these means, it is a 

 relatively straightforward matter to trace, qualitatively, changes in certain 

 oceanographic conditions through the Quaternary, and to correlate one core 

 with another in any suitable region; but few investigators have yet obtained 

 sufficient data to determine the actual positions of boundaries between water 

 masses during the various stages of the Quaternary. On the basis of their 

 foraminiferal investigations, Phleger et al. (1953) concluded that the northern 

 edge of the North Equatorial Current off the west coast of Africa may have 

 shifted southward through at least 10-15° of latitude during some parts of the 

 Quaternary; and Jouse (1960) found that the southern limit of the boreal 

 diatom assemblage east of the Kuriles, at present situated at about 42-45°N, 

 was displaced northward to about 50°N during the last interglacial period. 



6. Mieropaleontological Determination of Ages 



Passing from a consideration of the pelagic micropaleontology of the Quater- 

 nary to that of the Tertiary, we enter a field in which our knowledge is even 

 more fragmentary. The principal reason for this is that fossiliferous Tertiary 

 pelagic sediments are much less frequently encountered than those of Quater- 

 nary age; and when they are obtained it is difficult to date them accurately, or 

 to refer them to isochronous levels preparatory to applying them to the inter- 

 pretation of oceanic conditions at any given time. 



The degree to which any group of fossils is potentially useful for age deter- 

 minations depends primarily upon the areal extent of distribution of individual 

 species, and the length of time between origin and extinction of the species. 

 Species and assemblages of the planktonic micro-organisms here under con- 

 sideration have relatively large areas of distribution, and the species and genera 

 seem to have evolved at rates comparable with other fossil groups, and, there- 

 fore, all are potentially useful as age-indicators. Their application is limited by 

 such factors as inadequacy of the taxonomic systems currently in use, and the 

 small number of described, well-dated assemblages with which samples of 

 unknown age can be compared, 



A. Coccolithophorids 



The potentialities of these microfossils as indicators of age were pointed out 

 by Bramlette and Riedel (1954), and in recent years our knowledge of Tertiary 

 assemblages has been increased by the investigations of several European 

 workers (including Defiandre and Fert, 1954; Martini, 1958, 1959; Stradner, 



