836 



EBICSON 



[chap. 31 



though the total length of the profile is 1300 km. When the distances between 

 cores are much smaller, as for examj^le the distance between two cores from 

 the vicinity of a seamount, essentially complete time equivalence of faunal 

 zones is a reasonable assumption. 



Fig. 1 shows the general distribution of dextral and sinistral populations of 

 Globorotalia truncatulinoides in the North Atlantic. Here again the pattern of 

 distribution is on an ocean-wide scale. Study of widely scattered cores has 

 shown that this pattern has persisted for at least several thousand years, a 

 time interval amply sufficient to have permitted thorough mixing of the popula- 

 tions. Evidently there is some environmental condition which tends to maintain 

 the present distributional pattern. Coiling dominance may, therefore, be 

 regarded as characteristic of the waters of certain regions much in the same 

 way that a particular salinity, temperature or oxygen content may characterize 

 a water-mass. Presumably changes in coiling direction with depth in the cores 

 reflect wide geographical shifts in environmental conditions during the late 

 Pleistocene. On theoretical grounds it seems improbable that the areal distribu- 

 tion of controlling conditions in the open Atlantic could ever have been on any 

 other than a very broad scale. This implies that zones determined by changes in 

 coiling dominance in cores ought to be very nearly isochronous over distances 

 of at least hundreds of kilometers, if not thousands. 



4. Some Examples 



The general locations of the suites of cores discussed in the following section 

 are shown in Fig. 1. The geographical positions and depths of the individual 

 coring stations are given in the following table. 



Table I 

 Position and Depths of Coring Stations 



