SECT. 3] TURBIDITY CURRENTS 769 



the cores could be used to determine the areal distribution of various species at 

 the present day, while samples from lower levels of the cores would show, in an 

 orderly manner, the development of present-day species through at least 

 Pleistocene and Recent time". 



However, the results of his study forced him to conclude "that the situation 

 is actually more complicated than was expected. Late Tertiary Radiolaria were 

 found to be reworked into Recent sediments, and evidence of disconformities 

 were found in the sediment sequences. Re-examination of some of the samples 

 from the Challeiiger Expedition showed that Tertiary sediments outcrop on the 

 deep-sea floor. Erosion, and reworking of older Radiolaria into younger sedi- 

 ments, obscure phylogenetic developmental sequences in the long sediment 

 cores. ..." 



The "epic poem" contained in these layers of sediment had, it seemed, 

 been frequently reduced to a font of jumbled type as cryptic to read as a 

 typesetter's pie. 



Kolbe (1957, 1957a), who found pre-Quaternary diatoms in several Pacific 

 and Indian Ocean cores, concluded that their presence is "unimportant" and 

 "accidental, although no plausible explanation of the occurrence can be given". 

 Riedel, however, "speculated that a more or less continuous agency (? current) 

 was transporting Radiolaria on or near the sea floor " and that "the transporting 

 agency has acted either continuously, or at such short intervals as to obscure 

 intermittance". 



Recent seismic-reflection studies indicate that the thickness of sediments 

 beneath the abyssal floor is much greater than that beneath the adjoining 

 abyssal hills and Mid-Atlantic Ridge. This suggests that ocean currents may 

 gradually concentrate fine sediments in the deep ocean basins. 



In the light of Riedel' s conclusions, it is not surprising that difficulty was 

 encountered in the stratigraphic interpretation of the chemical analyses of 

 these cores (Berrit and Rotschi, 1956) and that, in general, the ionium and 

 radium methods of dating of deep-sea sediments have been more often a failure 

 than a success (Kroll, 1955; Volchok and Kulp, 1957). Probably the most 

 important result of the later studies has been the conclusion that conditions of 

 uniform, undisturbed, unreworked sedimentation required by these dating 

 methods only rarely prevail on the deep-sea floor. 



C. Sub-bottom Echoes 



The character of the echo returned to the echo-sounder from the deep-sea 

 floor can be examined in great detail by use of the high-resolution Precision 

 Depth Recorder. 



The echo may be sharp and discrete, diffuse and prolonged, single, double or 

 multiple. Over distances of a few hundred yards, the bottom may appear 

 absolutely smooth or completely disrupted on a minute scale. 



Sub-bottom echoes are frequently observed in deep-sea echograms. In some 

 cases the sediment horizon responsible for the sub-bottom echo has been identi- 



