Particle-by-particle, deep-sea deposition of pelagic clay, Area H, results In 

 the slowest rate of deposition, and this area shows the least slope of a straight-line 

 relationship between porosity and the clay-size fraction (Fig. 10). Each area In 

 this figure is represented by two lines marking iimlts of plotted values. Reference 

 Is made in the following discussion to an imaginary line, which is not shown in 

 Figure 10, located equidistant between the limit lines for each area. Area D, in 

 the vicinity of the Blake Plateau, is in a region of slow deposition, which is shown 

 by the presence of manganese nodules in the surficial layer of some cores and in 

 dredge hauls collected by the Hydrographic Office. A moderate amount of pelagic 

 deposition is indicated by the texture and mineralogy of Area C cores, and the re- 

 sulting relative rate probably was fairly rapid. Area G is close to land, but swept 

 by currents, and the expected rate of deposition is moderate. All other areas, A, 

 B, E, and F, have similar depositional environments on the continental slope. With 

 decreasing distance from land the relative rate of deposition should increase; this 

 results in the sequence: F - E - B and A. However, the amount of sediment contrib- 

 uted from land influences the sequence and a more probable one, from slow to fast, 

 is: E - F - B and A. The calculated relative deposition rate from these, and other 

 less important considerations, is (relatively slow to fast): H - D - F, F 6, and G 

 (about the same) - E - A, B, and C (about the same) . The observed sequence from 

 Figure 10 is (relatively slow to fast): H-D-F, F6, andG-A-E-C-B. 

 Differences between the two sequences are not considered significant. 



F . PLASTICITY AND THE ATTERBERG LIMITS 



Determination of limits — Measurement of the Atterberg liquid limit, LL, was 

 standardized three decades ago by use of a mechanical liquid limit machine designed 

 by Casagrande (1932a). Liquid limit is arbitrarily defined by the ASTM (1958, p. 1132) 

 as the water content at which two halves of a sediment cake will flow together for a 

 distance of 1 .25 cm (0.5 in) along the bottom of a groove separating the two halves 

 when the cup containing the cake is dropped 25 times for a distance of 1 cm at the rate 

 o'f two drops per second. An ASTM machine with a hard rubber base was used for all 

 tests, together with an ASTM grooving tool . The plastic limit, PL, test briefly con- 

 sists of measuring the lowest water content at which the sediment sample can be rolled 

 into threads 3 mm (one-eighth in) in diameter without them breaking into pieces (ASTM, 

 1958, p. 1137). 



Casagrande (1932a, p. 130; 1948, p. 922) reported that ovendrying fine-grained 

 samples, as prescribed by ASTM designations D 423-54T (Lll) and D 424-54T (PL), 

 radically affects the limits of organic sediment and less markedly the limits of inorganic 

 sediment; limits generally are higher for non-dried material . This relationship has been 

 confirmed by Selmer-Olsen (1953) and Rosenqvist (1955, p. 72) for marine clays of 

 normal salt content, although an opposite effect was found by Rosenqvist when clay 



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