fall within the limits of gravelly sand. 

 A sample containing less than 5 percent 

 silt and sand and 95 percent gravel 

 would be termed a gravel. 



In addition to the textural term, ap- 

 propriate modifiers are added to indi- 

 cate composition. If a sample is com- 

 posed of 95 percent sand-sized 

 particles, and if these grains are prin- 

 cipally fragments of shells, then the 

 type name would be shell fragment 

 sand. 



Heavy mineral contents 

 Reagents : 



1. Bromoform, density 2.9. 



2. Ethyl alcohol. 



Procedure : A dried sediment sample 

 of about 5 gms. weight is placed in a 

 central open-ended tube inside of a60- 

 ml. centrifuge tube. The sample is 

 stirred together with 40 ml. of heavy 

 liquid and then spun at 2000 r.p.m. for 

 5 minutes. The sample is stirred again 

 and the mixture spun for a second 5 

 minutes at 2000 r.p.m. After centri- 

 fuging, the inner tube containing the 

 light fraction is removed after 

 stoppering its upper end and then 

 drained into a paper filter. The heavy 

 liquid is caught and returned to use. 

 The filtered sedinnent is washed with 

 alcohol and the washings saved for 

 reclamation of the heavy liquid. After 

 the washing, the light minerals are 

 allowed to air dry. The outer tube con- 

 taining the heavy grains is poured into 

 a second paper filter and treated in 

 similar fashion. Both papers are folded 

 over and marked with the sample num- 

 ber. The weights of the light and heavy 

 mineral fractions are recorded and the 

 percentages of each calculated on the 

 basis of total sample weight. 



GENERAL CHARACTER OF 

 BOTTOM MATERIALS 



The pattern of the sediment textural 

 types is indicated on figure 4. It is 

 evident that the position of the Gulf 

 Stream has an important bearing on the 

 sediment character if one notes that 

 the boundary of the high carbonate 



sediment (shown by the 50 percentile 

 contour) essentially lies along the 

 boundary of the north-flowing stream 

 and the inner mainly south-flowing 

 currents. The sedinnents seaward of 

 this approximate boundary are domi- 

 nantly composed of the tests of 

 Foraminifera. Inshore from this zone 

 the sediments are varying combinations 

 of quartz sands and shell fragment 

 sands. In general, the proportion of 

 shell fragments and of carbonate per- 

 centages increases from shore to the 

 shelf edge, and from north to south 

 along the inner shelf. Off Florida, the 

 shelf sediments are dominantly shell 

 fragment (shell hash) sands and gravels. 



In a few of the stations closest to shore 

 a silty sand appears that probably rep- 

 resents the present sediment contribu- 

 tion from the land sources. The roughly 

 linear trend of gravelly sand patches 

 may indicate the approximate position of 

 old strand lines during lower sea levels 

 of the glacial ages. Finer sediments ap- 

 pear on the upper slopes and on the outer 

 portion of the Blake Plateau. However, 

 the coarsest bottom materials are pres- 

 ent along the inner margin of the Blake 

 Plateau and under the main portion of 

 the Gulf Stream. The finer sediments 

 present in this zone to the south may 

 represent finer materials lying behind 

 the gentle sill that fornns the terminus 

 of the Florida Straits. Beyond this low 

 barrier, the main current apparently 

 sweeps most fine nnaterial to the north. 



Organic contents are very low over 

 much of the shelf and outer Blake 

 Plateau. This plus the large proportion 

 of mollusk fragments would appear to 

 be the result of slow rates of deposition 

 and long exposure at the surface. It is 

 probable that the major part of the shelf 

 sediment cover is relict or residual 

 from the pre -Recent. This same pattern 

 is evidenced on many shelf areas of the 

 world (see Emery, 1952, 1954, and 

 I960). 



A striking feature of the Blake Plateau 

 is the presence of manganese nodules. 

 Bottom photographs in the files of the 

 Woods Hole Oceanographic Institute and 

 the Liamont Geological Observatory 

 show that much of the area of the Blake 

 Plateau is covered by cobble and pebble 



17 



