SECT. 3] ORGANIC TRANSPORTATION OF MARINE SEDIMENTS 787 



near five specimens of Holothuria atra from a reef flat at Guam showed that the 

 finer grains are dissolved first, causing the excreted residual sand to have a 

 coarser median diameter than the total which was ingested. Unlike worms, 

 holothurians move laterally as far as 50 m per day, producing more lateral than 

 vertical transportation of sand. 



Many other invertebrates also act as sediment transporters, but those whose 

 role is best known live in the intertidal region, and even there they are only of 

 minor importance as compared to inorganic and other organic agents. Accord- 

 ingly, they will not be further considered here. 



8. Comparison with Inorganic Transporting Agents 



Sand and stones are transported by inorganic agents over, on and under the 

 water surface to eventual deposition on the ocean floor. According to Rex and 

 Goldberg (1958) high-altitude jet streams may be responsible for the presence 

 of fine-grained quartz in sediments at mid-latitudes. Other winds carry seaward 

 so much desert silt and sand from the Sahara Desert as to discolor sails of 

 ships (Darwin, 1887, p. 5) and to make an appreciable contribution to the ocean- 

 floor sediment in the east central Atlantic (Radczewski, 1939). The writer has 

 noted similar contributions of wind-blown sediment in the Red Sea, Persian 

 Gulf and Yellow Sea. Darwin mentioned that the dust settling on to the At- 

 lantic consists largely of diatoms, 65 of 67 species being of freshwater origin. 

 Such may well be the origin of the thin layers of freshwater diatoms recently 

 found in cores in the east central Atlantic (Kolbe, 1957), rather than the 

 extreme eustatic or diastrophic movements proposed by Landis (1959) to 

 convert the sea floor into sites of freshwater lakes. 



Layers of volcanic ash are widespread in oceanic sediments, being well 

 known in cores from the North Atlantic (Bramlette and Bradley, 1942), 

 Mediterranean (Mellis, 1955) and Pacific (Menard, 1953; Worzel, 1959). The ash 

 owes its presence, of course, both to vulcanism and wind. Coarser ejecta from 

 volcanoes must make its way to distant sites by floating, an easy feat owing to 

 the high porosity of pumice. Probably the chief contributing volcanoes are 

 those which have grown upward from the sea floor to near sea-level. During the 

 past decade eruptions from such volcanoes have been reported off Japan 

 (Niino et al., 1953), off Mexico (Richards, 1958), near Guam and in the Azores. 

 Large quantities of pumice were produced in the nearly simultaneous eruptions 

 off Japan and Mexico, and rounded pieces floated as far as 12,000 km from the 

 Mexican source. The writer has noted much pumice stranded on beaches of the 

 Mediterranean Sea probably from Greek or Sicilian volcanoes. As pointed out 

 by Ladd (1960), floating pumice may transport for long distances mollusks, 

 corals and other attached invertebrates. The same high porosity permits pieces 

 of dried coral to float alone for long distances (Kornicker and Squires, 1962). 



Non-volcanic sediments denser than water can also float on calm bodies of 

 water owing to surface tension. Patches of floating sand are common on tide 

 pools (Emery, 1945) and have also been seen on a quiet bay of Japan (Kotaka 



