SECT. 2] 



MICKOTOPOGKAPHY 



445 



bands of vesicularity suggest successive flows of lava subsequently fractured 

 and rounded by the erosive effect of sediment-laden currents. Small angular 

 cobbles on a deep-sea ridge are shown in Fig. 9. On the Mid-Atlantic Ridge, 

 recent earthquake and volcanic activity breaks up and dislodges rocks and can 

 create screes of angular boulders and cobbles (Fig. 10). The variable covering 

 of manganese on the boulders suggests that they have been recently moved 

 and the pale uncovered basal surface exposed. 



Manganese nodules, formed in situ on the sea floor, have frequently been 

 photographed in the Pacific (Dietz, 1955; Menard and tShipek, 1958) (Fig. 11), 



Fig. 11. 42° 50'S, 125° 32'W. Depth 4560 m. S.W. Pacific Basin. Area of picture 2h by 

 2| m. (U.S. Navy Electronics Laboratory photo by C. J. Shipek.) 



Large and small manganese nodulea or thickly encrusted rocks resting on a cal- 

 careous ooze. Note complete absence of sediment cover. 



and more recently in the western Atlantic (Elmendorf and Heezen, 1957 ; 

 Heezen, Tharp and Ewing, 1959) (Fig. 12), and in the Scotia Sea. In the Pacific 

 they occur in sufficient quantities to constitute a potentially economic source 

 for manganese, copper, cobalt and nickel. Sometimes they are extremely well 

 rounded (in the Scotia Sea) and at other times they are moderately angular. 

 They vary in size from microscopic up to half a metre or more in diameter 

 and can be recognized in photographs from their uniform distribution over the 

 bottom and from the fact that they are found in areas where clastic and erratic 

 boulders are very rare. Nodules of phosphorite (Fig. 13) have been photo- 

 graphed, dredged and described by Emery and Dietz (1950). 



In high latitudes, the proportion of erratic rocks caused by melting icebergs 

 becomes appreciable and it is necessary to distinguish them from those locally 

 derived. Although it may be possible to do this solely on the proportions of 



