SECT. 2] 



MICROTOPOGRAPHY 



449 



by currents along the bottom. There are two jjossible explanations, neither of 

 which has any proof. It is possible that the particles, although pebble-size, are 

 of organic origin, such as empty shells, and therefore have a much lower density 

 than mineral particles. This would enable small currents to move them easily 

 into banks (cf. current ridges described below). Secondly, the pebbles may in 

 fact consist of a living colony of an abyssal mollusc which spreads laterally over 

 the normal pelagic sediment. Only closely integrated photography and sampling 

 can answer this question. 



Fig. 17. 37°- 20'N, 33° 25'W. Depth 838 m. Peak on Mid-Atlantic Ridge, southwest of 

 Azores. Area of picture 1| by 2| m. (Photo by L.G.O.) 



Sandy bottom with coarser dark particles heavily disturbed by the tracks of 

 surface organisms. 



d. Sand 



The identification of sand on the sea floor from photographs depends chiefly 

 on the eff"ects on the sediment surface of disturbances by currents and faunal 

 activity (Fig. 17). It is barely possible to distinguish it from oozes by its grain 

 size except in the case of very coarse sands. The essential physical proj^erty 

 that can be used to distinguish sand from ooze is its incoherence and, hence, its 

 inability to maintain steep sloj)es. After disturbance it will take up its charac- 

 teristic angle of repose. On this basis, a sandy sediment containing enough 

 clay material to make it coherent will be classed as mud or ooze. 



On the continental margins, the sands observed are primarily quartz sands 

 derived from continental erosion and calcareous detritus. In the jjresence of 

 comparatively strong currents they are well sorted and appear very frequently 



