SECT. 2] MICROTOPOGRAPHY 447 



rock types, photography can sometimes help by observing how they lie on 

 the bottom. Fig. 4 illustrates a case where dark erratic rocks, mainly igneous 

 and metamorphic, are lying at random on top of a crumbling bedrock of lime- 

 stone. 



c. Pebbles and granules 



Pebbles and granules generally occur in aggregates where they are con- 

 veniently described by the term gravel. In this size range it is much less easy 

 to classify the different rock types since the normal resolution of the photo- 

 graphs is insufficient to examine individual particles. Each of the three pro- 

 cesses which produce boulders and cobbles also produces finer gravel, and, 

 where it occurs together with coarser material, it is reasonable to surmise a 

 common origin (Fig. 10). But there are other areas where the largest particles 

 present are no more than a few centimetres in diameter and where identification 

 is extremely difficult unless samples are taken (Figs. 14 and 15). 



Fig. 14. 47° ll'N, 27° 19'VV. Depth 2798 m. East bank of median valley of Mid-Atlantic 

 Ridge. Area of picture 4 by 5 m. (Photo by N.I.O.) 



Shingle of very varied composition lying on a substratum of pteropod and Olobi- 

 gerina ooze. Samples of shingle recovered from the trigger weight of the camera 

 include quartz basalts and limestones. This is probably ice-rafted from the north. 

 Note the disturbance of the shingle by animal holes and two ophiuroids in the fore- 

 ground. 



One of the puzzles arising from underwater photographs is the formation of 

 banks of what appears to be pebble-size gravel, but which have extremely sharp 

 boundaries with what is evidently underlying Glohigerina ooze (Fig, 16). The 

 sharpness of the boundary precludes the possibility that the pebbles have 

 settled from above, and the pebble size makes it unlikely that they are moved 



