SECT. 2] MICROTOPOGRAPHY 461 



animal in the sediment. Many of the mounds show the imprint of a five- 

 cornered star on the top (Figs. 19 and 37), suggesting that they can be attributed 

 to an asteroid or ophiuroid. Where there is a sharp change of colour of the 

 sediment in the top few centimetres, the mounds have the colour of the lower 

 layer, showing that the material has been brought up from iinderneath (Fig. 30). 

 Another possible origin of the smaller mounds is that they have been formed 

 from excreted material after digestion by mud-feeding animals such as holo- 

 thurians. Studies of holothurians in the shallow waters around Bermuda 



Fig. 30. 35° ll'N, 15° 20.5'W. Depth 2248 m. Seamoimt northeast of Madeira. Area of 

 picture H by 4 m. (Photo by L.G.O.) 



Level sedunent covered by the workings of small bottom-burrowing organisms 

 which have brought mounds of light calcareous ooze on top of the darker surface. 

 Note the disturbed patches around the gorgonian (at the top) and the sponge (lower 

 right). 



(Crozier, 1918) showed that in 1.7 square miles, 500 to 1000 tons of sand passed 

 annually through them, being equivalent to a layer of 1 cm thickness every 100 

 years. In deep water the population is very much less, but the sedimentation 

 rate is low and excretions of this sort may contribute significantly to the 

 sediment disturbance, and may, in fact, alter the chemical composition 

 appreciably. 



The second group consists of holes or depressions in the sediment. They are 

 not so numerous in general as the mounds but in certain areas they are promi- 

 nent. Occasionally the photographs show some organism in the hole but there 

 is seldom sufficient detail for identification. An interesting feature of the holes 



