34 THE OCEANOGRAPHY OF THE NORTH-WESTERN OCEAN [PART I 



its chemical composition a preponderance of silica, this mineral 

 constituting from one-half to two-thirds of its Aveight, these 

 benthic deposits are characterised by their abundance in lime. 

 Calcium carbonate constitutes from one-half to two-thirds of their 

 weight. They consist largely of the broken down and com- 

 minuted remains of the skeletons of such animals and plants as 

 form calcareous shells or skeletons. These are corallines or 

 calcareous algae, corals, echinoderms such as starfishes, sea-urchins, 

 &c., polyzoa, the spicules of calcareous sponges, and the shells of 

 moUusca and Crustacea. When these animals and plants die their 

 remains form the sea floor, or at least a considerable part of it in 

 some localities. In speaking of the terrigenous deposits which are 

 characteristic of the sea bottom within the 2000-contour line we 

 must include these benthic or "neritic" deposits which are not 

 derived from the land, but from the bottom-living animals which 

 inhabit this region to a greater extent than that underneath 

 deeper water. (See Appendix.) 



The term "pelagic" was first applied to the deposits of the 

 ocean floor in the deeper parts of the ocean and at a great distance 

 from land. Outside the 2000-metre line the nature of the sea 

 floor is strikingly different from that within the continental area. 

 While the remains of animals living on the sea bottom do indeed 

 occur, these are not abundant and do not constitute a notable 

 proportion of these deposits, and the only material composing 

 them which has a terrestrial origin is such as has been derived 

 from the material discharged during volcanic eruptions. Pumice 

 from such a source finds its way into the sea directly or after 

 carriage by rivers, and after floating on the surface of the sea 

 becomes water-logged and sinks to the bottom there to undergo 

 decomposition. Volcanic ash too is carried in the atmosphere and 

 finally may fall on the sea and sink. These substances occur in 

 the muds or oozes which form the floor of the deep oceans, but 

 mud or sand derived from the detrition of the land is not found 

 there. Instead we find a material which is almost entirely derived 

 from the remains of the animals and plants which live in the body 

 of sea water extending from the surface down to the bottom. 



At a limited distance from the land and still within the limits 

 bounding the area of the terrigenous deposits may be found the 



