FEATURES OF THE OCEAN BOTTOM 19 



occasionally using purely geographical terms, at other times 

 names of ships or oceanographers. To depths exceeding 3,000 

 fathoms he applied the term " Deeps," and these were named 

 exclusively after persons. 



In some instances more than one system has been used for 

 the same ocean, much confusion thereby being caused. For 

 the Pacific Ocean in particular there are four distinct systems 

 of naming the features of the ocean floor. Even at the present 

 time there is no general agreement amongst oceanographers 

 as to the nomenclature to be adopted. 



The relatively shallow area bounding continental masses is 

 termed the " Continental Shelf." This shelf belongs to, and 

 may be considered as a part of, the continental land. The 

 seaward boundary of this shelf may be taken, in the majority 

 of cases, to be the loo-fathom line, where there is usually a 

 sharp descent to great oceanic depths. In recent geological 

 periods this shelf formed a part of the dry continental land. 

 In high latitudes this shelf is characterised by the presence of 

 erratic boulders due to the Ice Age ; in tropical areas by the 

 growth of corals. The continental shelf is in general free 

 from the traces of volcanic action. In addition to the true 

 continental shelf there are pseudo-continental shelfs, entirely 

 cut off by deep water from the main continental masses. Of 

 these the Seychelles may be taken as an example. The 

 continental shelf is of considerable economic importance, 

 since it constitutes the spawning-ground of all marine fish of 

 economic importance, and on it the great sea fisheries of the 

 world are carried on. 



The continental shelf forms about 8 per cent, of the sea 

 area. The sea bottom here is composed of material derived 

 from the land masses. All changes in level of 100 fathoms 

 above or below sea-level have taken place in recent geological 

 times. Only occasionally is the continental shelf extremely 

 narrow e.g., the west coast of South America between 35 

 and 14 S. Usually in such localities the land exhibits the 

 characteristics of a raised shelf. 



