IV DEPTHS AND DEPOSITS OF THE OCEAN 139 



land-slopes throughout the basin are, as in the Pacific, steeper 

 than those of the Atlantic. The ratio between the two areas 

 on either side of the 500-fathoms line is again much less than 

 in the case of the Atlantic, the area less than 500 fathoms in the 

 Indian Ocean being over 2 million square miles, as compared 

 with less than i million square miles for the area between 500 

 and 1000 fathoms. 



The Indian Ocean, unlike the other two, is completely land- 

 locked to the north. The area with depths less than 1000 fathoms 

 forms a zone of varying width along the main land-masses, a fairly 

 wide zone round the various island groups, and extends into the 

 Red Sea and Persian Gulf. The area with depths between Indian Ocean 

 1000 and 2000 fathoms is made up of the greater part of the ^i^p^ti,^''^ 

 Bay of Bengal and the Arabian Sea, a fairly wide belt along between 1000 

 the east coast of Africa, a much narrower one along the western fathom?" 

 shores of the Sunda Islands and Australia, a large expanse 

 between Tasmania and the Antarctic continent which narrows 

 considerably towards the west, and a large tract extending from 

 lat. 30" to 55' S. and long. 35° to 94 E., forming a plateau on 

 which are situated the islands of Prince Edward, Crozet, 

 Kerguelen, M'Donald, Heard, St. Paul, and Amsterdam, as 

 well as one or two small isolated areas. 



With the exception of a comparatively small area in the Indian Ocean 

 Southern Ocean, about lat. 60° S. to the south of Australia, the 2000 feAom"^ 

 area with depths between 2000 and 3000 fathoms is a continuous 

 one, though interrupted by areas of deeper and shallower water ; 

 it is continuous with the corresponding area of the Atlantic, but 

 distinct from that of the Pacific, being separated from it by the 

 rise that runs southwards from Tasmania to the Antarctic 

 continent. 



The areas exceeding 3000 fathoms in depth are referred to 

 under the next heading. 



Deeps. — As already indicated, those areas of the ocean-floor 

 covered by more than 3000 fathoms (5486 metres) of water 

 have been called Deeps, and, though occupying a relatively Deeps. 

 small proportion of the ocean-floor, estimated in the aggregate 

 at about 9 million square miles, they are extremely interest- 

 ing from an oceanographical point of view. Map II. shows 

 the distribution of these deeps throughout the great ocean 

 basins, according to the present state of our knowledge, and it 

 will be seen that the total number is fifty-seven, of which thirty- Number of 

 two occur in the Pacific, five in the Indian Ocean, nineteen in '"°^^" ^^^^' 



