196 The Realm of Nature CHAP. 



261. The Indian Basin. The Indian Basin, protected 

 on three sides by the inner edges of the great Eastern 

 World Ridge into which it penetrates, is only half the size 

 of the Atlantic, and one-third of the Pacific, to which it 

 bears some resemblance. The greatest depth, over 3000 

 fathoms, is found in the eastern angle between the coasts 

 of north-west Australia and Java. The basin grows 

 gradually shallower toward the south, most gradually toward 

 the south-east. The western half is greatly diversified by 

 narrow ridges running north-eastward from Madagascar to 

 Ceylon, and rising in numerous groups of low islands above 

 the surface of the water. 



262. Islands and Shoals. Those islands which are 

 merely parts of the crests of the World Ridges separated by 

 shallow water from the mainland, and composed of similar 

 rocks, are termed Continental Islands. Oceanic Islands are 

 those which rise from the depressed half of the Earth and 

 have no geological relation to the neighbouring land. 

 Many of them are composed of volcanic rocks, and must 

 be viewed simply as the summits of ridges or submerged 

 mountains. Others are built up of the remains of living 

 creatures, and rise only a few feet above the surface of the 

 water. These ( 280-282) require a foundation before they 

 can be formed, and the foundation is usually a submarine 

 peak or ridge. A submerged peak, rising within a few 

 hundred feet of the surface, is called an oceanic shoal. It 

 was supposed at one time that very few shoals of this kind 

 existed, the bed of the ocean being looked upon as an 

 almost unbroken plain, but the recent explorations of 

 telegraph ships have revealed a large number of shoals in 

 all the oceans, in some cases rising precipitously from vast 

 depths. 3 Probably many more remain undiscovered, for 

 unless the lines of soundings across an ocean are run at 

 very close intervals, they might be passed over. 



263. The Transitional Area. From mean sphere level 

 the upward slope of the World Ridges is at first gentle, but 

 after a certain height in almost all places it becomes com- 

 paratively steep, in rare cases even forming a succession of 

 rocky precipices. Fig. 37 shows that the average slope 



