212 The Realm of Nature CHAP. 



by volcanic upheaval and then reduced below sea-level by 

 the eroding action of waves, and some may have existed 

 originally at a suitable height. Others may have been 

 raised to the coral zone by ages of submarine sedimentation, 



being covered first by glob- 

 igerina ooze, then as the 

 depth was gradually dim- 

 inished by pteropod ooze, 

 and finally brought com- 

 paratively rapidly within 

 reach of reef- builders by 

 the accumulation of the 

 remains of sea-urchins, star- 

 fish, deep - sea corals, and 

 the like. The reef-building 



FIG. 4 i.-Murray's Theory of the origin of P ol 7P S raise a flat table f 



Coral Islands. The central volcanic solid rock, which, as it ap- 

 rock (solid black) is shown covered by 1,^*1, ( 



deep-sea deposits which build it up to proacnes tne SUrlace, grOWS 



the reef-building zone where an atoll is more rapidly on the circum- 

 formed. ' 



ference on account of the 



abundance of food supplied by ocean currents. The rim fin- 

 ally reaches the surface and cuts off the supply of food from 

 the polyps in the interior, which die, and the dead coral is 

 partly dissolved by the water, partly scoured out by tide and 

 waves, and so a lagoon is gradually hollowed. The outer 

 slope of the reef is alive, and ever growing outward. As it 

 becomes steep and wall-like, masses broken off by the waves 

 roll down to the bottom and form a more gentle slope or talus 

 on which the active corals continue to build seaward, always 

 increasing the diameter of the atoll. Meanwhile the sea-water 

 in the lagoon is at work dissolving and removing coral from 

 the inner edge, and the island does not increase in width 

 although its circle is continually widening. An atoll is thus 

 supposed to grow like a " fairy-ring " in the grass. Fringing 

 reefs growing seaward in the same way ultimately form 

 barrier reefs, in which the same process of active growth 

 seaward, and decay on the landward side has been observed. 5 

 In some cases barrier reefs have grown up directly far from 

 the island on the edge of a wide and shallow continental 



