1 68 THE SEAS 



and thirty degrees south of the equator. In this region they 

 are to be found everywhere — except on the western shoies 

 of continents — that the necessary shallow water conditions 

 prevail, and the coloured patches on the map shown in Plates 

 63 and 64 indicate how widespread are coral reefs and islands. 

 The Great Barrier Reef of Australia, the greatest coral 

 formation in the world, is 1,350 miles long, extending from 

 New Guinea southward along the entire coast of Queensland. 



Three types of coral reefs have been distinguished ever 

 since they were investigated by the early navigators. 

 Fringing Reefs develop around the shores of islands or 

 continents and have no extensive or deep-water channel 

 between them and the land. Barrier Reefs are also found 

 off land masses, but at a much greater distance from them, 

 so that there is a wide and relatively deep channel between 

 them and the shore, through which ships can safely pass. 

 Finally there are Atolls, which are not connected with land 

 at all and consist of rings of coral, part of which is raised 

 above the surface of the sea (Fig. 34). Within this 

 ring is invariably a lagoon, the depth of which — like that 

 which separates a barrier reef from the land — never exceeds 

 fifty fathoms and is usually much less, and varies in size 

 between a mere pool and an enclosed sea perhaps forty miles 

 from side to side. 



The formation of fringing and barrier reefs is clearly 

 bound up with the structure of the land they border, to the 

 constitution of which, however, they do not contribute, 

 although, as the result of the raising of the land in past ages, 

 dry land in many regions is composed of coral limestone, 

 originally formed in the sea and now in some cases within 

 mountains several thousands of feet above the level of the 

 sea. The formation of the real " coral islands " or atolls is 

 not associated with land, from which they may be separated 

 by hundreds of miles, the land formed being purely of coral 



