1 68 THE SEAS 



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

 are to be found everywhere — except on the western shores 

 of continents — that the necessary shallow water conditions 

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

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

 The Great Barrier Reef of Australia, the greatest coral for- 

 mation in the world, is 1,260 miles long, extending from New 

 Guinea southward along the entire coast of Queensland. 



Three types of coral reefs have been distinguished 

 since the first investigations by early navigators. Fring- 

 ing Reefs develop around the shores of islands or conti- 

 nents 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, partly 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 — seldom exceeds 

 thirty 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 



