xi The Bed of the Oceans 209 



clear, and warm water ; and, although they may live, 

 they do not form reefs where the temperature is less 

 than 70, or has a yearly range greater than 12, or a 

 depth greater than about 20 fathoms. They are par- 

 ticularly active on the margin of the Red Sea, where the 

 conditions of salinity, temperature, and depth are most 

 favourable. The distribution of reef-building corals is given 

 in the map of Plate XV. Corals are never found near the 

 mouths of great rivers on account of the water being fresh 

 and muddy. They do not build on the west coasts of the 

 tropical continents because of the cold upwelling water 

 (241). The part of the Somali coast in the Indian Ocean 

 against which the south-west monsoon raises cold water 

 ( 248) is free from corals on account of the great annual 

 range of temperature which results. ' Corals are confined to 

 the centre and western sides of tropical oceans, except in 

 warm currents such as the Gulf Stream, which enables them 

 to live luxuriantly far into the temperate zone, the Bermuda 

 islands, in 32^ N., having the highest latitude where coral 

 islands are now forming. There the polyps appear able 

 to form reefs at a temperature as low as 68, but these reefs 

 are largely composed of calcareous sea-weeds and worm- 

 tubes. 



280. Coral Reefs and Islands. The Gulf of Mexico 

 and the west coast of Florida, the western Indian Ocean, 

 and in particular the western Pacific, are the seats of very 

 active and typical coral growth. There are three distinctive 

 forms of coral structure, (a) The fringing reef, which 

 closely surrounds the shore, forming on .the seaward slope 

 of the land in shallow water, and as it grows older gradually 

 widening toward the sea. (b) The barrier reef, which 

 usually lies at a distance from the land, running parallel 

 to the coast, and on its seaward side often springing 

 abruptly from great depths. On the landward side a 

 shallow lagoon of still water is shut in by the reef, which 

 is always broken by one or more narrow channels, allowing 

 boats or even large vessels to enter. Innumerable volcanic 

 islands in the Pacific, such as the Solomon Islands, the Fiji 

 group, and Tahiti, are encircled with fringing and barrier 



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