THE BENTHAL 211 



The reefs frequently reach the water level, above which they are 

 unable to grow. Many kinds of coral, however, are able to withstand 

 a certain amount of exposure, so that the upper surface of the reef 

 may be laid bare at ebb tide. Islands projecting above high tide mark 

 are produced by the action of storms, which pile up loose blocks on the 

 low-tide platform. These hold the erosion materials of the platform, 

 small coral fragments and sand, and the accumulation may rise 1 to 2 

 m. above water level for stretches of varying extent. Seeds drift to 

 this beach, find a foothold, and a vegetation may develop which 

 increases in richness and variety with time. The cocoa palm thrives 

 especially in this situation. A multitude of terrestrial invertebrates, 

 hermit crabs, land crabs, insects, and snails, gather on the new terri- 

 tory, and these are soon followed by birds. Finally man invades the 

 island, bringing with him the animal and plant associates of human 



-"» 



Fig. 40. — The island of Bora Bora, in the Society Islands, with its barrier reef. 



After Agassiz. 



culture. It is not rare, however, that spring flood tides or great storms 

 wash all traces of life from such low islands into the sea, as in the 

 Tuamotus in 1878 and 1903. 61 



Coral reefs are distinguished as fringing reefs, barrier reefs, and 

 atolls, the last being ring-like coral islands. 



Fringing reefs are juxtaposed to the coasts and follow their out- 

 lines. They may be reached dry-shod from the shore at low tide, when 

 their platforms are exposed. They are often only a few meters wide, 

 but may reach a breadth of a kilometer, according to the slope of the 

 coast on which they are formed; the narrower reefs are on steep 

 slopes, the broad reefs on gentle ones, as for example at Dar-es- 

 Salaam. 02 



The barrier reefs are separated from the coast by a canal or 

 lagoon, of varying width. Islands may be surrounded by a ring-like 

 barrier reef (Fig. 40) . Mainland coasts may be paralleled by them for 

 great distances. The great barrier reef of Australia extends for about 

 1900 km., the width of its lagoon varying from 38 to 150 km., its depth 

 from 30 to 50 m. Such reefs have a steep outer and gentle inner slope. 



The atolls are ring or horseshoe-shaped reefs, extending only a few 



