68 THE FLOOR OF THE OCEAN 



KILOMETERS 



FIGURE 36. MAP OF BARRIER REEF AROUND THE TRUK ISLANDS. 

 DEPTHS IN FATHOMS. 



(reef -insets). The third pair, which are sections across Tagula 

 Island of the Pacific Louisiade archipelago, and Truk Islands 

 of the Pacific Caroline group, exemplify typical barrier reefs. 

 The last three sections refer to Peros Banhos of the Indian 

 Ocean, Funafuti of the Pacific Marshall group, and Koluma- 

 dulu of the Indian Ocean — all typical atolls. 



Maps and sections alike point to a conclusion which is ex- 

 tremely hard to avoid: the living reefs are veneers, grown up 

 on old banks that had been smoothed by wave and current 

 before the banks were colonized by the coral larvae. In all of 

 the cited cases, and at hundreds of other atolls and barriers of 

 the tropical belt, the lagoon depths never reach 50 fathoms or 



