390 



COELENTERATA ANTHOZOA 



CHAP. 



Ch 



ranging from 30 to 50 fathoms. 1 To give one example: In 

 the dredging No. 50, depth 32 to 35 fathoms, living examples of 

 the following genera of corals were obtained : Madrepora, 

 Montipora, Psammocora, Pavonia, and Astraeopora. 



Coral Reefs and Atolls. In many regions of the tropical 

 seas, banks and islands are found which are built up of blocks of 

 coral, coral detritus, and altered or modified limestone. These 

 are the famous coral reefs of which so much has been said and 

 written during the last half-century. There can be little doubt 

 that the superficial strata of these formations are entirely due 

 to the action of coral -forming animals and plants living in warm, 

 shallow sea-water. 



Three classes of coral reefs are usually recognised : the 

 " fringing reefs " which follow the contour of the coast at a distance 



of a few hundred yards, 

 and are separated from 

 the beach at low tide by 

 sand flats or a shallow 

 lagoon ; the " barrier 

 reefs," following the con- 

 tour of the coast less 

 regularly than the fring- 

 ing reefs, but at a much 

 greater distance, and 

 separated from the beach 

 by a lagoon of sufficient 

 depth to serve as a 

 harbour for ships of great 

 size ; and, finally, the 

 " atolls," which are ring- 

 shaped, or broken circlets 

 of low islands enclosing 

 a lagoon which is, in 

 some cases, of consider- 

 able depth. 

 It was observed by the early surveyors that in many cases 

 the sea-bottom slopes downwards steeply or almost precipitously 

 from the outer edge of the barrier reefs and atolls to very great 



Fig. 172. Plan of Minikoi Atoll in Laccadive Archi- 

 pelago. A, the land elevated above the level of 

 high-water mark ; Gh, the boat channel ; 5 fm, 

 the five fathom line ; 2 fm, the two fathom line ; 

 X. the lagoon with a maximum depth of 7 fathoms ; 

 R, the reef continuing the circle on the east side 

 of the atoll, awash at high tides. (After Stanley 

 Gardiner. ) 



1 "Report on the Results of Dredging on the Macclesfield Bank," Admiralty 

 Report, 1894. 



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