§ 29. CORAL ISLANDS. 629 



highest tides, above which the animalcules have no power to advance 

 and the reef of course no longer extends upwards." 



29. Coral islands.— Kotzebue, Flinders, MM. Quoi 

 and Gaimard, Mr. Darwin, and others, have severally de- 

 scribed the formation of Coral islands ; the following is an 

 abstract of their observations. 



The Coral banks are every where seen in different stages 

 of progress : some are become islands, but not yet habitable; 

 others are above high-water mark, but destitute of vegeta- 

 tion ; while many are overflowed with every returning tide. 

 When the polypes of the corals at the bottom of the ocean 

 cease to live, their skeletons still adhere to each other, and 

 the interstices being gradually filled up with sand and 

 broken pieces of corals and shells, washed in by the sea, a 

 mass of rock is at length formed. Future races of these 

 animalcules spread out upon the rising bank, and in their 

 turn die, and thus increase and elevate this wonderful 

 monument of their existence. 



The reefs which raise themselves above the level of the 

 sea, are usually of a circular or oval form, and surrounded 

 by a deep and oftentimes unfathomable ocean. In the 

 centre of each, there is generally a shallow lagoon with still 

 water, where the smaller and more delicate kinds of zoo- 

 phytes find a tranquil abode ; while the stronger species 

 live on the outer margin of the isle, where the surf dashes 

 over them. When the reef is dry at low water, the coral 

 animals cease to increase. A continuous mass of solid 

 stone is then seen, composed of shells, echinoderms, and 

 fragments of corals, united by calcareous sand, produced by 

 the pulverization of the shells and of the friable polyparia. 

 Fragments of coral limestone are thrown up by the waves ; 

 these are cracked by the heat of the sun, washed to pieces 

 by the surge, and drifted on the reef. After this the calca- 

 reous mass is undisturbed, and offers to the seeds of the 



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