68 NATURAL HISTORY HEADER. 



we examine, we will find small cavities which mark the 

 work of a single polyp, and which give to coral its char- 

 acteristic appearance. 



5. The coral-workers are found in all the tropical seas 

 where the water is shallow enough for their operations. 

 Although each one is tiny and insignificant, yet in the ag- 

 gregate, from their vast numbers and ceaseless workings, 

 they build up immense reefs in the ocean. Their building 

 goes on in the water from the surface to a depth of one 

 hundred and fifty feet, the hardier varieties building be- 

 low, and the tender ones carrying on the work successively 

 above. The necessary conditions in regard to depth of 

 water are found only in the vicinity of the land, and hence 

 the coral-workers are found near the coasts of continents 

 and islands in tropical regions. 



6. In many parts of the ocean, and especially in por- 

 tions of the Pacific, long-continued investigation has shown 

 that the land is gradually sinking. Among the islands, 

 the low lands along the shore will first disappear, and it 

 thus happens that the coral rock constructed next to the 

 land is gradually changed to a reef at some distance from 

 the land. The coral polyps meantime continue to build up- 

 ward, keeping the coral rock nearly even with the surface, 

 and this is changed to dry land above the surface by the 

 dashing of the waves, which break off fragments and pile 

 them upon the top of the undisturbed portion of the rock. 



7. Some of the islands have only sunk a little way, leav- 

 ing a reef partially or entirely inclosing it at a distance 

 varying from half a mile to several miles from the land ; 

 some have sunk so that only the tops of the hills appear 

 above the water, the coral-reef appearing now to surround 

 several islands ; and some of the islands have entirely dis- 

 appeared, leaving a ring of coral-reef inclosing a lagoon, 

 known as a lagoon-island, or atoll. 



8. These circular reefs are found from one mile to 



