CORAL FORMATIONS. 619 



Corals of the different groups here mentioned grow together promiscuously at 

 different depths up to low-tide level. The largest Aslreas, 3[eandrinas, and 

 Parties, with many Madrepores and other kinds, have been seen by the author 

 constituting the upper part of the growing reef. At Tongatabu there were 

 single masses of Porites twenty-five feet in diameter, along with Astreas and 

 Meandrinas ten to fifteen feet. But, while these different groups do not corre- 

 spond to different zones in depth, there are, without doubt, species in them which 

 belong to the deeper waters, and others to the more shallow. 



The Porites, and some species of the Astrea, Madrepora, and Pocillojyora groups, 

 continue to grow a little above low-tide level, equal to about one-third the 

 height of the tide, — as they will endure a temporary exposure to the sun without 

 serious injury. The Porites is an especially hardy group; for the corals suffer 

 less from impurity or silt in the waters than the species of other groups. 



All the reef-forming species grow within the limit of 100 feet. The Dendro- 

 phylliee, and a few other kinds that grow at greater depths, contribute but little 

 to the formation of reefs. 



The polyp-corals have the power of growing indefinitely upward, while 

 death is going on at equal rate either at the base of the structure (as in the 

 moss of which peat is made) or through its interior, and are only stopped in 

 upward progress by reaching the surface of the water. The hemispherical 

 Astreas, many feet in diameter, although covered throughout with living 

 polyps, may be alive to a depth of only half or three-quarters of an inch, 

 and the huge Porites to a depth of less than a quarter of an inch : that is, 

 only a thin exterior portion of the mass is really living. 



Besides corals and shells, there are also some kinds of calcareous 

 vegetation, called Nullipores, both branching and incrusting in 

 form, which add to the accumulation. They grow well over the 

 edge of the reef, in the face of the breakers, and attain consider- 

 able thickness. 



Action of the waves. — The waves, especially in their heavier move- 

 ments, sweeping over the coral plantations, may be as destructive as 

 winds over forests. They tear up the corals, and, by incessant tritu- 

 ration, reduce the fragments to a great extent to sand ; and the 

 debris thus made and ever making is scattered over the bottom, 

 or piled upon the coast by the tide, or swept over the lower parts 

 of the reef into the lagoon. The corals keep growing, and this 

 sand and the fragments go on accumulating ; the consolidation 

 of the fragmental material makes the ordinary reef-rock. Thus, 

 by the help of the waves, a solid reef-structure is formed 'from 

 the sparsely-growing corals. 



Where the corals are protected from the waves, they grow up 

 bodily to the surface, and make a weak, open structure, instead 

 of the solid reef-rock ; or, if it be a closely-branching species, so as 

 to be firm, it still wants the compactness of the reef that has been 

 formed amid the waves. 



