ii6 CORALS AND CORAL LSLANDS. 



an instance diving is found to be the only resource left for 

 freeing the foul anchor. 



The margins of the reefs in and about the inner channels are 

 often luxuriant with magnificent corals quite to the edge so that 

 while the reef is elsewhere solid rock to its very top, here at 

 the margin it is alive and may be said literally to be growing. 



The rock of the inner reefs seldom consists of rolled or 

 broken fragments of coral like a large part of that of the 

 outer reef. It is often made of dead corals, standing to a 

 great extent as they grew ; yet it is generally compact and 

 firm in texture. The cavities among the branches and masses 

 gradually become filled with coral sand, and the whole is 

 finally cemented and so made solid. At Tongatabu and 

 among the Feejee Islands, reefs thus formed of corals standing 

 in their growing positions are common. Though now mere 

 dead rock, and exceedingly firm and compact, the limits of 

 the several constituent coral masses may be distinctly made 

 out. Some individual specimens of Porites in the rock of the 

 inner reef of Tongatabu are twenty-five feet in diameter ; and 

 Astrseas and Maeandrinas, both there and in the Feejees, mea- 

 sure twelve to fifteen feet. These corals, when growing be- 

 neath the water, form, as has been stated, solid hemispheres, 

 or rounded hillocks ; but on reaching the surface, the top dies, 

 and enlargement takes place only on the sides; and in this 

 manner the hemisphere is finally changed to a broad cylinder 

 with a flat top. This was the condition of the Astrseas and 

 Porites in the reef-rock referred to. Such a platform looks 

 like a Cyclopean pavement, except that the calcareous ce- 

 menting material, filling in between the huge masses, is more 

 solid than in any work of art : it even exceeds in compactness 

 the corals themselves. Other portions of reefs consist of 

 branchi7ig corals, with the intervals filled in by sand and small 

 fragments ; for even in the stiller waters fragments are to some 

 extent produced. A rock of this kind is often used for build- 

 ings and for walls on the island of Oahu. It consists mainly 

 of Porites, and in miany parts is still cavernous, or but imper- 

 fectly cemented. 



