12 ON CORAL REEFS AND ISLANDS. 



side of them a depth of many fathoms. They sometimes seem 

 to grow up from a narrow base, like a mushroom ; and a ship 

 striking one with her keel may crush it and glide on. More fre- 

 quently, they are below like the solid reef above described, and 

 the contest is more likely to be fatal to the vessel than to the coral 

 patch. Corals grow over them, as in the shallow waters about 

 other reefs ; and, as elsewhere, there are deep cavities among the 

 congregated corals, in which a lead will sometimes sink to a depth 

 of several feet, or even fathoms. These holes about growing 

 reefs often give much annoyance to the boat which may venture 

 to anchor upon them; and in many an instance in the course of 

 the surveys, diving was the only resource left for freeing the foul 

 anchor. 



The rock of the inner reefs is peculiar in being but sparingly 

 fragmentary. The corals composing it stand to a great extent as 

 they grew ; yet it is not less compact or firm in its texture than 

 the rock of the outer reefs. The cavities among the branches 

 and growing masses gradually become filled with coral sand, and 

 the whole is finally cemented and thoroughly compacted. At 

 Tongatabu and among the Feejee Islands, reefs thus made of 

 corals standing in their growing positions are common. Though 

 now mere dead rock, 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 were twen- 

 ty-five feet in diameter; and Astraeas and Meandrinas, both there 

 and in the Feejees, measured twelve to fifteen feet. These corals, 

 when growing beneath the water, form solid hemispheres, or 

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

 enlargement takes place only on the sides. In this manner the 

 hemisphere is finally changed to a broad cylinder with a flat top. 

 This was the condition of the Astraeas and Porites in the reef-rock 

 referred to. The platform looks like a Cyclopean pavement, ex- 

 cept that the cementing material, filling in between the huge 

 masses, is more solid than any work of art: it even exceeds in 

 compactness the corals themselves. Other portions of these reefs 

 consist of branching corals, with the intervals filled in by sand 

 and small fragments; for even in the more still waters fragments 

 are to some extent produced.* There is also to be found here, 

 and frequently over large areas, the solid white limestone already 

 described, showing internally no evidence of its coral origin, and 

 containing rarely a few shells or imbedded fossils. 



The formation of the inner reefs goes on at a less rapid rate than 

 that of the outer, as the process depends on growth unaided, ex- 

 cept in a comparatively small degree, by the action of the waves. 

 Moreover, as we shall explain more particularly in another place, 



* A rock of this kind is often used for buildings and for walls on the island of 

 Oahu. It consists mainly of Porites, and in many parts is still cavernous, or but im- 

 perfectly cemented. It is the material of the large church at Honolulu. 



