214 CORALS AND CORAL ISLANDS. 



reduced, if we would deduce the rate of growth of the reef; 

 because a large part of the reef-grounds — that is, of the region 

 of soundings receiving the coral debris— is bare of growing 

 corals. This is the case with much the larger portion of all 

 lagoons and channels among reefs, the bottoms of which, as 

 already explained, are often sandy or muddy, and to a great 

 extent so because too deep for living corals ; and it is true 

 even of the coral plantations, these including many and large 

 barren areas. These unproductive portions of reef grounds 

 constitute ordinarily at least two-thirds of the whole ; and 

 making this allowance, the estimate of one-fourth of an inch 

 a year would become one-twelfth of an inch. 



Again, shells add considerably to the amount of calcareous 

 material, perhaps one-sixth as much as the corals ; but against 

 this we may set off the porosity of the coral. 



The rate of growth of the Mosandrina clivosa, stated on page 

 98, would make the rate of increase in the reef very much 

 less rapid. The specimen — the growth of fourteen years — 

 weighs 24 oz. avoirdupois, and has an average diameter of 

 7 inches. This gives for the amount of calcareous material 

 — the specific gravity being 2*523 (p. 75) — 16*45 cubic inches ; 

 which is sufficient to raise a surface seven inches in diameter 

 to a height of 0*428 inch ; and consequently the average j'<?(2/'/)' 

 increase would be about i-33d of an inch. Allowing for two- 

 thirds of the reef-ground being unproductive in corals, the 

 rate of increase for the whole would become i-iooth of an 

 nch. But supposing that shells add one-fourth as much as 

 the corals to the reef material, the rate of increase would be- 

 come about i-8oth of an inch per year. 



The specimen of Oculifia diffusa, referred to on page 98, 

 weighs 44 ounces, which is five-sixths more than that of the 

 Mgeandrina, while the average diameter of the clump is the 

 same. The average annual increase would consequently cover 

 a circular area of seven inches diameter i-i8th of an inch 

 deep. And making the same allowances as above, the rate 

 for the year for the whole reef-grounds would be i-44th of an 

 inch. The specimen of Maeandrina mentioned by Major 



