RATE OF GROWTH OF CORAL REEFS. 251 



part annually added. This ascertained, it would be easy to 

 calculate how much the added coral would, if ground up, raise 

 the area that is covered by the Madrepora. A rough esti- 

 mate gives the author an average increase to this surface of 

 a fourth of an inch a year. But this fourth must be much 

 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 Mceandrina clivosa, stated on 

 page 125, 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. 99) — 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 yearly 

 increase would be about 1-3 3d of an inch. Allowing for two- 

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

 rate of increase for the whole would become 1-1 00th of an 

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



