RATE OF GROWTH OF CORAL REEFS. 255 
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 débris—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 Maandrina 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 1-33d of an inch. Allowing for two- 
thirds of the reef-ground being unproductive in corals, the 
rate of increase for the whole would become 1-100th of an 
inch. But supposing that shells add one-fourth as much as 
