152 CORAL FORMATIONS. 
the fluorids and phosphates amount, on an average, to about 4th per 
cent., or 0-25 parts in a hundred of coral: and the amount in the same 
manner of the phosphates, is 0:05 per cent. A cubic foot of coral, 
as deduced from the average specific gravity, weighs one hundred 
and fifty-seven pounds, and consequently, in each cubic foot, there 
must be full six and one-fourth ounces of fluorids, and one and one- 
fourth ounces of phosphates: and in each cubic rod seventeen hun- 
dred pounds of fluorids, and three hundred and forty pounds of phos- 
phates. These fluorids are fluorids of calcium and magnesium, and 
the phosphates, phosphates of lime and magnesia. To obtain the 
amount of these ingredients in a reef a mile long, half a mile wide and 
a hundred feet deep, the estimate for a cubic rod should be multiplied 
by 320,000; which will give for the fluorids more than five hundred 
millions of pounds. 
Late geological researches have placed it beyond doubt that the 
various limestones consist mainly, like coral limestone, of animal re- 
mains, among which, in many instances, corals have a conspicuous 
place. These limestones often contain ¢rystallizations of fluorid of 
calcium (fluor spar) ; and in other beds which have been acted upon 
by heat, and thus rendered crystalline, there are, besides this mine- 
ral, crystallizations of apatite, (phosphate of lime,) and chondrodite 
(consisting of fluorine, magnesia and silica). Moreover, these are 
among the most common minerals of such limestones. ‘The above 
facts supply us with a full explanation of their origin. ‘The fluorine, 
phosphoric acid, magnesia, and silica present, are adequate for all re- 
sults, without looking to other sources for the elements of these dis- 
seminated minerals. Instead, therefore, of being extraneous minerals 
introduced into the limestone rock, they are an essential part of its 
constitution. And they have been separated from the general mass 
by a segregation of like atoms, under well-known principles, while 
the rock was subjected to an elevated temperature. The fluorid of 
calcium appears to crystallize without much heat: but apatite 
and chondrodite are found in granular limestones, which show, by 
their crystalline texture, that they have been subjected either to a 
very high temperature, or to one long continued of more moderate 
degree. 
Lord Byron, of the Blonde, states that specimens of phosphate of 
lime, (apatite,) were actually collected on Mauki, of the Hervey Group, 
one of the elevated coral islands. 
VIII. The cementation of coral sand along shores and beneath the sea 
