114 ON CORAL REEFS AND ISLANDS. 



certain areas during the deposition of the chalk strata. It is well 

 known that heated waters dissolve lime much less readily than 

 cold ; and this might be a reason for its inferior hardness and 

 earthy texture. The character of the cretaceous deposits presents 

 many interesting points bearing upon this subject; but a discus- 

 sion of them would be out of place here, as our object is simply 

 to state such inferences as the facts observed among existing reefs 

 may have suggested. 



This coral chalk has been examined microscopically by Pro- 

 fessor Bailey, for infusoria and polythalamia, without detecting 

 anything of this kind. It appeared to contain nothing organic. 



VI. The analyses have shown that ordinary corals consist 

 mainly of carbonate of lime. There is a small proportion of 

 flnorids and phosphates, with some silica, alumina, and oxyd 

 of iron. These flnorids and phosphates, existing in the coral, 

 must exist also in the limestone rock made from coral. It 

 is probable from some trials marie by Prof. Silliman, Jr., that 

 these, constituents may be found also in many shells. 



From the several analyses of corals by Mr. Silliman, we infer 

 that the flnorids and phosphates amount, on an average, to about 

 Jth per cent., or 0*25 parts in a hundred of coral : and the amount 

 in the same manner of the phosphates, is 005 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 fluo- 

 rids, and one and one-fourth ounces of phosphates : and in each 

 cubic rod seventeen hundred pounds of flnorids, and three hun- 

 dred and forty pounds of phosphates. These flnorids are fluo- 

 rids of calcium and magnesium, and the phosphates, phosphates 

 of lime and magnesia. To obtain the amount of these ingredi- 

 ents 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 ani- 

 mal remains, among which, in many instances, corals have a 

 conspicuous place. These limestones often contain crystalliza- 

 tions 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 mineral, crystallizations of apatite, (phos- 

 phate of lime,) and chondrodite (consisting of fluorine, magne- 

 sia 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 results, without 

 looking to other sources for the elements of these disseminated 



