48 James D. Dana, Esq., on the Structure^ 



2-523 was the average from fifteen specimens examined by 

 Professor B. Silliman junior.* 



In composition, tlie common reef corals, of which the branch- 

 ing Madrepora and the massive Astrseas are good examples, 

 consist almost wholly of carbonate of lime, the same ingre- 

 dient which constitutes ordinary limestone. In 100 parts, 

 90 to 96 parts are of this constituent ; of the remainder, 

 there are 3 to 8 parts of organic matter, with some earthy 

 ingredients, amounting in certain species to 2 parts, though 

 often less than 1. These earthy ingredients are silica, mag- 

 nesia, alumina, oxide of iron, phosphate of magnesia, and 

 fluorides of magnesium and calcium. The following is the 

 result of one of Mr Silliman's analyses from those made by 

 him for the Report on Zoophytes. t The specimen was a 

 Porites from the Sandwich Islan d s . It afforded, — Carbonate of 

 lime, 9584 ; Phosphates, fluorides, &c. 2*05 ; Organic matter, 

 2*11. The various earthy ingredients are included in the 

 second line of the analyses, in this species, amounting to 

 2-05 per cent. One hundred parts of the same, subjected to 

 exact analyses, gave the following result : — Silica, 22*00 ; 

 Lime, 13*03 ; Magnesia, 7*66 ; Fluoride of calcium, 7*83 ; Flu- 

 oride of magnesium, 12*48 ; Phosphate of magnesia, 2*70 ; 

 Alumina (and iron), 16*00 ; Oxide of iron, 18*30. In other 

 analyses, similar results were obtained, with sometimes a 

 larger proportion of fluorides. 



The horny corals (axes of Gorgoniee and Antipathi) were 

 found by Hatchett to have nearly the constitution of ordi- 

 nary horn. J 



The sea-water and the ordinary food of the polyps are 

 evidently the source from which the ingredients of coral are 

 obtained. As coral is an animal secretion, there is no good 

 reason for the surprise with which this subject is sometimes 

 approached. The same powers of elaboration which exist 



* Report on Zoophytes, p. 713. On page 711, it is suggested by the author 

 that the high degree of hardness, which characterises the corals and also the 

 shells of many molluscs, may arise from the structure of the calcareous secretions 

 being like that of Aragonite, instead of common calc spar. The hardness is 

 near that of Aragonite, though sometimes a little exceeding it. 



t Op. cit., p. 712, and this Journal (2), 1-189. 



X Report on Zoophytes, p. 56. 



