56 ON CORAL REEFS AND ISLANDS. 



usually with faint evidences of a concentric structure. It is thus 

 that the red coral of commerce, used in jewelry, differs from the 

 Madrepore or common white coral : it is the axis of a species of 

 Corallium ; and the polyps constituted a layer about it, in the 

 same manner as the polyps of Gorgoniae cover the horny axis of 

 these species. 



In hardness, the common calcareous corals are a little above 

 ordinary limestone or marble, the degree being represented in the 

 mineralogical scale of Mohs by 3-5 to 4, while, in limestone, it 

 is about 3. The ringing sound given when coral is struck with 

 a hammer, indicates this superior hardness. It is a common 

 error of old date to suppose that coral when first removed from 

 the water is soft, and afterwards hardens on exposure. But, in 

 fact, there is scarcely an appreciable difference ; the live coral has 

 a slimy feel in the fingers; but if washed clear of the animal 

 matter, it is found to be quite firm. The waters with which it 

 is penetrated may contain a trace of lime in solution, which 

 evaporates on drying, and adds slightly to the strength of the 

 coral, but the change is hardly appreciable. A branched Madre- 

 pore rings on being struck when first collected ; and a blow in 

 any part puts in hazard every branch throughout it, on account of 

 its elasticity and brittleness. Its specific gravity varies from 2*5 

 to 2-8 : 2*523 was the average from fifteen specimens examined 

 by Prof. B. Silliman, Jr.* 



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

 ing Madrepora, and the massive Astrseas are good examples, con- 

 sist almost wholly of carbonate of lime, the same ingredient 

 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, magnesia, alumina, oxyd of iron, 

 phosphate of magnesia, and fiuorids 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.f The 

 specimen was a Porites from the Sandwich Islands. It afforded — 



Carbonate of lime, . . . 95*84 

 Phosphates, fiuorids, &c, . . 2*05 



Organic matter, . . . . 2*11 



The various earthy ingredients are included in the second line 

 of the analysis, in this species amounting to 2*05 per cent. One 



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

 that the high degree of hardness, which characterizes corals and also the shells of 

 many molluscs, may arise from the structure of the calcareous secretions being like 

 that of arragonite, instead of common caic-spar. The hardness is near that of arra- 

 gonite, though sometimes a little exceeding it. 



| Op. cit.,p. 712, and Silliman's Journal [2], i, 189. 



