90 CORAL FORMATIONS. 
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 Madrepore 
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 spectfic gravity varies from 2°5 to 2:8: 2-523 was 
the average from fifteen specimens examined by B. Silliman, Jr.* 
Composition. 'The common reef-corals, of which the branching 
Madrepora, and the massive Astreas are good examples, consist 
almost wholly of carbonate of lime, the same ingredient which con- 
stitutes 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, oxyd of iron, phosphate of magnesia, and fluorids of 
magnesium and calcium. ‘The following is the result of one of Mr. 
Silliiman’s analyses from those made by him for the Report on 
Zoophytes.t The specimen was a Porites from the Sandwich 
Islands. It afforded— 
Carbonate of lime : - - - - 95°84 
Phosphates, fluorids, &c. - - - - - 2°05 
Organic matter - - - - - - 2-1 
The various earthy ingredients are included in the second line of 
the analysis, and in this species amounted to 2-05 per cent. One 
hundred parts of the same, subjected to exact analysis, gave the fol- 
lowing result :— 
Silica - - - = 2 - - 22:00 
Lime - - - - - - - 13°03 
Magnesia - - - - - - - 7°66 
Fluorid of calcium - - - - - 7°83 
Fluorid of magnesium - - - - : 12°48 
* 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 calc-spar. The hardness is near that of arragonite, though 
sometimes a little exceeding it. 
7 Op: cit. p. 712. 
