CONSTRUCTION OF COllAL REEFS. 381 



Polypiers continue to grow until they have reached 

 the surface of the water. The construction of coral 

 may be likened to a forest : intervals are left com- 

 parable to those between the branches of a tree and 

 the trees of the forest. Animal remains — partly de- 

 rived from the decay of some parts of the coral, partly 

 consisting of the debris of molluscs and fish — fill up 

 the gaps, and, in the manner of a chemical cement, 

 serve to bind the whole into a compact mass. 



Tlie coral insects absorb, particle by particle, the 

 carbonate of lime from the water, and they de[)Osit 

 it afterwards. The carbonate sometimes appears in 

 a muddy form, and, hardening by exposure to air, 

 appears very similar to chalk. This phenomenon is 

 very remarkable in the Bermudas, where it has been 

 studied by the naturalist Nelson: "After having 

 observed the decomposition of shells and polypiers 

 from the less calcareous to the clumps of meandrinae 

 and astreae, not only in situ, but m the masses which 

 had been detached by means of the diving-bell for 

 the works of the arsenal, I do not hesitate to affirm 

 a common origin for the chalk of the Bermudas and 

 the banks of stone, more or less solid, which constitute 

 the islands themselves— only that the latter result from 

 the accumulation of fragments mechanically broken, 

 whereas the rock or chalky paste is due to the de- 

 struction, owing to prolonged submersion, of the mem- 



