92 



ISrOTES ON COEAL REEFS, WITH SPECIAL REFER- 

 ENCE TO THE FUNAFUTI BORINGS. 



By T. Stephens, M.A., F.O.S- 



It may be premised that the object of tbis paper is, not to 

 announce any new discovery, but merely to give a brief 

 summary of what is known of the history of coral reefs, with 

 notices of the chief theories that have been advanced in con- 

 nection with the subject. 



The coral polyps belonging- to the family of the Astrseidse, 

 the chief reef-builders, flourish only in the warmer Avaters of 

 the ocean within the tropics. Their range extends from 

 about 20deg. south to 20deg. north of the Equator; but, 

 under the influence of the Gulf Stream, they are found as far 

 north as the Bermudas. The action of the carbonic acid 

 derived from these minute organisms upon the lime always 

 present in sea water enables them to secrete the carbonate 

 of lime which forms the stony framework popularly known 

 a " coral," and this is all that remains after the death 

 of successive generations of reef-builders working uj^^^^i-'^s 

 within tlieir appointed range. 



Coral reefs are classed under three heads, fringing reefs, 

 barrier reefs, and atolls. The fringing reef lies close to the 

 land in fiat beds traversed in all directions by shallow 

 channels and pools, and with larger breaks opposite the 

 mouths of rivers. Building upwards from the sea bottom, 

 where the depth is not too great, the coral polyps die on 

 reaching the surface, and the growth of the reef is then con- 

 fined to its seaward face. Barrier reefs lie at a distance from 

 land in lines more or less parallel with the nearest coast, and 

 generally rising from great depths on the seaward side. The 

 atoll is a partially-closed ring of cnral formation witli no 

 land in the centre, the typical coral island, though, as at 

 Funafuti, the ring often consists of numerous detached 

 islands. 



Darwin's theory may be briefly stated. A careful and 

 widely extended study of the life and work of the reef-build- 

 ing coral polyps had proved that their operations cannot be 

 carried on at a greatar depth than about 25 fathoms, and that 

 when the formation gradually built up by successive genera- 

 tions on the stony remains of their predecessors reached the 

 surface, all upward growth of live coral ceased. Darwin 

 knew of numerous instances where the remains of old cora]; 



