V 



I 





4 



On Coral Reefs and Islands. ISl 



Latitude. Depth of 6° Fahrenheit. 



15° - . 40—60 fath 



. lOV . . 50—75 



6° . . 75 



Equator. 0° - - 75—100 



S. Latitude. 5° . . 50—75 



10° .. - 50 



(( 



(( 



cc 



4( 



C( 



C( 



cc 



15^ . - 50 



20" . - 40 



25" - . 25 



28°— 30^ - - Surface. 



It appears, therefore, that among the causes limiting the range 

 of corals in depth; light and hydraulic pressure must have great 

 influence. The proportion of atmospheric air present may be 

 another cause. Yet according to Daroijdeau, the deeper waters 

 contain more atmospheric air and also more carhonic acid, — the 

 difference being as much as -pi^th the volume of the water * 



Quoy j^ud Gaymard were the first authors who ascertained that 



reef-forming corals were confined to small depths, contrary to the 



account of Forster and the early navigators. The mistake of 



previous voyagers was a natural one, for coral reefs were proved 



to staiid^ in an aufathomable ocean; yet it was from the first a 



niere opmion, as the fact of corals growing at such depths had 



never been ascertained* It is now considered altogether probable 



that the bottom of the ocean in its deeper parts is mostly without 



me of any kind. The few Caryophyllice and other species which 



are met with in deep waters, have been shown to be sparsely 



scattered, mostly of small size, and nowhere form accumulations 

 or beds. 



.^ ^bove-mentioned authors, who explored the Pacific in the 

 Uranie under D'Urville,f conchided from their observations that 

 five or six fathoms (30 or 36 feet) hmited their downward dis- 

 tribution. Ehrenberg, by his observations on the reefs of the 

 ■K-ed Sea, confirmed the observations of Q.uoy and Gaymard ; he 

 concluded that living corals do not occur beyond six fathoms. 

 JWr- Stutchbury, after a visit to some of the Paumotus and Tahiti, 

 remarks that the living clumps do not rise from a greater depth 

 than 16 or 17 fathoms.| Mr. Darwin, who traversed the Pacific 

 ^ith Captain Fitzroy, R. N., gives 20 fathoms as not too great a 

 "^ange, and mentions reported instances of growing reefs in 25 or 

 even 30 fathoms. He states that in the Red Sea^ according to 

 y^ptain Morehead, living corals occur at 25 fathoms. At Keel- 



i"g Atoll, growing corals are described by him as wholly disap- 



, -^nimation of Sea Waaler collected during the Voyage of the Bonite, Jame- 

 4^A f *"^* Jour., July, 1838, p, 164. Darondeau's observations require confirniatioii. 

 I ^*^^"^ards also in the Astrolabe. 

 4 b. Stutchbury. West of England Journal, i, 48. 



