56 James D. Dana, Esq., on the Structure, 



Latitude. Depth of 60° Fahrenheit. 



Equator 0° 75-100 fathoms. 



S. Latitude 5° 50-75 



10"^ 60 



15° 60 



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 Darondeau, 

 the deeper waters contain more atmospheric air, and also 

 more carbonic acid, — the difference being as much as too^^^ 

 the volume of the water.* 



Quoy and Gaymard were the first authors who ascertained 

 that reef-forming corals were confined to small depths, con- 

 trary to the account of Forster and the early navigators. The 

 mistake of previous voyagers was a natural one, for coral 

 reefs were proved to stand in an unfathomable ocean ; yet it 

 was from the first a mere opinion, as the fact of corals grow- 

 ing 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 life of any kind. The 

 few Caryophyllise, and other species which are met with in 

 deep waters, have been shewn to be sparsely scattered, 

 mostly of small size, and nowhere form accumulations or beds. 



The above-mentioned authors, who explored the Pacific in 

 the Uranie, under D'Urville,t concluded from their observa- 

 tions, that five or six fathoms (30 or 36 feet) limited their 

 downward distribution. Ehrenberg, by his observations on 

 the reefs of the Red Sea, confirmed the observations of 

 Quoy and Gaymard ; he concluded that living corals do not 

 occur beyond six fathoms. Mr Stutchbury, after a visit to 

 some of the Paumotus Tahiti, remarks, that the living clumps 

 do not rise from a greater depth than 16 or 17 fathoms.J 



* Examination of sea water, collected during the voyage of the Bonite. — 

 Jameson's Edin. Jour., July 1838, p. 164. — Darondeau. Observations require 

 confirmation. 



t Afterwards also in the Astrolabe. 



I S. Stutchbury, West of England Journal, i, 48. 



