116 CORALS AND CORAL ISLANDS. 



zebue states that in some lagoons of the Marshall group he ob- 

 served living corals at a depth of twenty-five fathoms, or one 

 hundred and fifty feet. 



Prof. Agassiz observes that about the Florida reefs, the 

 reef-building; corals do not extend below 10 fathoms. Mr. L. 

 F. de Pourtales states that he found species of Oculina and Clad- 

 ocora off the Florida reefs living to a depth of 15 fathoms. 



It thus appears that all recent investigators since Quoy and 

 Gaymard have agreed in assigning a comparatively small depth 

 to growing corals. The observations on this point, made dur- 

 ing the cruise of the Wilkes Exploring Expedition, tend to 

 confirm this opinion. 



The conclusion is borne out by the fact that soundings in the 

 course of the various and extensive surveys afford no evidence 

 of growing coral beyond twenty fathoms. Where the depth 

 was fifteen fathoms, coral sand and fragments were almost uni- 

 formly reported. Among the Feejee Islands, the extent of 

 coral-reef grounds surveyed was many hundreds of square 

 miles, besides the harbors more carefully examined. The reefs 

 of the Navigator Islands were also sounded out, with others 

 at the Society Group, besides numerous coral islands ; and 

 through all these regions no evidence was obtained of corals 

 living at a greater depth than fifteen or twenty fathoms. 

 Within the reefs west of Viti Lebu and Vanua Lebu, the anchor 

 of the Peacock was dropped sixty times in water from twelve 

 to twenty four fathoms deep, and in no case struck among 

 growing corals ; it usually sunk into a muddy or sandy bottom. 

 Patches of reef were encountered at times, but they were at a 

 less depth than twelve fathoms. By means of a drag, occasion- 

 ally dropped in the same channels, some fleshy Alcyonia 

 and a few Hydroids were brought up, but no reef-forming 

 species. 



