Ch.IL bareiee eeefs. 57 



blance between the reefs of the barrier and atoll 

 classes may be seen in the small, but accurately re- 

 duced charts on Plate I., 1 and this resemblance can be 

 further shown to extend to every part of their struc- 

 ture. Beginning with the outside of the reef; many 

 scattered soundings off Grambier, Oualan, and some 

 other encircled islands, show that close to the breakers 

 there exists a narrow shelving margin, beyond which 

 in most cases, the ocean suddenly becomes unfathom- 

 able. Off the west coast of New Caledonia, Captain 

 Kent 2 found no bottom with 1 50 fathoms, at two ship's 

 lengths from the reef; so that the slope here must be 

 nearly as precipitous as off the Maldiva atolls. 



I can give little information regarding the kinds 

 of corals which live on the outer margin* When I 

 visited the reef at Tahiti, although it was low-water, 

 the surf was too violent for me to see the living 

 masses ; but, according to what I heard from some in- 

 telligent native chiefs, they resemble in their rounded 

 and branchless forms, those on the margin of Keeling 

 atoll. The extreme verge of the reef which was 

 visible between the breaking waves at low-water, con- 

 sisted of a rounded, convex, artificial-like breakwater, 

 entirely coated with Nulliporse, and absolutely similar 

 to that which I have described at Keeling atoll. 

 From what I heard when at Tahiti, and from the 



1 The authorities from which these charts have been reduced, 

 together with some remarks on them, are given in a separately ap- 

 pended page, descriptive of the Plates. 



2 Dalrymple, Hydrog. Mem. vol. iii. 



