Cel. H. 
BARRIER-REEFS. 
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 Gambier, Ualan, and some 
other encircled islands, shew 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 150 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. 
