1836.4 BARRIER-REEFS. 471 
3032 F5 
A 
0 
g Hy Wie 
NM 
1, Vanikoro. 2. Gambier Islands. 3. Maurua. 
The horizontal shading shows the barrier-reefs and lagoon-channels. The inclined 
shading above the level of the sea (AA), shows the actual form of the land; the inclined 
shading below this line, shows its probable prolongation under water. 7 
Are we to suppose that each island is surrounded by a collar- 
like submarine ledge of rock, or by a great bank of sediment, 
ending abruptly where the reef ends? If the sea had for- 
merly eaten deeply into the islands, before they were protected 
by the reefs, thus having left a shallow ledge round them under 
water, the present shores would have been invariably bounded by 
great precipices; but this is most rarely the case. Moreover, on 
this notion, it is not possible to explain why the corals should 
have sprung up, like a wall, from the extreme outer margin of 
the ledge, often leaving a broad space of water within, too deep 
for the growth of corals. The accumulation of a wide bank of 
sediment all round these islands, and generally widest where the 
included islands are smallest, is highly improbable, considering 
their exposed positions in the central and deepest parts of the 
ocean. In the case of the barrier-reef of New Caledonia, which 
extends for 150 miles beyond the northern point of the island, in 
the same straight line with which it fronts the west coast, it is 
hardly possible to believe, that a bank of sediment could thus have 
been straightly deposited in front of a lofty island, and so far be- 
yond its termination in the open sea. Finally, if we look to other 
eceanic islands of about the same height and of similar geological 
constitution, but not: encircled by coral-reefs, we may in vain 
search for so trifling a cireumambient depth as 30 fathoms, except 
21 
