500 
BA RRIER - REEFS 
CHAP. 
building corals, which cannot live at great depths, based their 
encircling structures ? This is a great apparent difficulty, 
analogous to that in the case of atolls, which has generally been 
overlooked. It will be perceived more clearly by inspecting 
the following sections, which are real ones, taken in north and 
south lines, through the islands with their barrier-reefs, of 
Vanikoro, Gambier, and Maurua ; and they are laid down, both 
vertically and horizontally, on the same scale of a quarter of an 
inch to a mile. 
It should be observed that the sections might have been 
taken in any direction through these islands, or through many 
i. 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. 
SECTIONS OF BARRIER-REEFS. 
other encircled islands, and the general features would have 
been the same. Now bearing in mind that reef-building coral 
cannot live at a greater depth than from 20 to 30 fathoms, and 
that the scale is so small that the plummets on the right hand 
show a depth of 200 fathoms, on what are these barrier-reefs 
based ? 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 
formerly 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 
