470 BARRIER-REEFS. [cHAP. xx. 
wall sometimes between two and three hundred feet under water 
in height: externally the reef rises, like an atoll, with extreme 
abruptness out of the profound depths of the ocean. What can 
be more singular than these structures? We see an island, 
which may be compared to a castle situated on the summit of a 
lofty submarine mountain, protected by a great wall of coral- 
rock, always steep externally and sometimes internally, with a 
broad level summit, here and there breached by narrow gate- 
ways, through which the largest ships can enter the wide and 
deep encircling moat. 
As far as the actual reef of coral is concerned, there is not the 
smallest difference, in general size, outline, grouping, and even 
in quite trifling details of structure, between a barrier and an 
atoll. The geographer Balbi has well remarked, that an encircled 
island is an atoll with high land rising out of its lagoon ; remove 
the land from within, and a perfect atoll is left. 
But what has caused these reefs to spring up at such great 
distances from the shores of the included islands? It cannot be 
that the corals will not grow close to the land; for the shores 
within the lagoon-channel, when not surrounded by alluvial soil, 
are often fringed by living reefs; and we shall presently see 
that there is a whole class, which I have called Fringing Reefs 
from their close attachment to the shores both of continents and_ 
of islands. Again, on what have the reef-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 per- 
ceived 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 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 ? 
