which bound the central expanse: these, I think, it can be shown, have
resulted from a cause analogous to that which has produced the bifurcating
channel across Mahlos Mahdoo.


CHAPTER II.--BARRIER REEFS.

Closely resemble in general form and structure atoll-reefs.--Width and
depth of the lagoon-channels.--Breaches through the reef in front of
valleys, and generally on the leeward side.--Checks to the filling up of
the lagoon-channels.--Size and constitution of the encircled islands.--
Number of islands within the same reef.--Barrier-reefs of New Caledonia and
Australia.--Position of the reef relative to the slope of the adjoining
land.--Probable great thickness of barrier-reefs.

The term "barrier" has been generally applied to that vast reef which
fronts the N.E. shore of Australia, and by most voyagers likewise to that
on the western coast of New Caledonia. At one time I thought it convenient
thus to restrict the term, but as these reefs are similar in structure, and
in position relatively to the land, to those, which, like a wall with a
deep moat within, encircle many smaller islands, I have classed them
together. The reef, also, on the west coast of New Caledonia, circling
round the extremities of the island, is an intermediate form between a
small encircling reef and the Australian barrier, which stretches for a
thousand miles in nearly a straight line.

The geographer Balbi has in effect described those barrier-reefs, which
encircle moderately sized islands, by calling them atolls with high land
rising from within their central expanse. The general resemblance between
the reefs of the barrier and atoll classes may be seen in the small, but