CHAPTER II. 
BARRIER-REEFS. 
Closely resemble in general form and stricture atoll-reefs — Width 
and dentil 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 con- 
venient 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 en- 
circling 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 resem- 
