56 



CHAPTEE 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 Cale- 

 donia and Australia — Position of the reef relative to the slope of the 

 adjoining land — Probable great thickness of barrier-reefs. 



The term 6 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- 



