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distance, there was none with 210 fathoms. Small steep-sided banks or knolls, covered with luxuriantly- growing coral, rise from the interior expanse to the same level with the external rim, which, as we have seen, is formed only of dead rock. It is impossible to look at the plan (fig. 1, Plate II.), although reduced to so small a scale, without at once perceiving that the Great Chagos Bank is, in the words of Captain Moresby,1 'nothing more than a half-drowned atoll.' But of what great dimensions, and of how extraordinary an internal structure ! We shall hereafter have to consider both the cause of its submerged condition, a state common to other banks in the group, and the origin of the singular submarine terraces 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. 1 This officer has had the kindness to lend me an excellent MS. account of the Chagos Islands; from this paper, from the published charts, and from verbal information communicated to me by Captain Moresby, the above account of the Great Chagos Bank is taken. chapter{Section 456 CHAPTER II. BAERIER-REEFS. Closely resemble in general form and structure atoll-reefsWidth and depth of the lagoon-channelsBreaches through the reef in front of calleys, and generally on the leeward sideChecks to the filling up of the lagoon-channelsSize and constitution of the encircled islandsNumber of islands within the same reef Barrier-reefs of New Caledonia and AustraliaPosition of the reef relatice to the slope of the adjoining land Probable great thickness of barrier-reefs. The term ' barrier' has been generallyapplied to that vast reef which fronts the N.E. shore of Australia, and by most...
