DESCRIPTION OF THE PLATES.

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PLATE I.

In the several original surveys, from which the small plans onthese plates have been reduced, the coral-reefs are engravedin very different styles. For the sake of uniformity, I haveadopted the style used in the charts of the Chagos Archi-pelago, published by the East Indian Company , from thesurvey by Capt. Moresby and Lieut . Powell. The surfaceof the reef, which dries at low water, is representedby a surface with small crosses : the coral-islets on the reefare marked by small linear spaces, on which a few cocoa-nut trees, out of all proportion too large, have beenintroduced for the sake of clearness. The entire annularreef which when surrounding an open expanse of water,forms an ‘ atoll , 1 and when surrounding one or more highislands, forms an encircling ‘barrier-reef,’ has a nearlyuniform structure. The reefs in some of the originalsurveys are represented merely by a single line withcrosses, so that their breadth is not given ; I have hadsuch reefs engraved of the width usually attained by coral-reefs. I have not thought it worth while to introduce allthose small and very numerous reefs, which occur withinthe lagoons of most atolls and within the lagoon-channelsof most barrier-reefs, and which stand either isolated, orare attached to the shores of the reef or land. At PerosBanhos none of the lagoon-reefs rise to the surface of thewater; a few of them have been introduced, and aremarked by plain dotted circles. A few of the deepestsoundings are laid down within each reef; they are infathoms, of six English feet.