DESCRIPTION OF THE PLATES. 



PLATE I. at end of Volume. 



In the several original surveys, from which the small plans on this 

 plate have been reduced, the coral-reefs are engraved in very different 

 styles. For the sake of uniformity, I have adopted the style used 

 in the charts of the Chagos Archipelago, published by the East 

 India Company, from the survey by Capt. Moresby and Lieut. 

 Powell. The surface of the reef, which dries at low water, is 

 represented by a stippled surface with small crosses : the coral- 

 islets on the reef are marked by small linear unstippled spaces, on 

 which a few cocoa-nut trees, out of all proportion too large, have 

 been introduced for the sake of clearness. The entire annular reef, 

 which when surrounding an open expanse of water, forms an ' atoll,' 

 and when surrounding one or more high islands, forms an en- 

 circling ' barrier-reef,' has a nearly uniform structure, and has been 

 tinted, in order to catch the eye, of a pale blue colour. The reefs 

 in some of the original surveys are represented merely by a single 

 line with crosses, so that their breadth is not given ; I have had 

 such reefs engraved of the width usually attained by coral-reefs. I 

 have not thought it worth while to introduce all those small and 

 very numerous reefs, which occur within the lagoons of most atolls 

 and -within the lagoon-channels of most barrier-reefs, and which 

 stand either isolated, or are attached to the shores of the reef or 

 land. At Peros Banhos none of the lagoon-reefs rise to the surface 

 of the water ; a few of them have been introduced, and are marked 

 by plain dotted circles. A few of the deepest soundings are laid 

 down within each reef; they are in fathoms, of six English feet. 



Fig. 1. — Vanikoeo, situated in the western part of the S. Pacific; taken 



from the survey by Capt. D'Urville in the Astrolabe ; the scale is ^ 



of an inch to a geographical mile ; the soundings on the southern 



side of the island, namely from 30 to 40 fathoms, are given from 



a 



