DESCRIPTION OF THE PLATES. 

 ^ 



PLATE I. 



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

 these plates 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 Archi- 

 pelago, published by the East Indian Company, from the 

 survey by Capt. Moresby and Lieut. Powell. The surface 

 of the reef, which dries at low water, is represented 

 by a surface with small crosses : the coral-islets on the reef 

 are marked by small linear 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 encircling ' barrier-reef,' has a nearly 

 uniform structure. 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. 



