CORAL-REEFS. 59 



exaggerated. Its longest axis is ninety nautical miles, and 

 another line drawn at right angles to the first, across the 

 broadest part, is seventy. The central part consists of a 

 level muddy flat, between forty and fifty fathoms deep, 

 which is surrounded on all sides, with the exception of 

 some breaches, by the steep edges of a set of banks, rudely 

 arranged in a circle. These banks consist of sand, with a 

 very little live coral ; they vary in breadth from five to 

 twelve miles, and on an average lie about sixteen fathoms 

 beneath the surface ; they are bordered by the steep edges 

 of a third narrow and upper bank, which forms the rim to 

 the whole. This rim is about a mile in width, and with the 

 exception of two or three spots where islets have been 

 formed, is submerged between five and ten fathoms. It 

 consists of smooth hard rock, covered with a thin layer of 

 sand, but with scarcely any live coral ; it is steep on both 

 sides, and outwards slopes abruptly into unfathomable 

 depths. At the distance of less than half a mile from one 

 parr, no bottom was found with 190 fathoms; and off 

 another point, at a somewhat greater 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 IV.), although reduced to 

 so small a scale, without at once perceiving that the Great 

 Chagos Bank is, in the words of Capt. Moresby, 1 " nothing 

 more than a half- drowned atoll." But of what great 

 dimensions, and of how extraordinary an internal structure ? 



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 Capt. 

 Moresby, the above account of the Great Chagos Bank is taken. 



