54 ATOLLS. Ch. I. 



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 slopes abruptly outwards into 

 unfathomable depths. At the distance of less than half 

 a mile from one part, 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 w T e 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 c no- 

 thing more than a half-drowned atoll.' But of what 

 great dimensions, and of how extraordinary an internal 



1 This officer has had the kindness to lend me an excellent MS. 

 account of the Chagos Islands ; from this paper, from the published 

 charrs, and from verbal information communicated to me by Captain 

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



