Sect. II. ATOLLS. 29 



breach through the reef, has been noticed by several 

 authors. There are some atoll-formed reefs, rising to 

 the surface of the sea and partly dry at low water, on 

 which from some cause islets have never been formed ; 

 and there are others, on which they have been formed, 

 but have subsequently been worn away. In atolls of 

 small dimensions the islets frequently become united 

 into a single horse-shoe or ring-formed strip ; but 

 Diego Garcia, although an atoll of considerable size, 

 being thirteen miles and a half in length, has its 

 lagoon entirely surrounded, except at the northern end, 

 by a belt of land, on an average a third of a mile in 

 width. To show how small the total area of the annu- 

 lar reef and the land is in islands of this class, I may 

 quote a remark from the voyage of Lutke, namely, that 

 if the forty-three rings, or atolls, in the Caroline Archi- 

 pelago, were put one within another, and over a steeple 

 in the centre of St. Petersburg, the whole would not 

 cover that city and its suburbs. 



The form of the bottom, as given by Captain 

 Beechey in his sections of the atolls in the Low 

 Archipelago, exactly coincides with that already de- 

 scribed in Keeling atoll : it gradually slopes to about 

 twenty fathoms, at the distance of between one and 

 two hundred yards from the edge of the reef, and 

 then plunges at an angle of 45 9 into unfathomable 

 depths. l The nature, however, of the bottom seems 



1 The slope of the bottom round the Marshall atolls in the Northern 

 Pacific is probably similar: Kotzebue (First Voyage, vol. ii. p. 16) 

 says, ' We had at a small distance from the reef, forty fathoms depth, 

 which increased a little further so much that y/q could find no bottom.' 



