CORAL-REEFS. 39 



The slope round Christmas atoll (lat. i° 4' N., 157 45' W.), 

 described by Cook, 1 is considerably less ; at about half a mile 

 from the edge of the reef, the average depth was about four- 

 teen fathoms on a fine sandy bottom, and at a mile, only 

 between twenty and forty fathoms. It has no doubt been 

 owing to this gentle slope, that the strip of land surrounding 

 its lagoon has increased in one part to the extraordinary 

 width of three miles ; it is formed of successive ridges of 

 broken shells and corals, like those on the beach. I know 

 of no other instance of such width in the reef of an atoll ; 

 but Mr. F. D. Bennett informs me that the inclination of the 

 bottom round Caroline atoll in the Pacific, is like that off 

 Christmas Island, very gentle. Off the Maldiva and Chagos 

 atolls, the inclination is much more abrupt; thus at 

 Heawandoo Pholo, Lieutenant Powell 2 found fifty and sixty 

 fathoms close to the edge of the reef, and at 300 yards dis- 

 tance there was no bottom with a 300-yard line. Captain 

 Moresby informs me, that at 100 fathoms from the mouth 

 of the lagoon of Diego Garcia, he found no bottom with 

 150 fathoms; this is the more remarkable, as the slope is 

 generally less abrupt in front of channels through a reef, 

 owing to the accumulation of sediment. At Egmont 

 Island, also, at 150 fathoms from the reef, soundings were 

 struck with 150 fathoms. Lastly, at Cardoo atoll, only 

 sixty yards from the reef, no bottom was obtained, as I am 

 informed by Captain Moresby, with a line of two hundred 

 fathoms ! The currents run with great force round these 

 atolls, and where they are strongest, the inclination appears 

 to be most abrupt. I am informed by the same authority, 



1 Cook's Third Voyage^ vol. ii. chap. 10. 



2 This fact is taken from a MS. account of these groups lent me by 

 Captain Moresby. See also Captain Moresby's paper on the Maldiva 

 atolls in the Geographical Journal > vol. v. p. 401. 



