40 CORAL-REEFS. -m 



that wherever soundings were obtained off these islands, 

 the bottom was invariably sandy : nor was there any reason 

 to suspect the existence of submarine cliffs, as there was at 

 Keeling Island. 1 Here then occurs a difficulty ; can sand 

 accumulate on a slope, which, in some cases, appears to 

 exceed fifty-five degrees? It must be observed, that I 

 speak of slopes where soundings were obtained, and not of 

 such cases, as that of Cardoo, where the nature of the 

 bottom is unknown, and where its inclination must be 

 nearly vertical. M. Elie de Beaumont 2 has argued, and 

 there is no higher authority on this subject, from the 

 inclination at which snow slides down in avalanches, that a 

 bed of sand or mud cannot be formed at a greater angle 

 than thirty degrees. Considering the number of soundings 

 on sand, obtained round the Maldiva and Chagos atolls, 

 which appears to indicate a greater angle, and the extreme 

 abruptness of the sand-banks in the West Indies, as will 

 be mentioned in the Appendix, I must conclude that the 

 adhesive property of wet sand counteracts its gravity, in a 

 much greater ratio than has been allowed for by M. Elie de 

 Beaumont. From the facility with which calcareous sand 

 becomes agglutinated, it is not necessary to suppose that 

 the bed of loose sand is thick. 



Captain Beechey has observed, that the submarine slope 



1 Off some of the islands in the Low Archipelago the bottom appears 

 to descend by ledges. Off Elizabeth Island, which, however, consists 

 of raised coral, Capt. Beechey (p. 45, quarto ed.) describes three 

 ledges: the first had an easy slope from the beach to a distance of 

 about fifty yards : the second extended two hundred yards with twenty- 

 five fathoms on it, and then ended abruptly, like the first; and 

 immediately beyond this there was no bottom with two hundred 

 fathoms. 



2 Memoires pour servir a une description Geolog. de France, tome iv. 

 p, 216. 



