PREFA TOR Y NOTE. 



VI 1 



This last example of reef has been compared by Jukes 1 to 

 'a great submarine wall or terrace, fronting the whole north-east 

 coast of Australia, resting at each end on shallow water, but 

 rising from very great depths about the centre ; its upper 

 surface forming a plateau covered by 10 to 30 fathoms of 

 water, but studded all over with steep-sided block-like masses 

 which rise up to low water-level. These masses are especially 

 numerous, and most linear along the edge of the great bank 

 on which they rest; the passage between them being often very 

 narrow, like regular embrasures opened here and there through 

 the parapet wall of a fortress. These 'individual reefs' 

 running along the outer edge protect the comparatively shallow 

 water inside, and with the numerous inner reefs that are 

 scattered over its space make it one great natural harbour.' 



The third and last class, or 'atoll' (Fig. 3), is an elliptical, oval 

 or roundish ring of coral, with here and there a break in its 



Fig. 3.— Stewart Atoll or Sikiana (lat. 8° 22' S. ; long. 162° 58' E.). 

 C, Reef Channel ; F, Faule Island. 



continuity, and with a central lake-like expanse of water, known 

 as the lagoon. The outside water is generally very deep, and 

 the inside shallow ; thus off the Cocos-Keeling Atoll the 



1 Manual of Geology ; p. 131 ; Voyage of H.M.S. Fly, vol. i. 

 chap. xiii. 



