64 BARRIER REEFS. Ch. II. 



average distance from the land is between twenty and 

 thirty miles, but in parts from fifty to ninety. The 

 great arm of the sea thus included, is from ten to 

 twenty-five fathoms deep, with a sandy bottom ; but 

 towards the southern end where the reef is further 

 from the shore, the depth gradually increases to forty, 

 and in some parts to more than sixty fathoms. Flinders 

 has described the surface of the reef as consisting of a 

 hard white agglomerate of different kinds of coral, 

 with rough projecting points. A few low islets have 

 been formed on it. The outer edge is the highest 

 part ; it is traversed by narrow gullies, and at intervals 

 by ship- channels. The sea close outside is in most 

 parts profoundly deep ; but to the north, near New 

 Guinea, and to the south, the depth is much less, and 

 here the bottom slopes gradually from the reef, as it 

 generally does in front of the ship-channels. 1 



There is one important point in the structure of 

 barrier-reefs which must here be considered. The 

 accompanying diagrams represent north and south ver- 

 tical sections, taken through the highest points of Vani- 

 koro, Grambier, and Maurua islands, as well as through 

 their encircling reefs. The scale both in the horizontal 

 and vertical direction is the same, namely, a quarter of 

 an inch to a nautical mile. The height and width of 

 these islands are known ; and I have attempted to repre- 



1 The foregoing details are taken chiefly from Flinders' Voyage to 

 Terra Australis, vol. ii. p. 88 ; but these have been corrected by the 

 account given by Prof. Jukes, Narrative of the Voyage of the Fly, 

 vol. i. 1847, chap. xiii. 





