68 CORAL-REEFS. 



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 1 has 

 described the surface of this reef as consisting of a hard 

 white agglomerate of different kinds of coral, with rough 

 projecting points. The outer edge is the highest part ; it 

 is traversed by narrow gullies, and at rare intervals is 

 breached by ship-channels. The sea close outside is 

 profoundly deep; but, in front of the main breaches, 

 soundings can sometimes be obtained. Some low islets 

 have been formed on the reef. 



There is one important point in the structure of barrier- 

 reefs which must here be considered. The accompanying 

 diagrams represent north and south vertical sections, taken 

 through the highest points of Vanikoro, Gambier, and 

 Maurua Islands, and 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 is known ; and I have 

 attempted to represent the form of the land from the 

 shading of the hills in the large published charts. It has 

 long been remarked, even from the time of Dampier, that 

 considerable degree of relation subsists between the in- 

 clination of that part of the land which is beneath water 

 and that above it; hence the dotted line in the three 

 sections, probably, does not widely differ in inclination from 

 the actual submarine prolongation of the land. If we now 

 look at the outer edge of the reef (AA), and bear in mind 

 that the plummet on the right hand represents a depth of 

 1,200 feet, we must conclude that the vertical thickness of 

 these barrier coral-reefs is very great. 



1 Flinders' Voyage to Terra Australis, vol. ii. p. 88. 



