64 
BARRIER-REEFS. 
Cii. II. 
thirty miles, but in parts from fifty to ninty. The 
great arm of the sea thus included, is frou ten to 
twenty-five fathoms deep, with a sandy botfim ; but 
towards the southern end where the reef it further 
from the shore, the depth gradually increasesto forty, 
and in some parts to more than sixty fathoms. iiinders 
has described the surface of the reef as consistng of a 
hard white agglomerate of different kinds ol coral, 
with rough projecting points. A few low isleti have 
been formed on it. The outer edge is the hghest 
part ; it is traversed by narrow gullies, and at intervals 
by ship-channels. The sea close outside is in nost 
parts profoundly deep ; but to the north, near ^ew 
Guinea, and to the south, the depth is much less, f,nd 
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, Gambier, 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- 
sent the form of the land from the shading of the hills 
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. 
