Oh. II. 
BARRIER-REEFS. 
59 
analogous to that low and inner part of the islets in 
many atolls, which is formed by the accumulation of 
matter from the lagoon. At Hogoleu (fig. 2, Plate I.), 
in the Caroline Archipelago, 1 the reef on the south 
side is no less than twenty miles ; on the east side, 
five ; and on the north side, fourteen miles from the 
encircled islands. 
The lagoon-channels may be compared in every 
respect with true lagoons. In some cases they are 
open, with a level bottom of fine sand ; in others they 
are choked up with reefs of delicately branched corals, 
which have the same general character as those within 
Keeling atoll. These internal reefs either stand sepa- 
rately, or more commonly skirt the shores of the in- 
cluded high islands. The depth of the lagoon-channel 
round the Society Islands varies from two or three, 
to thirty fathoms ; in Cook’s 2 chart of Ulietea, how- 
ever, there is one sounding laid down of 48 fathoms : 
at Yanikoro there are several of 54 and one of 56^ 
fathoms (English), a depth which even exceeds by a 
little that of the interior of the great Maldiva atolls. 
Some barrier-reefs have very few islets on them ; whilst 
others are surmounted by numerous ones ; and those 
round part of Bolabola (Plate I., fig. 5), form a single 
linear strip. The islets first appear either on the 
angles of the reef, or on the sides of the breaches 
1 See Hydrographical Mem. and the Atlas of the Voyage of the 
Astrolabe, by Capt. Dumont D’Urville, p. 428. 
2 See the chart in vol. i. of Hawkesworth’s 4to ed. of Cook’s First 
Voyage. 
