Ch. II. BARRIER REEFS. 59 



from the hills ; it is an encroachment on the channel, 

 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 

 separately, or more commonly skirt the shores of the 

 included high islands. The depth of the lagoon-chan- 

 nel 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 Vanikoro 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 



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 Hawkes worth's 4to ed. of Cook's First 

 Voyage. 



