AGASSIZ: FIJI ISLANDS AND CORAL EEEFS. 55 



in length. From the extremities of the islands off the south face projects 

 a horn of the outer reef, forming a small enclosed lagoon separated from 

 the principal lagoon and with a greatest depth of ten fathoms. The 

 greatest depth of the principal lagoon is 23 fathoms. The northern face 

 of the outei' reef is much broken, and flanked by large patches of corals 

 and heads, with a wide entrance into the northwestern end of the lagoon. 

 The western face of the lagoon is three miles long. The inner edge of 

 the western face is flanked by a broad belt of coral patches and heads. 

 The coral patches are growing upon sunken heads of elevated reef rock, 

 the remnants of the elevated reef once covering the area now occupied 

 by the Aiwa lagoon. The reef flats consisted wherever we saw them of 

 elevated reef rock planed off" to a general level, upon which coral patches 

 were growing. 



This group is hfstorically interesting, as Dana in his Corals and Coral 

 Islands (p. 2G4) gives a section across the islands and lagoon, which 

 according to him illustrates very forcibly the effect of subsidence in 

 the formation of atolls. A more unfortunate selection could not have 

 been made. But Dana did not visit this group, and took his information 

 from the charts, or he would not have chosen as an illustration an island 

 group which consists of elevated limestone rock, — islands which after 

 their elevation have suffered most extensive denudation and erosion, and 

 upon the outer edge of the surrounding platform the corals of the present 

 epoch have found a footing and formed a sheet of very moderate thick- 

 ness. The lagoon between the encircling reef and the islands has 

 been hollowed out by mechanical causes similar to those which T have 

 alluded to elsewhere, and which have in my opinion shaped the lagoons 

 of all the atolls and barrier reefs of the Fijis. 



Dana assumes the lagoon of Aiwa to have been formed during the 

 slow subsidence of the island enclosed within its reef, while there is every 

 evidence that the lagoon of Aiwa has been scooped out by submarine 

 erosion, and the island, consisting of late tertiary limestones, has been 

 lowered by denudation, and that the limestones after their elevation have 

 remained at the present level, and that the corals now growing upon the 

 outer rim and upon the reef flats, as well as the coral heads insid» of the 

 lagoon, have but little thickness, and that well within the greatest depth 

 at which reef-building corals can grow. 



