OF THE TONGA ISLANDS. 613 



The raised reef-platform of which the islands to the north are 

 formed, as well as the raised coral-island Fotiiha, and those of the 

 Kotu (Jronp, show that elevation has taken i)laco, tliough only to a 

 comparatively small extent, in the northern and western parts of the 

 group. On the other hand, the absence of islands of any consider- 

 able size from the southern parts of tlie reefs appears to indicate 

 that this region was not affected by the elevation. 



The island of Nomuka in the southern part of the Hapai Group is 

 triangular in shape, the sides, which are about equal, measuring 

 some 2 miles in length. To the south lies the little island Nomuka- 

 iki, separated by a channel a mile and a half wide, and 15 fathoms 

 deep. 



A large shallow lagoon occupies a great part of the larger island ; 

 it is less than a fathom deep, and its level does not rise and fall 

 with the tide, while the water it contains is much denser than sea- 

 water *. At the bottom of the lagoon there is a thick layer of black 

 mud, which gives out a strong smell of decomposing matter when 

 it is stirred. The land border is broad on the west and north- 

 west of the lagoon, but narrow on the other sides. A ridge on the 

 western side attains a height of over 160 feet at two points. On 

 the east the land is much lower, varying from 15 to 96 feet above 

 the sea, and on the south-west it is often hardly higher than the 

 top of the beach. The formation consists of coral rock. 



Xomuka and jS'omuka-iki are surrounded by fringing reefs, which 

 in the case of the latter stretch out a considerable distance (in one 

 part over a mile) from the shore. The twenty-fathom line encloses 

 both islands : on the eastern and north-western sides of I^omuka 

 it is about half a mile from the edge of the reef, while on the west 

 and south-west of iS^omuka-iki it is a mile away and encloses some 

 outlying coral patches. Outside the twenty-fathom line the slope is 

 very gradual. Eight miles from land on the west and south-west 

 (the only directions in which distant soundings have been taken) the 

 depth is not greater than 50 fathoms t. 



Xomuka is evidently an atoll which has been formed in shallow 

 water, though it is only in the last stage of its elevation that the 

 ring has been completed ; and here the shallowness of the surround- 

 ing sea appears to exclude subs^'dence as a factor in its formation : 

 for the summit of the elevation on which, on that hypothesis, the 

 limestone formation rests would lie beneath the lagoon ; but the 

 floor of the lagoon is only some i) fathoms higher than the bottom 

 of the surrounding sea outside the reef. 



To the east of the Nomuka Group, a reef dotted with islands is 



* On comparing the density of the lagoon-water with that of sea-water and 

 distilled water by means of a hydrometer, the readings were : — 



Distilled water — 2 



Sea-water -|-25 



Lagoon-water -|-o9 



t Opposite the western end of the channel between the islands 1 obtained a 

 branch of a living madrepore at a depth of 81 fathoms, and a living fragment 

 of one of the Poritidce caaie up on the ' Egeria's ' sounding-lead from the samo 

 depth. 



2it2 



