OF THE TONGA ISLANDS. 611 



Tho summit of tlic other three-storied hill, Mo'unga Lafa, is, I 

 believe, as the name implies, flat. The uniform covering- of bush 

 obstructs any jj;eneral view of it ; but in traversing it I was unable 

 to tind any depression corresponding to that in the summit of 

 Talau. 



In the short visits that I paid to tho Vavau (xroup, 1 landed on a 

 few only of the islands, and had not the means of making satis- 

 factory observations of contour. I have drawn attention to the 

 features above described, with the hope that when the survey of the 

 Tonga Islands which was begun by H.M.8. ' Egeria ' is continued, 

 the heights and contours of the islands and terraces occurring at 

 different levels in this remarkable group may receive special 

 attention. 



The peculiar shape of the group, penetrated by long, narrow, and 

 deep inlets of the sea, may perhaps be accounted for by supposing 

 a much-denuded basis, probably of volcanic nature, whose sur- 

 face was deeply scored by valleys previous to its subsidence. While 

 the island was submerged the deposits which formed the limestone 

 rock were laid down, and the lower layers vrould have already begun 

 to accumulate during the downward movement. But, though the 

 base of the limestone deposit may very probably have been formed 

 during subsidence, the upper part and the contour of the land must 

 be chiefly referred to the succeeding period, which has, in the main 

 at least, been one of elevation. 



During intervals in the elevation, the terraces and plateaus which 

 constitute so marked a feature have been formed, first on the highest 

 and later on the lower peaks and ridges, while the intervening- 

 valleys still penetrate far among the islands and promontories. 



It is interesting that, both at Yavau and at Eua, the reefs which 

 have been formed during intervals in the elevation of the islands 

 have in some cases acquired atoll- or barrier-like contours. 



The larger islands of the Hapai Group lie in a line extending 

 from Haauo in the north to Uiha and Alefa in the south. The}' 

 are indeed the elevated parts of a long reef, which, after extending 

 in a south-westerly direction for some 30 geographical miles, sweeps 

 round in an even curve to the west and north-west, and is finally lost 

 among the reefs and islands of the Kotu Group. At two points in 

 the northern part of the reef, gaps are indicated in the chart. The 

 northern of these is bordered by reefs which are continued for a 

 considerable distance in a westerly direction. To the south of the 

 southern part of this reef there is an outer and more interrupted 

 line of reef concentric with it. On the convex or weather side the 

 margin of the reef is even, but on the opposite side the coral grows 

 in irregular patches and the margin is ill-defined. 



The island of Lefuka may be taken as a type of the islands 

 situated on the reef. It is about three quarters of a mile in breadth, 

 and some three or four miles in length. Prom the beach on the 

 western side there is a very gradual slope towards the eastern side 

 f)f the island, which is about 20 feet above high water. Near the 



Q. J. G. S. No. 188. 2 u 



