THE TONGA ARCHIPELAGO. 177 



three terraces. The northern edge of Yavau Island rises to a height of 

 more than 500 feet, and slopes in a general way southward and inland. 

 The southern shore is deeply indented by bays and sounds, and flanked by 

 innumerable islands and islets, some of considerable height (150 to 250 feet), 

 they gradually become smaller and smaller toward the southward and east- 

 wai'd. These islands have been formed by the denudation and erosion of the 

 greater Vavau ; they form tongues of land and sea and sounds of all shapes 

 and sizes, showing the traces of the former land connections of the islands 

 and islets, and their disintegration to the eastward and southward by the 

 action of the sea. The islands and i-slets to the southward of the main island 

 rise from more or less extensive reef flats which stud the whole plateau, and 

 on which corals grow in great profusion (mainly Millepora, Porites, Pavonia, 

 Pocillipora, Fungia, and Astrea), and to a depth of five to six fathoms in the 

 sounds. In the Nomuka group they extend in the more open waters to 

 fourteen and sixteen fathoms. 



It is evident that in the Tonga group, which is a very extensive area of 

 elevation, recent corals have played iio part in the formation of the masses 

 of land and of the plateau of the Tonga Kidge, and that here, as in the 

 Society, the Cook and Fiji Islands, also in areas of elevation, tliey are a 

 mere thin living shell or crust, growing at their characteristic depths, upon 

 platforms which are either volcanic or calcareous, the formation of which 

 has been independent of their growth. 



It appears probable to Lister that the atoll-shaped islands of the southern 

 part of the Tonga group have grown on a bank of volcanic material laid out 

 in shallow water, and that there is no necessity to call on the hypothesis of 

 subsidence to account for their formation. The distribution of the land masses 

 of the Tonga Archipelago is quite different in the four plateaus. In Vavau 

 the main land mass occupies the northern edge of the plateau ; in Tonga- 

 tabu it flanks the southern edge, while in the Haapai group the narrow 

 eastern rim is edged by a row of broad islands, and in Nomuka the Otu 

 Tolu group on the east face and Nomuka Island on the northern face con- 

 stitute the elevated area of that plateau. 



To the westward of the Tongas, from Ilonga Hapai to Fauna lai, a line of 

 volcanic islands extends for nearly 200 miles ; some of them have been active 



12 



