Febeuaky 23, 1900.] 



SCIENCE. 



291 



wbicli corals grow in great profusion. In 

 a distance of nearly 10 miles northward 

 of ISTuku-Alofa the plateau is nowhere 

 more than 15 fathoms deep ; and a long 

 tongue runs northward, gradually deepen- 

 ing into 20 to 50 fathoms to the 100-fathom 

 line. 



The Tonga-Tabu plateau is separated 

 from the Namuka Group Plateau by a 

 funnel-shaped channel with a depth passing 

 rapidly into 300 fathoms from the 100- 

 fathom line. The Namuka Plateau is rec- 

 tangular. The principal island is Namuka, 

 where we anchored. We found the island 

 to be composed of tertiary elevated corallif- 

 erous limestone with a shallow sink, filled 

 with brackish water, occupying the south- 

 eastern part of the island. The sink is 

 separated by a high sand beach, about 200 

 yai'ds wide, from the sea. 



ITamuka Iki, the island next to Namuka, 

 we found to consist, at its southern ex- 

 tremity, of stratified volcanic material re- 

 sembling somewhat the so-called soapstone 

 of Fiji. I was told that other islands in 

 this group, near Tonumeia, in the center 

 of the Namuka Plateaii, were volcanic. 

 Mango, as we could see it from our an- 

 chorage, appeared to be volcanic. So that 

 this part of the Tongas is, like the Lau 

 Group in Fiji, made up of islands in part 

 volcanic and in part composed of elevated 

 coralliferous limestone. The eastern edge 

 of the Namuka Plateau (whicli we did not 

 visit) is edged with small low islands. We 

 merely steamed past the western islands of 

 the Hapai Group, but close enough to see 

 that Tongua, Kotu, and Fotuhaa, which are 

 respectively 120, 120 and 200 feet high, are 

 composed of elevated limestone. The east- 

 ern flank of the Hapai Plateau is edged 

 with long low islands with extensive coral 

 reefs along the reef flats of these islands. 



The Hapai Plateau is triangular, with 

 isolated islands rising ou the northwestern 

 side from the deep water separating it from 



the Vavau Plateau. It is separated from 

 the Namuka Plateau by a narrow channel 

 with over 300 fathoms of water. 



The northernmost plateau of the broad 

 ridge of the Tonga Islands is the Vavau 

 Plateau. This is elliptical, with a long 

 tongue extending on the eastern face of the 

 ridge toward the northern point of the 

 Hapai Plateau, ending in isolated banks 

 (the Disney and Falcon banks), lying to 

 the northward of the broad channel, with 

 over 400 fathoms separating it from the 

 Hapai Group. The Vavau Group is by 

 far the most picturesque of the Tonga 

 Islands. It consists of the principal island 

 of Vavau, extending across the northern 

 part of the Vavau Plateau. Several parts 

 of the island of Vavau are finely terraced ; 

 four terraces are indicated there, and other 

 fiat-topped smaller islands show traces of 

 two or three terraces. The northern edge 

 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 fianked 

 by innumerable islands and islets, some of 

 considerable height (150 to 250 feet), which 

 gradually become smaller and smaller as 

 they rise toward the southward and east- 

 ward, these islands having been formed 

 from 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 on the eastward and 

 southward by the action of the sea. The 

 islands and islets to the southward of the 

 main islands rise from more or less exten- 

 sive reef flats which stud the whole plateau, 

 and on which corals grow in great profusion 

 (mainly Millepora, Porites, Pavonia, Pocil- 

 lopora, Fungia, and Astrea), to a depth of 

 5 to 6 fathoms in the sound. In the 

 Namuka Group they extended in the more 

 open waters to 14 and 16 fathoms. 



