THE TONGA ARCHIPELAGO. 179 



extremity of Vavan, rises the island of Toku, and nearly ten miles farther 

 the volcanic island of Fanua lai, at the extremity of a line of volcanic 

 islands separated from the Tonga Ridge by a channel less than 1000 fathoms. 

 Falcon rises from a depth of about 1000 fathoms, Tofua and Kao and Late 

 from less than 500 fathoms. The islands on the northern extension of the 

 Vavau Plateau rise from somewhat greater depth. The volcanic islands, 

 as far as Late, are on the edge of a bank, parallel with the Vavau Plateau, 

 separated from it by a channel about 900 fathoms deep. 



At the Tongas the volcanic outbursts are mainly indicated by the summits 

 of a volcanic ridge running parallel to the greater Tonga Plateau, with the 

 exception of the few outbursts through the limestone area which have been 

 traced in the Nomuka group. 



Of course, until all the islands have been carefully examined, it is impos- 

 sible to say that other points may not show volcanic outbursts, as those 

 observed in Nomuka.^ But it is very evident that the distinction, which 

 was first made by Lister, of islands partly volcanic and partly of corallifer- 

 ous limestone, or wholly volcanic or limestone, is one of a very genei'al 

 character. We find scattered over the Pacific volcanic islands in the Cook, 

 the Society, the Hawaiian, the Samoan, and the Fiji Island groups, or we 

 find coralliferous limestone islaiids, in the Paumotus, the Fijis, the Tongas, 

 and the Cook Islands. Or we find a combination of the two in the vai'ious 

 groups we have mentioned. 



The existence of such a gigantic plateau of elevated limestone, like that 

 of the Tongas, which extends for nearly 200 miles in length and with an 

 average width of over twenty miles, indicates that the formation of these 

 huge masses of limestone must have taken place over extensive areas of the 

 sea during Tertiary times. In the case of the Tongas, the limestone was 

 probably deposited at a depth not much greater than 1000 fathoms, and 

 has been raised to form the gigantic ridge known as the Tonga or the 

 Friendly Islands. 



Of course this thick mass of coralliferous limestone could only have been 

 formed during a period of subsidence ; but it was only after its elevation, 



1 Mango, Tanoa, Nomuka iki, Tonumeia, and Kelefesia are stated by Lister, to be formed of vol- 

 canic tuff. 



