THE TU^'GA AKCHirELAGO. 175 



THE TONGA ARCHIPELAGO. 



Plates lll-ms, 213-219. 



The Tonga Islands we found a most interesting group. The elevated 

 tertiary coralliferoas limestones take here their greatest development, and 

 are on a scale far beyond that of their development in Guam, the Lau 

 group of the Fijis, or the Paumotus. The first island of the Tongas we 

 visited, Eua, is perhaps the most interesting of the islands composed mainly 

 of tertiary elevated coralliferous limestone I have visited. The four 

 plateaus of Tongatabu, Nomuka, Haapai, and Vavau (Pis. 214-219), which 

 constitute the Tonga Ridge, rise abruptly from the 100-fathom line ; they are 

 separated by deep valleys connecting the eastern and western flanks of the 

 ridge. The extremity of the southern one is occupied by Tongatabu Island. 

 The land behind the cliffs of its southern coast rises to a height of over 270 

 feet, and slopes northward very gradually to form the low land which occu- 

 pies the northern coast of the island, where it is, except at Mount Zion and 

 Cook's Hill, not more than from five to twenty feet above the level of the 

 sea. At Cook Point and along the southern coast three terraces are in- 

 dicated. The northern coast is deeply indented by shallow bays, full of 

 islands, reef flats, and reef patches, on which corals grow in great profu- 

 sion. For a distance of nearly ten miles northward of Nukualofa the 

 plateau is nowhei'e more than fifteen fathoms deep ; it then runs as a long 

 tongue northward, gradually deepening into twenty to fifty fathoms to the 

 100-fathom line. 



The Tongatabu Plateau is separated from the Nomuka Plateau by a 

 funnel-shaped channel with a depth passing rapidly into 300 fathoms from 

 the 100-fathom line. The Nomuka Plateau is rectangular (PI. 216) ; its prin- 

 cipal island is Nomuka. We found the island to be composed of tertiary 

 elevated coralliferous limestone, with a shallow sink, filled with brackish 

 water, occupying the southeastern part of the island. The sink is separated 

 from the sea by a high sand beach, about 200 yards wide. 



