82 EXPEDITION OF THE “ALBATROSS,” 1899-1900. 
To the westward of the Tonga Islands a line of volcanic islands 
extends nearly 200 miles, from Honga Hapai to Fanua lai, some of 
which have been active very recently. Falcon Island disappeared in 
1898, and Letté is still active. This line of voleanoes runs at a distance 
of from fifteen to twenty miles parallel with the trend of the four 
irregularly shaped plateaux upon which rise the Tonga Islands. They 
are the summits of a great ridge, over 200 miles in length, sloping very 
gradually to the westward into deep water, and being somewhat steeper 
to the eastward, towards the smaller platforms from which rise the 
voleanic peaks of the group. The plateaux of Tongatébu, Nomuka, 
Haapai, and Vavau, are separated by deep valleys connecting the eastern 
and western flanks of the ridge. These four plateaux rise abruptly from 
the 100-fathom line. The extremity of the southern one is occupied by 
Tongatébu Island. The land behind the cliffs of its southern coast 
rises to a height of over 250 feet, and slopes northward very gradually 
to form the low land which occupies the northern coast of the island, 
and is, except at Mount Zion and Cook 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 indicated. The northern coast is 
deeply indented by shallow bays, full of islands, reef flats, and reef 
patches, on which corals grow in great profusion. For a distance of 
nearly ten miles northward of Nukualofa the plateau is nowhere more 
than fifteen fathoms deep; and a long tongue runs northward, gradually 
deepening into twenty to fifty fathoms to the 100-fathom line. 
The Tongatébu plateau is separated from the Nomuka Group plateau 
by a funnel-shaped channel with a depth passing rapidly into 300 
fathoms from the 100-fathom line. The Nomuka plateau is rectangular. 
The principal island is Nomuka, where we anchored. 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 by a high sand-beach, about 
200 yards wide, from the sea. 
Nomuka Iki, the island next to Nomuka, we found to consist, at its 
southern extremity, of stratified voleanic material, resembling somewhat 
