614 MR. J. J. LISTER ON THE GEOLOGY 



marked on the chart, resembling on a small scale the great Hapai 

 barrier to the north. 



Tongatabu is the largest island of the group. It is some 22 miles 

 in greatest length, and of an irregular crescent shape, with its 

 convexity presented to the south. Considerable tracts along the 

 northern shore are not above the reach of the highest tides ; but the 

 level gradually rises towards the convex border, and the highest part 

 is at the south-east, opposite Eua. 



An extensive shallow area lies to the north of the island. 

 Opposite the middle of the northern shore, and extending for about 

 a third of the whole of its length, there is a large basin, which forms 

 the harbour of Nukualofa. It has an average depth of 15 fathoms, 

 and communicates with the outer water by a deep channel on the 

 eastern side. 



A wide shallow area, having a depth of from 6 to 9 fathoms, 

 extends from the western point of the island eastward, bounding 

 the basin and eastern channel on the north. Along the outer or 

 seaward margin of this area coral reefs grow up to the surface, 

 forming a more or less continuous barrier on the east and north-west. 

 To the north, however, the line of reefs is interrupted, and here 

 shallow banks (though apparently without reefs) extend for some 

 15 miles in the direction of the Nomuka Group. Several islands, 

 some of sand, some of coral rock elevated about 15 feet, are dotted 

 along the reefs on the north of the harbour and eastern channel. 



The interior of Tongatabu is partly occupied by an irregular 

 and shallow lagoon, which communicates with the sea on the north. 



Hegarding the island and the shallow tract to the north as a 

 whole, we find an area whose circumference, represented by the 

 convex border of the island on the south, and the shallow 9-fathom 

 area, on which reefs have grown, to the north, stands at a higher 

 level than the central part, represented, by the lagoon and the 

 15-fathom basin. 



In mentioning this island, Darwin pointed out* that "it re- 

 sembles either an up-raised atoll, with one half originally imperfect, 

 or one unequally elevated." The gaps in the line of reef along 

 the northern margin show that the atoll was imperfect : but the 

 gradual rise of the land towards the convex side, and the fact that 

 it attains its highest point where it most nearly approaches the 

 greatly elevated island of Eua, appear to point strongly to the con- 

 clusion that it has also been unequally elevated. 



The island is formed of coral limestone throughout, and in many 

 l)laces this rock lies at the surface. In the grassy roads of the town 

 of Nukualofa one often meets with flat patches of coral, many feet 

 in diameter, on which the lines of astreids or madrepores are seen 

 radiating from a common centre. Elsewhere the rock is covered 

 with a layer of reddish-brown clay, which in some cases attains 

 many feet in thickness t. 



The contour of Tongatabu is flat or presents gentle undulations, 



* ' Coral Eeefs,' 3rd ed. p. 177. 



t A similar clay overlies the limestone formation of Vavau. 



