612 MK. J. J. LISTEK ON THE GEOLOGY 



eastern side there is a rather rapid descent, teiminating in a liti^le 

 cliff some 10 feet high, whose base is much excavated by the waves. 

 The rock of which the cliff is made consists of masses of coral of 

 various sizes, cemented together into a conglomerate. Its structure, 

 in fact, resembles that of a shore-platform. Extending from the foot 

 of the cliff some 300 yards to seaward is the present shore-platform, 

 on which, at low tide, there is about a foot of water near the shore, 

 while the outer part lies bare. The cliff of raised rock extends 

 along the whole of the eastern shore of the island. 



Some distance from the concave side of the long Hapai reef there 

 are numerous shoals and small islands. The island of Fotuha, north 

 of the Kotu Group, displays the abrupt undermined cliffs of a 

 raised coral-island, and stands at an elevation of about 100 feet 

 above the sea. 



The few soundings that are given in the chart show depths of 

 10 to 24 fathoms along the greater part of the western boundary of 

 the reef, though 30 fathoms is marked opposite the northern break 

 in the reef above alluded to, and 56 fathoms within its northern 

 border. 



The Kotu Group consists of a number of small islands, from whose- 

 shores considerable tracts of reef extend at sea-level. 



The only island of this group on which I landed was Hafeva. I 

 found no raised coral-rock, but a loose sandy formation which 

 attained a height of some 12 feet at the shore. The interior of the 

 island is depressed and occupied by a marsh. There is a wide gap 

 among the reefs on the western side, with a depth of some 8 fathoms, 

 forming an anchorage which was at one time used by whalers.- 

 Of the other islands of the Kotu Group, three show the deeply 

 undermined cliff's characteristic of raised coral-islands ; but two of 

 them, Kotu and Matuku, as already stated, recall, in their reddish- 

 brown clifi's and peculiar outline, the tuff islands of Mango and 

 jS^omuka-iki of the southern part of the Hapai Group (see p. 598). 



It seems clear that the long reef of the Hapai Group must he 

 regarded as a barrier-reef or an imperfect atoll. The shoals and 

 islands situated within the concavity of the reef, both the northern 

 ones and those of the Kotu Group, probably either rest on or are 

 parts of the mass on whose eastern and southern slopes the reef has 

 grown, and the volcaric character which I thought I recognized in 

 two of the latter offers some evidence as to the kind of formation of 

 which the basis is composed. 



The outer line of reefs, lying some four miles to the south of the 

 main one, and concentric with it, is a very remarkable feature. Its 

 relation to that reef seems to imply that it is of an older date, and 

 accords very well with the view that the reefs are slowly growing 

 outwards on the slopes of a submerged mound. It must be ob- 

 served, however, that in the northern part of the great reef, in the 

 neighbourhood of the larger islands, there appears no reason for 

 supposing that the reef is moving away from the centre of the group, 

 for it is bordered by flourishing coral-reefs on the western as well 

 as on the eastern side. 



