OF TllK TOXGA ISLANDS. ^15 



but there are no streams or stream-channels on tlie surface. The rain 

 soaks through thci porous rock, and finds its way to the sea b}' 

 underp^round channels excavated in the limestone. The caverns and 

 subterranean j>'alleries which have been thus hollowed out by the 

 water are in some places very extensive, and in walking about the 

 island one is often reminded of their presence by the hollow sound 

 of one's footsteps. 



8ome remarkable features are presented by the narrow reef 

 fringing the southern and eastern shores of Tongatabu. The 

 land gradually slopes towards the ocean, and ends abruptly in a 

 low cliff of coral rock. From the foot of the cliff there extends 

 towards the sea a narrow reef-platform, whose outer edge is raised, 

 forming a barrier * several feet in height. This barrier is in the 

 ])Osition of the mound formed of nuUipores which is commonly 

 found near the seaward edge of a reef-platform f. It presents a 

 steep surface towards the platform, and a vertical one towards the 

 sea. In many places there is on the platform a deijth of five feet 

 or more of water, whose level is above that of the sea, and which is 

 prevented from flowing away by the barrier on the seaward side. 

 The upper surface of the barrier is formed of a number of shallow 

 basins — the upper ones several feet in width — whose rims are per- 

 fectly level and which stand at different heights, the lower basins 

 surrounding the upper, stage below stage, strongly recalling the 

 structure which was presented by the well-known pink and white 

 terraces of Lake Eotomahana in New Zealand. The outer part of 

 the barrier is penetrated by fissures which open above by rounded 

 apertures left between the margins of the basins. 



The rollers, coming up from the open ocean, break against the 

 steep face presented to them by the reef, and are thrown aloft in 

 clouds of spray, while great jets and spouts of water are forced up 

 through the fissures. The water falls into the basins on the barrier, 

 streaming over their level edges along the whole of their circum- 

 ference, to be caught by the basins beneath, and so down from basin 

 to basin. 



The edges and outer sides of the basins are covered with living 

 nuUipores, which grow in encrusting laminae, and flourish in the 

 freshly aerated water which streams over them. Wherever the 

 water flows, the nuUipores grow; and thus the edges of the basins 

 are kept level, and are constantly growing upwards and outwards. 



It appears probable that such a barrier is a form of the nuUipore 

 mound. Its unusual height and the remarkable basin-like formation 

 may be due to the steepness of the outer slope of the reef, causing 

 the waves which break against it to be thrown upwards, and thus 

 the water in which the nuUipores flourish always falls on them from 

 above. 



The reef on the eastern shore of Eua presents similar features. 



The little island of Eua-iki lying opposite the mouth of the eastern 



* This term is used ■without any reference to a barrier-reef, 

 t The nullipore mound at Keeling Island is described in Darwin's ' Coral 

 Reefs,' ;3rd ed. pp. KJ, 14. 



