474 Mr Gardiner, The Coral Reefs of [Mar. 7, 



PART V. 

 Special Features in the Natural History of Corals. 



Section 1. The Structure of a, Reef 



I have already described fully the external appearances of the 

 coral reefs of Funafuti and Rotuma. In Fiji the reefs, which are 

 freely exposed to the ocean, have all a well-defined rim and reef- 

 flat ; some also have a boulder zone, which is naturally absent 

 from the reef at Funafuti owing to the elevation of that atoll. 

 Few have such a luxuriant growth of corals as are found on the 

 reef-flat off the windward end of Rotuma, but otherwise their 

 differences are merely differences in detail. 



The boulder zone shows, I think, the distance from the edge of 

 a reef, at which boulders can be piled up on the reef-flat to form, 

 subsequently, land. From the Penguin the outer reefs on the 

 windward side of Nukulailai, an atoll about 65 miles south of 

 Funafuti, could be seen to be as narrow as in the similar position 

 at that atoll ; on the other hand the same reefs off the leeward 

 island are as broad as on the leeward side of Funafuti, but with no 

 rough zone. In this position at Nukulailai there is a rim much 

 broken up by fissures, followed by a reef-flat upwards of 200 yards 

 broad, hollowed out for 2 — 3 feet with, in places, pools of water 

 5 — 6 feet deep ; this extends to the island itself, which is not more 

 than 2 — 3 feet above the high-water level. The island is formed 

 principally of small boulders and sand, and rests on the reef-flat 

 all round, so that the latter has a total breadth of over 600 yards. 

 This atoll differs greatly from Funafuti, as its lagoon is almost 

 completely encircled by the reef and has everywhere a sandy 

 bottom. Land too encloses the greater part, the broadest gaps in 

 it being to the north-west and south-west. There are a few 

 isolated blocks of rock on the reef, but these are not regular in 

 position and seem rather to be negroheads than pinnacles ; they 

 are more abundant to leeward. 



From the appearance of the atoll of Nukulailai I should not 

 judge that any upheaval is necessary to explain its characters ; 

 there may, however, have been one of 2 — 3 feet. The reef to the 

 south-west, where there is no land, bears a close resemblance to 

 that on the west side of Wakaya, Fiji. It consists of pools of 

 water 1 — 10 feet deep, fringed with corals, nodulose, and laminated 

 nullipores ; Millepora is everywhere very common, as also is 

 Heliopora, a genus only found at 35 fathoms off Funafuti. The 



