44 



J. STANLEY GARDINER. 



some of its masses attain a height of 3 feet above the reef-flat, but the general level is 

 1 to 2 feet less. What appears to be the inner pai-t of the reef-flat forms the foundation ; 

 it is to some extent worn into holes, in which the larger loose masses lie, imbedded in sand. 

 The three islands merge at their ends into this zone, masses of the conglomerate extending 

 out from each for 100 yards or more along it. It has a twofold origin, consisting of 

 boulders cast into this position from the reef and lagoon, and also of masses of the con- 

 glomerate, some loose and some attached to the solid platform below. Relatively little of 

 the whole, however, consists of masses attached to the reef, the greater part being formed 

 of loose blocks of the old coral-rock. Towards the outer side a certain number of coral 

 blocks are thrown up from seaward, and on the lagoon side also a few may be washed 



S?s 





^^ 



Fig. 11. Boulder zone and reef-flat between Wiringili and Eagandi. {From a photograph.) 



up. The sand beneath is coarse, consisting of fragments of the conglomerate and the 

 washings of the reef The boulders are singularly free from all boring and other destructive 

 organisms ; their exposed surfaces are absolutely bare of any animal or plant life. Little 

 can grow on the basal reef on account of the sand, and indeed no fixed organisms of any 

 sort are found save on the undersides of boulders, which may be almost covered with thin 

 sponge, Tunicate or Polyzoa colonies in numerous small incrustations with possibly a little 

 Hydroid and Actinian growth as well. Brilliantly coloured Turbellaria and similar looking 

 Nudibranchiata browse on these. Crabs of thin compressed genera and Ophiurids hurry to 

 escape on every side ; numerous Macrurans dart for shelter. Under the boulders a few sharp- 

 spined Echinids, thin-skinned Holothurians and large Cyprids lie free on the surface, and 

 in the sand may be found an occasional Polychaet and Sipunculid. In places the boulder 

 zone may be lower, where the water of the rising tide finds its earliest access to the lagoon, 

 and its surface practically smooth and bare. 



While the boulder zone has the above enumerated characters as far as Ragandi, it ceases 

 further north to be nearly so well defined. Indeed it is merely the inner part of the 

 reef-flat, sparsely strewn with boulders with a few masses of the conglomerate here and 

 there. The latter do not exist opposite the boat channels, and are most numerous inter- 

 mediate to these. Close to the north passage there are a large series of such masses, 

 many of great size, forming almost a bare conglomerate flat, covered with about a foot of 

 water at high tide. 



