AGASSIZ: FIJI ISLANDS AND CORAL EEEFS. 27 



— remnants left from the disintegration and erosion of the former o-reater 

 Mbengha. 



The bottom in the lagoon is a mixture of volcanic mvxd and coral sand 

 in the vicinity of the islands, but as we pi-oceed towards the reef it car- 

 ries a greater admixture of coralline aXgse and of coral sand, and in the 

 belt adjoining the inner edge of the outer reef is made up entii'cly of 

 fragments of coi-al, of coral sand, and of coralline algte. In the central 

 parts of the lagoon it is algse and corallines. 



The reef rises very gradually from seven fathoms to a depth of from 

 two to three feet on the reef flat. This is covered w'ith fragments of 

 dead corals which increase in number towards the sea edge of the reef. 

 The fragments are covered with algte, corallines, and nullipores, which 

 cement them together. In depths of seven to eight fathoms heads and 

 clusters of corals begin to grow. They are separated by wide lanes of 

 coral sand, and as we rise on the slope of the reef they grow more closely, 

 forming a wide belt of thriving corals from six to three or two fathoms 

 in depth, when they grow less profusely, and finally pass into the wide 

 flat area of the outer reef, made up of broken corals and fragments, and 

 large masses thrown up on the sea face of the reef, which are gradually 

 being broken up by the surf beating upon the reef flat. 



Storm Islet ^ (Plate 49) is an excellent specimen of a sand key 

 thrown up by the waves upon the outer reef flats. It is somewhat 

 less than three hundred yards 

 long and about eighty yai'ds 

 wide. The beach is quite steep, 

 protected by lai'ge patches of 

 beach I'ock, which surround the 

 southern extremity of the isl- 

 and. Tiie crest of the island 

 is covered with cocoanut trees, screw pines, and casuarinas, as well as 

 witli an outer fringe of bushes and shrubs. 



The greater part of the shores of ]\Ibengha Island are edged with a 

 fringing reef, and coral patches forming an irregular belt extend into six 

 or seven fathoms in depth. As the sea breaks but little on the outer 



1 Dana has called attention (Coral Islands, p. 241 ) to the advantage which coral 

 island accumulations have over other shore deposits, "owinpr to the ready aggluti- 

 nation of calcareous grains," and, as he suggests, with the formation of coral sand 

 rocks along the beaches and reef rock in the water a rock defence against en- 

 croachment is produced. So that limestone rocks thus formed will prove a most 

 effectual barrier to tlie destructive action of the waves. 



STORM ISLET. 



