AGASSIZ: FIJI ISLANDS AND CORAL REEFS. rtf 
——remnants left from the disintegration and erosion of the former greater 
Mbengha. 
The bottom in the lagoon is a mixture of volcanic mud and coral sand 
in the vicinity of the islands, but as we proceed towards the reef it car- 
ries a greater admixture of coralline alge and of coral sand, and in the 
belt adjoining the inner edge of the outer reef is made up entirely of 
fragments of coral, of coral sand, and of coralline alge. In the central 
parts of the lagoon it is algee 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 with fragments of 
dead corals which increase in number towards the sea edge of the reef. 
The fragments are covered with alg, 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 yards 
wide. The beach is quite steep, 
protected by large patches of 
beach rock, which surround the 
southern extremity of the isl- 
and. The crest of the island 
is covered with cocoanut trees, screw pines, aud casuarinas, as well as 
with an outer fringe of bushes and shrubs. 
The greater part of the shores of Mbengha 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 
STORM ISLET. 
1 Dana has called attention (Coral Islands, p. 241) to the advantage which coral 
island accumulations have over other shore deposits, ‘owing 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 the destructive action of the waves. 
