STRUCTURE OF CORAL REEFS. 105 



even in the Argo reef, and such other examples as offer now 

 but a single rock above the surface of the inclosed lagoon, we 

 shall endeavour to make apparent, if not already so, when the 

 cause of the forms of coral islands is under discussion. Yet it 

 is also evident that this correspondence is not exact, for many- 

 parts of the shores, and sometimes more than half the coasts, 

 may be exposed to the sea, while other portions are protected 

 by a wide barrier. 



In recapitulation, we remark, that reefs around islands may 

 be (i) entirely incircling ; or they may be (2) confined to a 

 larger or a smaller portion of the coast, either continuous or 

 interrupted ; they may (3) constitute throughout a distant 

 barrier ; or (4) the reef may be fringing in one part and a bar- 

 rier in another ; or (5) it may be fringing alone : the barrier 

 may be (6) at a great distance from the shores, with a wide sea 

 within, or (7) it may so unite to the fringing reef that the 

 channel between will hardly float a canoe. These points are 

 sustained by all reef regions. 



It is to be noted that the fringing and barrier reefs here 

 pointed out are not the whole of the coral reef; they are only 

 the portions that have been built up to the water's level. 

 Between them, and also outside of all, there are the submerged 

 coral banks which are continuous with the higher portions, and 

 all together make up the coral reef-ground of an island. 



A wide difference in the extent of reef-grounds follows from 

 the above-mentioned facts. On some coasts there are only 

 scattered groups of corals, or rising knolls, or mere points of 

 emerged coral rock ; but again, as, for example, west of the two 

 large Feejee Islands, there may be three thousand square miles 

 of continuous reef-ground, occupied with coral patches and in- 

 termediate channels or seas. The inclosing barrier off Vanua 

 Levu alone is more than one hundred miles long. The Ex- 

 ploring Isles, in the eastern part of the Feejee group, have a 

 barrier eighty miles in circuit. New Caledonia has a reef 

 along its wliole western shores, a distance of two hundred 

 and fifty miles, and it extends one hundred and fifty miles 

 further north, adding this much to the length of the island. 



