STRUCTURE OF CORAL REEFS. 13 



impure or fresh waters and currents often operate to retard their 

 growth. 



Owing to the last mentioned cause, the inner reefs are not usu- 

 ally joined close to the beach. They stand off a little, separated 

 by an interval of shallow water. At Mathuata, in the Feejees, 

 however, the reef extends quite up; and it is the more remarka- 

 ble as the country is a plain, the site of a Feejee village, and a 

 mile or two back stands a high bluff. On an island off this part 

 of ^anua Lebu is another example of this fact, and many more 

 might be cited. In such cases, however, there is evidence that 

 the shores upon which the corals grew were bare rocks, instead 

 of moving beach-sands. 



From these descriptions it appears that the main distinction 

 between the inner and outer reefs consists in the less fragmentary 

 character of the rock in the former case, the less frequent accumu- 

 lations of debris on their upper surface, and the more varied fea- 

 tures and slopes of the margin. Moreover, the Nullipores, which 

 seem to flourish best in the breakers, are of less extent, or but 

 sparingly met with elsewhere. 



The inner margin of a barrier reef, it should be observed, is 

 entitled to rank with inner reefs, as its corals grow in the same 

 quiet waters, and under like circumstances. The variety of coral 

 zoophytes is also greater in the stiller waters, and there are species 

 peculiar to the different regions, as explained in another plaee. 



Channels among reefs. — To complete this review of the gen- 

 eral appearance and constitution of reef formations, it remains to 

 add some particulars respecting the channels which intervene be- 

 tween coral patches, or separate them from the shores of an island, 

 and also to describe the coral accumulations forming beaches. 



The reef of New Holland has been instanced as affording an 

 example of one of the larger reef-channels, varying from thirty 

 to sixty miles in width, and as many fathoms in depth. The 

 reefs west of the large Feejee Islands offer another remarkable 

 example, the reef-grounds being in some parts twenty-five miles 

 wide, and the waters within the barrier, where sounded, twelve to 

 forty fathoms in depth. The barrier in this instance may be from 

 a few hundred yards to a half a mile in width ; and some of the 

 inner patches are of the same extent ; but by far the larger part 

 of the reef-ground is covered with deep waters, mostly blue like 

 the ocean, and as clear and pure. The sloop of war Peacock 

 sailed along the west coast of both Viti Lebu and Vanua Lebu, 

 within the inner reefs, a distance exceeding two hundred miles. 

 The island of Tahiti on its northern side presents us with a good 

 illustration of a narrow channel, and at the same time exhib- 

 its the usual broken or interrupted character of reefs. This is 

 seen in the following cut in which the reefs, both fringing and 

 barrier, are the parts enclosed by dotted lines. The outer reef 



