148 CORALS AND CORAL ISLANDS. 



V. CHANNELS AMONG REEFS. 



To complete this review of the general appearance and 

 constitution of reef formations, it remains to add some partic- 

 ulars respecting the channels which intervene between coral 

 patches, or separate them from the shores of an island, and 

 also to describe the coral accumulations forming beaches. 



The reef of Australia has been instanced as affording an 

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

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

 average distance from the land is twenty to thirty miles, and 

 the ordinary depth ten to twenty-five fathoms ; but toward the 

 southern end, where the channel is widest, the depth exceeds 

 sixty fathoms. " The new Caledonia barrier reefs, 400 miles in 

 length," says Darwin, " seldom approach within eight miles of 

 the shore." The reefs west of the large Feejee Islands are 

 another remarkable example, the reef grounds being in some 

 parts twenty-five miles wide, and the waters within the bar- 

 rier, where sounded, twelve to forty fathoms in depth. The 

 barrier in this instance may be from a few hundred yards to 

 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. In the course of the cruise of the Wilkes 

 Exploring Expedition, 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 a good 

 illustration of a narrow channel, and at the same time one 

 that exhibits 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 



