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- 
lars 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 
