520 CCELENTERATES. 



is seen to dip down precipitously into deep water, and here, on account of the 

 depth of water, the reef is wanting. The barrier-reef also is broken through at 

 one point, forming the entrance to a harbour, such as is often found in reef- 

 surrounded islands. Many islands are bordered by a reef which protects the 

 land from the sea like a mole. The barrier-reef may occasionally be ten or fifteen 

 miles from land, and enclose several high islands. Various forms of reefs are 

 found between the two extremes presented by such a barrier-reef and the simple 

 fringing-reef. 



The channel within the reef at low tide is sometimes hardly deep enough for 

 navigation, or else it is blocked by masses of coral which render its passage 

 dangerous. At other times a reef encloses mijes of open water, ten, twenty, or 

 forty fathoms deep, but not free from hidden sources of danger ; masses of living 

 coral, from a few square feet to several square miles in extent, rising from the 

 bottom. In the Fijis all these kinds of channel formations occur. 



The extent of the reefs, which include scattered banks and masses far below 



ISLAND WITH FRINGING- AND BARRIER-REEFS. 



the surface of the water, varies greatly. On some coasts there are merely 

 scattered groups or mounds of coral-rock, the tips of which project as rocks; 

 while, on the other hand, to the west of the Fijis there is an area covered with 

 reef of about three thousand square miles. Other reefs are one hundred or one 

 hundred and fifty miles long, and the Australian barrier-reef attains a length of 

 one thousand two hundred and fifty miles. 



Passing from such a tropical island girt with coral-reefs, we come to what is 

 more especially known as a coral-island proper, or atoll, which may be described as 

 the encircling reef without any island to encircle. It surrounds a calm lake of 

 blue water, in striking contrast to the restless ocean outside the solid circle. The 

 ring of solid reef in this case is usually only one hundred to two hundred yards 

 broad, and at some parts so low that the waves break over it into the lagoon. At 

 other parts it is covered with tropical vegetation, but it rarely rises more than 

 three to four yards above high-water mark. Seen in the distance from a ship, a 

 coral island looks like a row of dark points, which are the tops of the cocoanut- 

 trees first seen above the horizon. On nearer approach, the lagoon with its green 

 border is a wonderfully beautiful sight. Outside of the reef is the heavy surf, and 

 within the white coral strand, the thick band of verdure, and the enclosed lake with 

 its minute islands. In colour, the water of the lagoon, where it is deep (ten to 



