THE EIGHT-RAYED POLYPS 



3495 



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 danger- 

 ous. At other times a reef incloses miles 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 

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

 groops 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 



ISLAND WITH FRINGING AND BARRIER REEFS. 



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 won- 

 derfully beautiful sight. Outside of the reef is the heavy surf, and within the 

 white coral strand, the thick band of verdure, and the inclosed lake with its minute 

 islands. In color, the water of the lagoon, where it is deep (ten to twelve fathoms), 

 matches the blue of the surrounding ocean, but delicate apple-green and yellow 

 tints mingle with the blue wherever the sand or coral rocks approach the surface. 

 Although the girdle of reef covered with vegetation occasionally surrounds the 

 lagoon, it is more often broken up into a ring of separate islets of various sizes; be- 

 tween some of which navigable channels are found, giving admittance to the lagoon. 

 The submarine fields of living coral spread along the coasts of the islands and 

 the mainland. Just as the accumulated remains of the primitive forests add layer 



