520 
CCELENTERA TES. 
is seen to dip down precipitously into deep water, and here, on account of the 
depth of water, the reef is wanting. The harrier-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 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 
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 
