XV.] CORAL LAND. 253 



peat-bog, described in the last chapter. It was there 

 shown that the bog-moss dies below, while it continues to 

 grow above ; and, in like manner, the coral-polypes die 

 below, leaving their calcareous skeletons, while they con- 

 tinue budding and growing above : hence, a coral-island 

 can be said to be " built " by the polypes only in the same 

 sense that a peat moss can" be said to be " built " by the 

 plants of the remains of which it consists. 



Many islands in tropical seas are skirted by low banks of 

 coral-formed rock. At high tide, the surface of the rock 

 is, for the most part, submerged, and its position is then 

 marked only by a white line of heavy breakers. But, at 

 low water, the surface is more or less exposed, forming a 

 broad and bare platform, which rises slightly above sea-level. 

 Some islands are completely bordered by a margin of this 

 coral-rock, while others are fringed only at certain points. 

 Where a stream runs down from the land, and carries 

 sediment to sea, the reef is generally absent, for the coral- 

 polypes do not thrive in muddy water. Rocky ridges, 

 which fringe a shore in the manner just described, are known 

 as frittging-reefs. 



In other cases, the coral-rock is not directly attached to 

 the coast, but stands off at some distance, so 'as to form a 

 barrier, perhaps many miles from land. Such reefs are 

 consequently called barrier-reefs. Between the coast and 

 the reef there is a channel of comparatively shallow water, 

 forming a harbour, to which entrance is gained by a breach 

 here and there in the reef, the reef itself constituting a 

 natural breakwater. Patches of coral-rock, forming small 

 isolated reefs, may be scattered about the quiet channel, 

 and the barrier itself may be broken up into a cliain of 

 detached reefs. Along the north-eastern coast of Australia 

 there is a chain of these barrier-reefs stretching for a length 



