SOME ANIMAL ARCHITECTS. 141 



coral-extensions of the ordinary beach. A typical reef of this- 

 description is seen to surround the island of Mauritius, and 

 another skirts the coast of Cuba. A sounding-lead allowed 

 to descend on the seaward face or edge of a fringing reef 

 would strike the true sea-bottom at a depth not exceeding 

 twenty-five fathoms. Its outer edge is fcrmed of true reef- 

 building corals, which seem to thrive best amid the spray 

 and surf. Near the shore, different and less hardy corals 

 live ; and in the shallow water which intervenes between the 

 reef and the shore, a whitish mud, consisting of the debris of 

 the dead corals, is found, together with blocks of coral which 

 have been torn from the reef and cast up on the shore by 

 the violence of storms. " A fringing reef," says Mr. Darwin, 

 " if elevated in a perfect condition above the level of the sea ? 

 would present the singular appearance of a broad, dry moat,, 

 bounded by a low wall or mound." The breadth of a 

 fringing reef depends on the slope of the beach ; the more 

 gradual the slope, the further seawards will the reef extend ; 

 whilst a steep beach, preventing a great depth of water 

 nearer the shore than the sloping form, will proportionally 

 limit the seaward growth of the corals. On very steep 

 coasts, fringing reefs may not exceed fifty yards in width, 

 that measurement representing the distance from the shore 

 at which the coral-polypes reach their furthest limit of depth. 

 The barrier reef is an erection of a very different kind from 

 the preceding variety of reef. In its most typical form, well 

 seen on the north-east coast of Australia, or on the western 

 coast of New Caledonia, a barrier reef appears as a great 

 bank or reef of coral, separated from the adjoining land by 

 a belt of water named the " inner channel." Sometimes an 

 island like Tahiti is surrounded by a barrier reef, which 

 stands like a great wall around the land, but is separated 

 from the latter by a channel. In the latter case the barrier 

 reef receives the appropriate name of " encircling reef." 

 Some of the barrier reefs are of immense extent. The great 

 barrier reef on the north-east coast of Australia extends un- 

 interruptedly for 1000 miles, and in nearly a straight line, 



