io8 CORALS AND CORAL LSLANDS. 



• 

 along the outer margin of the reef, and in the adjoining 

 shallow sea«;. Here they grow in profusion ; but yet the eager 

 lover of coral landscapes will be often disappointed by finding 

 among the crowded plantations extensive areas of coral sand. 



The outer margin of the reef receives the plunging waves, 

 and under this action, and the consequent unequal growth of 

 the corals, the outline is very irregular, being often deeply cut 

 into, and hence having sometimes long channels that give 

 entrance to the surging tide, and to the currents that flow back 

 in preparation for the next breaker. From it, seaward, the 

 depth of water usually sinks off rapidly from three to six 

 fathoms, and then falls away more gradually for many rods, or 

 it may be some hundreds of yards ; over the bottom in these 

 shallow waters are spread out the coral plantations, down to 

 a depth of 80 or 100 feet. Finally there is a rather abrupt 

 descent to depths beyond the reach of an ordinary sounding- 

 lead. The great difference in the rapidity with which the 

 water deepens depends chiefly on the varied character of sub- 

 marine slopes. Shallow waters may extend out for miles, 

 especially off the prominent points or angles ; but it is more 

 common to meet with the opposite extreme — great depths 

 within a few hundred feet. 



The outer reef or coral platform is generally a little the 

 highest at its seaward margin, owing partly to the growth of 

 ordinary corals and other species on this part, and also to the 

 accumulations which naturally would there be piled up by the 

 waves and become cemented. This part is therefore first laid 

 bare by the retreating tide ; and though a tempting place for 

 a ramble, it is often a dangerous place on account of the heavy 

 breakers. There is not only greater height, but often also 

 a remarkably smooth surface to the reef-rock, looking as 

 if water-worn, and frequently a blotching of the rock with 

 various shades of pink and purple. These colours and the 

 smoothness, as observed by Chamisso, are due to incrusting 

 Nullipores; and to the same calcareous sea-weeds, as Darwin 

 first observed, is often owing the increased height. The 

 material of the incrusting plant is more solid than ordinary 



