136 COEALS AND CORAL ISLANDS. 



4. Beaches and beach formations, produced by coral accu- 

 mulations on the shores through the action of the sea and 

 winds. 



The outer and inner reefs, channels, and beaches, act each 

 their part in producing the coral formations in progress about 

 islands. 



II. OUTER REEFS. 



The barrier and other outer reefs are always submerged at 

 high tide, except where elevated at surface by accumulations 

 of beach sands. The level is generally that of about one third 

 tide. The coral rock is built up by the agencies at work to 

 this level, and hence the existence of the broad platform-like 

 top of the barrier. The surface is however not even, for there 

 are many pools of water over it, even at the lowest tides, espe- 

 cially toward its outer limits, where corals of various kinds are 

 growing luxuriantly, with fit associates of shells, star-fishes, 

 echini, holothurias with their large flower-bearing heads, 

 sponges, corallines and sea- weeds, making scenes of rare beauty. 

 The growing corals are, however, most abundant along the outer 

 margin of the reef, and in the adjoining shallow seas. Here 

 thev grow in profusion ; but yet the eager lover of coral land- 

 scapes will be often disappointed by finding among the crowd- 

 ed 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 en- 

 trance 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 fath- 

 oms, and then falls away more gradually for many rods, or it 



