176 CORALS AND CORAL LSLAND8. 



breakers. In these shallow waters are the growing corals; 

 yet, as before stated, a large part is often barren sand or coral 

 rock, especially where the depth is over fifty feet. 



From a to b is the shore platform or reef-rock, nearly at 

 low-tide level, with the margin (a) slightly elevated, and usually 

 much incrusted at top with Nullipores. From the platform 

 there is a rise, by a steep beach (b c), of six or eight feet, to 

 the wooded part of the coral belt represented between c and d. 

 From d to e there is a gently sloping beach bordering the 

 lagoon. Beyond e, the waters of the lagoon at first deepen 

 gradually, and then fall off more or less abruptly. 



In the Paumotus, the shore platform, the steep beach, and 

 the more gently sloping shore of the lagoon are almost con- 

 stant characteristics. 



The width of the whole rim of land, when the island gives 

 no evidence of late elevation, varies from three hundred yards 

 to one-third of a mile, excepting certain prominent points, more 

 exposed to the united action of winds and waves and often 

 from opposite directions, which occasionally exceed half a mile. 



The shore platform is from one to three hundred feet in 

 width, and has the general features of a half -submerged outer 

 reef. Its peculiarities arise solely from the accumulations 

 which have changed the reef into an island. . Much of it is 

 commonly bare at low tide, although there are places where it 

 is always covered with a few inches or a foot of water ; and 

 the elevated edge, the only part exposed, often seems like an 

 embankment preventing the water from running off. The 



