{76 CORALS AND CORAL ISLANDS. 
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. 


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is g SECTION’OF “THE RIM OF AN ATOLL. 
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From « to 6 is the shore platform or reef-rock, nearly at 
low-tide level, with the margin (@) slightly elevated, and usually 
much incrusted at top with Nullipores. From the platform 
there is a rise, by a steep beach () c), of six or eight feet, to 
the wooded part of the coral belt represented between ¢ and d. 
From d to ¢ 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 
