STRUCTURE OF CORAL ISLANDS. 57 
b. Structure of Coral Islands. 
The descriptions of reefs and their islets apply with equal force 
to coral islands. By transferring here the statements respecting the 
former, we should have a nearly complete account of the latter. The 
same causes, with scarcely an exception, are at work :—the growing 
of coral-zoophytes, the action of the waves, oceanic currents, and the 
winds. This resemblance will be rendered more apparent by a review 
of their characters ; the description will be found to be a simple recapi- 
tulation of a former paragraph. 
The reef or the coral atoll, as it lies at the surface still uncovered 
with vegetation, is a platform of coral rock, usually two to four 
hundred yards wide, and situated so low as to be swept by the waves 
at high tide. The outer edge, directly exposed to the surf, is gene- 
rally broken into channels and jagged indentations, along which the 
waters of the resurging wave drive with great force. ‘Though in the 
midst of the breakers, the edge stands a few inches, and sometimes a 
foot, above other parts of the platform; the incrusting Nudlpores cover 
it with varied tints, and afford protection from the abrading action of 
the waves. ‘There are usually three to five fathoms water near the 
margin; and below over the bottom, which gradually deepens out- 
ward, beds of corals are growing profusely among lifeless patches 
of coral sand and fragments: often the dead areas much exceed those 
flourishing with zoophytes, and not unfrequently the clusters are 
scattered like tufts of vegetation in a sandy plain. The growing 
corals extend up the sloping edge of the reef, nearly to low tide level. 
For ten to twenty yards from the margin, the reef is usually very 
cavernous or pierced with holes or sinuous recesses, a hiding-place 
for various crabs, or a-retreat for the echini, asterias, the sea-anemones, 
and many a pretty mollusc; and over this portion, the gigantic 
Chama or Tridacna is generally found lying more than half buried 
in the solid rock, with barely room to gape a little its ponderous 
shell, and expose to the waters a gorgeously coloured mantle. Far- 
ther in are occasional pools and basins, alive with all that lives in 
these strange coral seas. 
The reef-rock, wherever broken, shows a detritus origin. Parts 
are of compact homogeneous texture, a solid white limestone, without 
a piece of coral distinguishable, and rarely an imbedded shell. But 
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