STRUCTURE OF CORAL REEFS. 129 
strongly contrasting with the steep slopes of the encircled 
island. 
Nearing in a vessel a coral-bound coast, the first sign of the 
reef, when the tide is well in, is a line of heavy breakers, per- 
haps miles in length, off a great distance from the land. On 
closer view, some spots of bare reef may be distinguished as 
the waves retreat for another plunge; but the next moment 
all again is an interminable line of careering waters. Happy 
for the cruiser in untried reef-regions, if the surging waves con- 
tinue to mark the line of reef; for a treacherous quiet some- 
times intervenes, which seems to be evidence of deep waters 
ahead, and the unsuspecting craft dashes onward ; but soon it 
is grinding over the coral masses, then thumping heavily at 
short intervals, and, in a few moments more, is landed helpless 
on the coral reef. The heavier billows as they roll by a vessel 
in such a plight—the author’s experience attesting—have a 
way of lifting it and then letting it drop with all its 
weight against the bottom, and hence, unless prompt escape is 
in some way secured, the assaulting waves gain speedy posses- 
sion, and soon after make complete the work of destruction. 
At low tide the breakers often cease, or nearly so. But the 
reef for the most part, is then in full view, and, with a good 
lookout aloft, favorable winds, and plenty of daylight, navi- 
gation is comparatively safe. 
Some idea of the features of a tropical island thus bor- 
dered, may be derived from the following sketch. The reef 
to the right is observed to fringe the shore, making a simple 
broad platform, as an extension, apparently, of the dry land. 
To the left there is the same coral platform at the surface, but 
it is divided by a channel into an inner and an outer reef—a 
Jringing and a barrier reef, as these two parts are called. At 
a single place the sea is faced by a cliff; and here, owing to 
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