STRUCTURE OF CORAL REEFS. 120 



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 authors 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 

 fringing 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 



