BARRIER REEFS. 



291 



nut trees, can be seen separating the dark heaving waters of the 

 ocean from the light green expanse of the lagoon, the still waters 



of which, within the reef, usually hatlie a fringe of low alluvial 

 soil, upon which the varied and beautiful productions of the tropi- 

 cal regions flourish at the foot of the abrupt and wild central 

 peaks. In the sketch given above, the barrier-reef may be seen 

 in the distance skirting around the island. These reefs are of all 

 sizes from three to forty miles in diameter; and the one which 

 encircles both ends and fronts one side of New Caledonia is up- 

 wards of 400 miles long. Externally the reef ris^s like an atoll 

 with abruptness out of the profound depth of the ocean, but in- 

 ternally it either slopes gradually into the channel, or terminates 

 in a perpendicular wall 200 or 300 feet in height. 



There is one remarkable feature connected with the circular 

 reefs, and that is, a deep and narrow passage almost invariably 

 opening from the sea into the lagoon, and kept open by the efflux 

 of the sea at low tides, and it has long been remarked in the 

 case of the barrier reefs, that this channel or opening alway 

 faced valleys in the included land. 



The third great class are the Fringing reefs, these, so far as the 

 coral reef itself is concerned, do not differ materially from the 

 others, except that the encircling belt of coral is much narrower. 

 Where the land slopes abruptly into the water the reefs are but a 

 few yards in width, forming a mere ribband or fringe around the 

 island, but when the slope is gradual, the width is much increased 



