140 REEF-BUILDING CORALS. PART in. 



An atoll is a ring or chaplet of coral, enclosing a 

 lagoon or portion of the ocean in its centre. The average 

 breadth of that part of the ring which rises above the 

 surface of the sea is about a quarter of a mile, often less, 

 and it is seldom more than from six to ten or twelve feet 

 above the waves : hence the lagoon islands are not visible 

 even at a very small distance, unless when they are 

 covered by the cocoa-nut palm or the pandanus, which 

 is frequently the case. On the outside, the ring or 

 circlet shelves down for a distance of one or two hundred 

 yards from its edge, so that the sea gradually deepens to 

 about twenty-five fathoms, beyond which the sides of the 

 ring plunge at once into the unfathomable depths of the 

 ocean with a more rapid descent than the cone of any 

 / volcano. Even at the small distance of some hundred 

 \ yards no bottom has been reached with a sounding line 

 \ a mile and a half long. All the coral on the exterior of 

 the ring, to a moderate depth below the surface of the 

 water, is alive ; all above it is dead, being the detritus 

 of the living part washed up by the surf, which is so 

 heavy on the windward side of the tropical islands of the 

 Pacific and Indian Oceans, that it is often heard miles 

 off, and is frequently the first warning to seamen of 

 their approach to an atoll. 



On the inside, these coral rings shelve down into the 

 clear calm water of the lagoon by a succession of ledges 

 of living corals, but of much more varied and delicate 

 kinds than those on the exterior wall and foundation of 

 J the atoll. The corals known as Porites are the chief 

 ( agents in building the exterior face of the ring : they 

 form great rounded irregular masses, like the Astrsea, but 

 much larger, being many feet in thickness ; and as the 

 polypes are only alive on the surface, numberless gene- 

 rations must have lived and died before they could have 

 arrived at that size. The rays of the stars are toothed 

 at the edges, so that they present rows of little points ; 



