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 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 Astræa, but much larger, being many feet in thickness; and as the polypes are only alive on the surface, numberless generations 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; in some species the rays are almost invisibly slender, the interstitial matter is full of pores, and the polypes have twelve tentacles. The Millepora complanata or palmipora is very commonly associated with the Porites; it is the largest coral known. It grows in thick vertical plates, intersecting each other at various angles, and forms an exceedingly strong honey-combed mass, generally affecting a circular form, the marginal plates alone being alive. Instead of stars, the polypes live in simple pores: myriads of these small cylindrical pores penetrate the surface of the plates perpendicular to their axes; sometimes they are so minute as to be scarcely visible.