ON CORAL AND CORAL REEFS 125 



nary hill, or mountain, girded by a vast parapet, within 

 which would lie a shallow moat. And the dry bed of 

 the Pacific might afford grounds for an inhabitant of 

 the moon to speculate upon the extraordinary sub- 

 terranean activity to which these vast and numerous 

 "craters" bore witness! 



When the structure of a fringing reef is investigated, 

 the bottom of the lagoon is found to be covered with 

 fine whitish mud, which results from the breaking up 

 of the dead corals. Upon this muddy floor there lie, 

 here and there, growing corals, or occasionally great 

 blocks of dead coral, which have been torn by storms 

 from the outer edge of the reef, and washed into the 

 lagoon. Shellfish and worms of various kinds abound ; 

 and fish, some of which prey upon the coral, sport in 

 the deeper pools. But the corals which are to be seen 

 growing in the shallow waters of the lagoon are of a 

 different kind from those which abound on the outer 

 edge of the reef, and of which the reef is built up. Close 

 to the seaward edge of the reef, over which, even in 

 calm weather, a surf almost always breaks, the coral 

 rock is encrusted with a thick coat of a singular vege- 

 table organism, which contains a great deal of lime 

 the so-called Nullipora. Beyond this, in the part of the 

 edge of the reef which is always covered by the breaking 

 waves, the living, true, reef-polypes make their appear- 

 ance; and, in different forms, coat the steep seaward 

 face of the reef to a depth of one hundred or even one 

 hundred and fifty feet. Beyond this depth the sounding- 

 lead rests, not upon the wall-like face of the reef, but 

 on the ordinary shelving sea-bottom. And the distance 

 to which a fringing reef extends from the land corre- 

 sponds with that at which the sea has a depth of twenty 

 or five-and-twenty fathoms. 



