FRINGING REEFS.— ATOLLS. lol 



Millepores (Seriatopora) ; while, deeper still, all living coral 

 ceases ; the lead bringing up either dead branches, or show- 

 ing the existence of a flat, gently-sloping floor, the true sea- 

 bottom, covered with fine coral sand and mud. Passing from 

 the edge of the reef landward, the Poriticlce cease, and are 

 replaced by a ridge of agglomerated dead branches and sand, 

 coated with N-ullipore / the floor of the shallow basin, or 

 "lagoon,"" inclosed between the reef and the land, is formed 

 by a conglomerate, composed of fragments of coral cemented 

 by mud ; and, on this, Meandrinoe and Funguv rest and 

 flourish, exhibiting the most gaudy coloration, and sometimes 

 attaining a great size. During storms, masses of coral are 

 hurled on to the floor of the lagoon, and there gradually in- 

 crease the accumulation of rocky conglomerate ; but in no 

 other wav can a fringing" reef, which has once attained its 

 limit in depth, increase in size, unless, indeed, the talus ac- 

 cumulating at the foot of its outer wall should ever rise suffi- 

 ciently high to afford a footing for the corals within their pre- 

 scribed limits of depth. 



Such is the structure cf a fringing reef ; but the great 

 majority of reefs in the Pacific are very different in their 

 character. Along the northeastern coasts of New Holland, 

 for instance, a vast aggregation of reefs lies at a distance 

 from the shore which varies from a hundred to ten miles ; 

 forming a mighty wall or barrier against the waves of the 

 Pacific. At a few hundred yards outside this " barrier reef ' 

 no bottom can be obtained w T ith a sounding-line of a thousand 

 fathoms ; between the reef and the mainland, on the con- 

 trary, the sea is hardly ever more than thirty fathoms deep. 

 Manv of the islands of the Pacific, again, are encircled w T ith 

 reefs corresponding exactly in their character with the barrier 

 reef ; separated, that is, by a relatively shallow channel from 

 the land, but facing the sea with an almost perpendicular wall 

 which rises from a very great depth. 



Finally, in many cases, especially among the single reefs, 

 which taken together constitute the great iVustralian barrier, 

 there is no trace of any central island ; but a circular reef, 

 usually having an opening on its leeward side, stands out in 

 the midst of the sea. These reefs, apparently unconnected 

 with other land, are what are called " Atolls." 



How have these barrier reefs, encircling reefs, and atolls, 

 been formed ? It is certain that the fabricators of these reefs 

 cannot live at a greater depth than in the fringing reefs. 

 How can they have grown up, then, from a thousand fathoms 



