FRINGING REEFS.— ATOLLS. 151 



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 Poritidce cease, and are 

 replaced by a ridge of agglomerated dead branches and sand, 

 coated with Nullipore / 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, Meandrince and Fungice 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, way 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 of 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 with 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, aorain, are encircled with 

 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 Australian 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 



