105 



The summit of the outer wall of an Atoll is entirely composed 

 of masses of living Actinozoa; numerous species of Porites, 

 Milleporoe, Astrece, and Meandrince, flourish there in luxuriance, 

 forming mounds of Coral from four to eight feet wide, and as 

 many thick, and separated from each other by narrow winding 

 channels, six feet deep, which intersect the entire line of reef 

 at right angles. The cells in the upper tier of the rock are 

 empty, as the Polyps eamiot endure exposure to the sun's rays 

 at low water; hence this check to their upward development 

 occasions their lateral expansion into masses having broad flat 

 summits; all below low water-mark, the Coral wall teems with 

 life; and the dead portions of Coral are covered over with layers 

 NuUipore, one of the calcareous Algse, which can bear any 

 amount of exposure; this marine plant envelopes the exposed 

 Coral like Lichens coating an old tree. The reef-forming 

 Polyps thrive best in the surf occasioned by the breakers, and 

 hence, where the surge rages most furiously, there these tiny 

 architects work with the greatest activity. The violence of 

 the waves frequently breaches the reef, and detached portions 

 are driven inwards towards the lagoon, where they form, with 

 other masses of similar origin, an inner reef; thus the ring 

 of rock is enlarged along its inner circle, and the active 

 development of the Porites living on the outer wall soon repairs 

 the damage done by the storm. So rapidly does the wall of 

 Keeling Atoll shelve downward, that Admiral Pitzrot, at a 

 distance of 2200 yards from the breakers, foxmd no bottom with 

 a line 7200 feet in length; hence the submarine slope of this 

 Atoll is steeper than that of any volcanic cone. 



The lagoon, it would appear, has a fauna of its own; distinct 

 species of Actinozoa with delicate branching stems, multitudes 

 of Radiata and Mollusca indigenous to it nestle among the 

 shallow waters, and fishes breed abundantly in the winding 

 channels and crooked crannies of the Coral rock. 



II. Barrier Reefs are similar to Atolls in structure, but 

 differ in physical arrangement. They run parallel with the 

 shores of some continent, or larger island, separated from the 



* Naturalists' Voyage, p. 46G. 



