1 6 THE GREAT BARRIER REEF. 



coral-reef formations. In tlie largest (central) islet there is figuratively represented that com- 

 monest type of reef formation which consists of more or less elevated land, — here represented by 

 the spheroidal coral-masses, — surrounded by a low platform or fringing reef. In the second 

 (nearer) islet is typified the encircling or " barrier " structural plan, in which an outer wall of 

 growing coral is separated from the "land" by a lagoon channel. The third islet, to the 

 right, requires but slightly further central hollowing to become like a typical lagoon island, 

 or atoll, reposing, as described by previous writers, like a garland on the surface of the water. 



NO. 2.-P0RITES MD MIXED SPECIES, FILM ISLANDS, 



The area delineated in this illustration represents a portion of the Palm Islands reef lying 

 midway between the isolated islets in the preceding plate and the foreshore reef that forms the 

 subject of Plate IV., No. i. The entire foreground and a large portion of the central-ground 

 in this reef-view consists of a huge mass of Porites some thirty or forty feet in diameter, and 

 having a depth of from two to three fathoms around its outer margin. Its growing edges, in 

 this instance completely submerged, are eroded and broken up into the most irregular outline, 

 though it may be surmised that in its pristine condition, long ages back, it presented the 

 smooth symmetry and modest proportions of such a coral-stock as the spheroidal Goniastrsea 

 growing near the centre of its weathered horizontal plateau. A large colony of specific 

 varieties have established themselves, and are flourishing, on this extensive plateau, including, 

 in addition to the Goniastraea above referred to, a second species of the same genus, and 

 numerous representatives of the several genera Mussa, Symphyllia, Coeloria, Mseandrina, and 

 other Astraeaceae. 



PLATE VII. 



FRINGING REEF, PORT DENISON. 



This reef represents one out of several of a series illustrating the characteristic aspect and 

 composition of the fringing reefs skirting Saddle-back Island in the vicinity of Port Denison. 

 The most noteworthy coral entering into the composition of this reef is the luxuriant growth of 

 Millepora alcicornis, partly submerged, towards the right in the middle line. This species, as 

 explained at length in the chapter specially dealing with coral organisms, belongs to a distinct 

 order, that of the Hydrozoa, which is but rarely associated with a hard, calcareous skeleton or 

 corallum. A second species, Millepora ramosa, is illustrated in Plate X., No. i. It is a genus, 

 however, that is by no means abundantly developed in the Great Barrier system, its zenith of 

 development being associated with the tropical Atlantic or West Indian region. Other corals 

 conspicuously represented in this reef-view include, near the central foreground, a large mass 

 of a finely convoluted Brain-stone coral, Coeloria, having intercalated between it and the 



