70 THE GREAT BARRIER REEF. 



The coral-reef-building polyps, it is tolerably well known, are practically confined 

 to the seas of the tropics, as limited by the parallels of latitude of 23^'' north and 

 south of the Equator. The main factor in their distribution is that of a uniformly 

 hio-h temperature, which, in the coldest winter months, shall not fall below that of 

 b%° F. Isothermal lines, coincident with this temperature, are found, however, to be by no 

 means in precise harmony with the above latitudes. On the west sides of the large conti- 

 nents of Africa and South America, in particular, the southern isothermal line is thrust 

 high up towards, and in the latter instance actually to the north of, the Equator. On the 

 Australian continent, as given in J. D. Dana's isothermal chart,* these limiting lines of 

 tropical temperature are practically uniform with that of 26° S. latitude on both the eastern 

 and the western shores, dropping down, however, as low as 30° some little distance off the 

 eastern coast. As a matter of fact, a few reef-corals are found in Moreton Bay, in lati- 

 tude 270, while the most southern living reefs are those which surround Lady Elliot Island, 

 just north of latitude 24°. One species of reef-coral, Plesiastrcea nrvillei, has been obtained in 

 Port Jackson, close to Sydney, New South Wales, just above the latitude of 34° south ; and 

 an allied species, P. Peronii, has been reported by Mr. Tenison-Woods as occurring at various 

 points along the south coast of Australia in latitudes as low as 38° and 39°. This genus 

 apparently represents the most southern Australian representative of the reef-forming class, 

 though there are, as in the European, or even in the British, seas, several solitary, temperate 

 or cold-water types. 



The proportion that the growing corals bear to the aggregate reef-masses is very small. 

 On such a reef, by way of example, as the one represented by Plate XXX., depicting the scene of 

 the wrecked mission schooner Harrier, and marked on the Admiralty charts as F reef, the 

 view altogether lacks the luxuriant crops of living corals that characterise the majority of the 

 reef-scenes selected for the illustration of this work. The lower figure of the same plate 

 affords an almost parallel illustration of a reef devoid of visibly growing corals, the lifelessness of 

 its general superficies being further accentuated by the huge rock-masses torn off the edge and 

 thrown high upon the surface of the reef. The interpretation of the phenomena is that the soft, 

 moist, anemone-like polyps that lay the foundations of these vast reefs cannot survive the long 

 exposure to the evaporating action of the tropical sun, that would be inevitably associated with 

 their growth at this high plane of elevation, and that, as a matter of fact, represents low-water 

 conditions at an ordinary neap-tide. The living corals in these reef-scenes lie entirely out of 

 sight, and at a lower depth, on the outer margin of the reef. The constituent elements of the 

 reef-areas visible in the two illustrations referred to, represent the consolidated wave-, 

 storm-, and current-accumulated debris of decayed or broken-down corals originally detached 

 from the outside growing margin. And this same constructive formula expresses a dominant 

 peculiarity of the majority of living coral-reefs. 



* J. D. Dana, "Corals and Coral Islands," p. 298. 1872. 



