THE AUSTRALIAN GREAT BARRIER REEF. gg 



the main branches and the minor branchlets are perfectly preserved ; and it is only on account 

 ot the absence, through erosion, of the slender projecting edges of the component corallites, tliat 

 any difficulty attends their identification. As previously remarked, the most abundantly repre- 

 sented types are closely allied to, if not absolutely identical with, the essentially cosmopolitan 

 Barrier spocies, Madrepora millcpora and M. dccipicns. The most painstaking search, including 

 numberless dredging operations, and the offer of substantial rewards, to fishermen and others, 

 utterly failed to bring to light evidence of the present existence of any living coralla of these 

 varieties, and it seems to be unquestionably certain that the genus Madrepora is now extinct in 

 Moreton Bay. No knowledge, moreover, could be elicited with reference to their having 

 occurred in the living state within the epoch of the colony's settlement, some fifty years ago 

 Thus, a very considerable interval has apparently elapsed since they flourished in this locality. 

 The first opinion entertained concerning the remarkable decadence of this coral group was that 

 either the climatic conditions had altered, ushering in a temperature colder than these corals could 

 withstand, or that a local elevation of the district had raised their growth-site to a plane in 

 which, during the winter months more especially, they would be subjected at low spring-tides to 

 similarly fatal atmospheric influences. A further consideration of all the conditions has, how- 

 ever, led to a somewhat different interpretation. 



The geographical configuration of Moreton Bay was briefly summarised in a previous page, 

 the circumstance of its area being almost completely hemmed in by a chain of three long 

 narrow islands, which stretched between the mainland points at the head of the bay being 

 especially alluded to. These islands, Bribie, Moreton, and Stradbroke, although rising to 

 several hundred feet, are composed almost entirely of drifted siliceous sand, thrown up by 

 the outside ocean. The peripheral contours of the said islands and of those of the labyrinths 

 of sandbanks that abound in their vicinity are continually changing, and their bulk is 

 undoubtedly augmenting. Dredging operations have, in fact, to be continually carried on in 

 order to maintain navigable channels through the gaps between Moreton and Bribie, and 

 between Moreton and Stradbroke Islands. The sand accumulation still in progress has, it may 

 confidently be assumed, been in operation for centuries, indicating at the same time a period, 

 not so very remote, when the existing accumulations were much less developed, and there 

 was consequently more open communication with the outer ocean. It was under these 

 conditions, it may be supposed, that the Madreporae flourished, although it is not here sug- 

 gested that the gradual shutting out of the freer access of the open ocean was the most potent 

 cause of their extinction. 



The fact remains as yet unnoticed that two considerable rivers, the Brisbane and the 

 Logan, in addition to many minor streams, discharge their waters into Moreton Bay. During 

 heavy floods the amount of fresh water pent up in the bay, and hindered from escaping through 

 the presence of the chain of islands, and the associated sand-banks, above referred to, is very 



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