98 THF. GREAT BARRIER REEF 



and consequently cannot form a reef. The foregoing evidence is practically conclusive in sup- 

 port of the view put forward in association with the descriptive account of the plate just quoted : 

 namely, that coral-reefs are produced in the tropics, not with relation so much to the intrinsic 

 reef-constructing properties of the specific coral polyps, but with relation to the rule that reef 

 consolidation (or the amalgamation of coral debris into a more or less solid, coarse or fine, con- 

 crete, or into a finer-grained, compact limestone) is associated only with the rapid evaporation 

 ot the lime-saturated sea-water on inter-tropical, tidally-exposed, coral banks or beaches. As 

 related in the descriptive text quoted, granite-like lumps of the size of cobble-stones are, on the 

 beach at Thursday Island, bound together, under like conditions, by the same tenacious calcareous 

 cement. 



Should further investigations prove that this interpretation of coral-rock conglomerate lime- 

 stone construction is correct, it will assist materially towards elucidating the extent to which sub- 

 sidence has been associated with the building up of any particular reef For, in such case, the 

 occurrence of typical consolidated coral-rock at a lower level than that of ordinary low spring- 

 tides will be a certain indication of degradation from its original plane of elevation. As inti- 

 mated in an carlj' page (p. 71 ) of the preceding chapter, this same phenomenon logically accounts 

 for the non-formation of reefs by the many luxuriantly growing Madreporaria, Lophohelia, 

 Amphihelia, Dendroph3'llia, &c., that form thickly-covered banks, often of many miles in extent, 

 in abyssal depths off the European coast. It was postulated in the previous reference to this 

 subject that, if these abyssal corals could be transported to shallow seas within the tropics, 

 they would participate as substantially as Madreporae, or other generally recognised reef-species, 

 in the function of reef-construction. Approaching, and proving, the same postulate {i.e., that 

 coral-reef conglomerate and limestones are formed within inter-tropical tidal areas only, and 

 altogether independently of the associated coral species, from a diametrically opposite avenue of 

 access), it has now been incontestably shown that typical reef-corals, specifically identical with 

 those which, a degree or two farther north, enter extensively into reef-construction, are, in extra- 

 tropical waters, unassociated with such a function. Temperature, therefore, and not the specific 

 varieties of Madreporaria, represents the prime factor in reef-construction ; and in this connection 

 it may be confidently affirmed that the presence of coral conglomerate and limestone, in any 

 fossil deposit, indicates with tolerable certainty that a tropical climate prevailed during the epoch 

 of its formation, concurrent, in all probability, with its occupation of a plane of elevation above 

 that of ordinary low spring-tide. 



The second question raised with relation to the Moreton Bay coral-growth is that of the 

 cause of the wholesale destruction of the originally abundant and luxuriant colony-stocks ol at 

 least two distinct species of the genus Madrepora. The coralla of these Madreporae may still be 

 obtained in abundance on their original site, exhibiting every appearance of having gradu- 

 all}' and quietly succumbed to some newly invading conditions inimical to their welfare. All 



