CHAPTER VIII. 



CORAL REEFS. 



FROM a very early period in the history of stratified formations, coral 

 reefs have been formed in the old seas ; but there is now no evidence 

 to demonstrate the conditions under which those corals flourished. It 

 is quite possible that the extinct forms may have known no limitation 

 of depth or of temperature such as influence the distribution of 

 existing reef-building corals. It is even possible that they may have 

 been able to withstand the influence of muddy waters, so fatal to the 

 animals forming coral growths at the present day. In any case, it- 

 would be unsafe to infer that the old forms of coral life were 

 governed by laws similar to those which now influence the group, in 

 face of the evidence that the generic types have in the long lapse of 

 geological time undergone considerable modification of structure, as 

 well as change in geographical distribution. The simple form of coral 

 is not limited in its distribution by depth of sea ; and some species 

 live off our own coasts and far north, as well as in the tropics. And 

 since the compound form of coral is a consequence of the fact that 

 the polyps increase by buds which rise from the margin of the parent 

 cup, and thus grow into a dense mass, it must be regarded as a geo- 

 graphical accident resulting from the distribution of land and water 

 that reef-building corals are now almost entirely confined to the tropics. 



In its simplest form, the coral is closely allied to the sea anemone, and 

 in fact differs essentially from the latter in possessing a skeleton 

 within the base and tubular investing substance which usually sends 

 converging plates towards the centre, which correspond to the radiating 

 partitions of the sea anemone called septa. In both groups the 

 tentacles, which when expanded give the flower-like appearance to the 

 polyp, remain un calcified, and form soft fleshy substance. Some of 

 these simple corals, such as the mushroom-like forms of Fungia, attain 

 a considerable size ; and such simple forms, represented by genera like 

 Paleocyclus and Petraia, occur in the Silurian and Cambrian rocks. 

 Considerable beds of limestone formed of corals are found in the 

 Palaeozoic strata. 



Rocks formed of Corals. Corals found fossilised do not appear to 

 have formed accumulations of a thickness to be compared with existing 

 coral reefs, but to have grown more after the manner of corals which 

 occur around the shore of a rising or stationary land. The oldest 



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