COS 



FOEMATION OF COEAL EEEFS. 



[Ch. XLIX. 



on 



negative evidence, and opposed to a theory wliicli so 



admirably 



-emember 



feet of atolls in wliicli the coralline limestone would be ^000 

 feet thick, implies, first, a slow subsidence of 4,000 feet, and, 

 secondly, an elevation of the same amount. Even if the re- 



movement 



ward one ceased, we must allow a great lapse of ages for 

 the accomplishment of the whole operation. 



We 



commencement 



the equatorial regions were as fitted as now for the support 

 of reef-building zoophytes. This postulate would demand 



variety of conditions 

 than they are usually 



com 



the continuance of a 

 throughout a much longer period 

 persistent in one place. 



To show the difficulty of speculating on the permanence of 



climatal circum 



growth of reef-building corals, we have only to state the fact 

 that there are no reefs in the Atlantic, off the west coast of 



amon 



Helena 



at 



Witl 



the exception of Bermuda, there is not a single coral reef in 

 the central expanse of the Atlantic, although in some parts 

 the waves, as at Ascension, are charged to excess with cal- 

 careous matter. The capricious distribution of coral reefs is 

 probably owing to the absence of fit stations for the reef- 

 building polypifers, other organic beings in those regions 

 obtaining in the great struggle for existence a mastery over 

 them. Their absence, in whatever manner it be accounted 

 for, should put us on our guard against expecting upraised 

 reefs at all former geological epochs, similar to those now in 

 progress. 



Lime, tvJience derived. — Dr. 



MaccuUoch 



of 



Geology, vol. i. p. 219, expressed himself in favour of the 



some 



have originated in organised substances. If we examine, he 

 says, the quantity of limestone in the primary strata, it will 



sm 



and argillaceous rocks than in the secondary ; and this maj 



I 



! 



( 

 I 



f 



f 



e 



fit 



of 



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