76 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



off the south coast of Cuba we have along the sides of Bartlett Deep 

 fully as steep a pitch as that of any coral reef. 



Gardiner ^ has noticed the atoll-shaped shoals inside the great barrier 

 reef of Fiji (Plate 23^), and compares them to the smaller atolls inside 

 the large atolls of the Maldive group. See also the Serpuline, atolls of 

 the Bermudas,- which hold the same relation to the lagoon inside the 

 outer barrier as the atoll-shaped shoals just mentioned hold in Fiji, 

 and they certainly are not due to the effects of the debris of the outer 

 reefs of the present day, as is claimed by Dana. 



As long as it was taken for granted that the coral reefs of the present 

 da}- were of great thickness, it was natural to compare them with the 

 great beds of coralliferoas limestones occurring iu jjast ages from the 

 time of the Devonian to the end of the Tertiary period. While there is 

 abundant proof that some of these limestone beds have been formed 

 during a period of subsidence, there is also ample proof that they must 

 have greatly increased in thickness by accretion. But it does not follow 

 from this that atolls and the lagoons of barrier reefs have been formed 

 during a period of subsidence, now that we know that a great many of 

 the coral reefs of the present day attain a comparatively moderate thick- 

 ness, well within the bath^-metrical limits at which reef building corals 

 thrive. Xor is there anything to prove that these ancient limestones 

 represent such modern structures as the atolls, or bordered the lagoons 

 of barrier reefs, and even if they did it is more natural to suppose that 

 the lagoons of these ancient atolls, as well as those within the ancient 

 barrier reefs, must have been formed b}- the same agencies which have 

 shaped the atolls and the lagoons of barrier reefs iu our days. So that 

 the analogy between ancient and modern reefs, such as is upheld by Dr. 

 R. von Lendenfeld,^ while undoubtedly true as far as it applies to the 

 formation of beds of coralliferous limestones of great thickness, vet has 

 no application to the actual formation of the lagoon of an atoll or of a 

 barrier reef. 



This still leaves open the question of the mode of formation of such 

 thick masses of coralline and coralliferous limestone, which, though they 

 may originally have been formed partly by subsidence, may also have 

 been formed partly by the gradual pushing out to seaward of the outer 

 edge of a reef increasing both in height (depth) and in width by the con- 

 stant pushing out of the mass of debris and of blocks detached from the 



1 Loc. at., p. 493. 



2 Bull. Mus. Comp. Zoul., Vol. XXVI. No. 2, 1805, p. 253. 



3 Nature, No. 1071, May 8, 1890. p. 29. 



