ACTINOZOA. 193 



external conditions which seem favourable, if 

 not essential, to the growth of reef-building 

 Corals. 



But to what other influences do the above three 

 classes of reefs, which present so much in common, 

 owe their occurrence? It has been said that they 

 are of Coral origin ; yet how is it that some of 

 them rise from depths so considerable, seeing that 

 those living Corals, by which they have been 

 constructed, build only in seas comparatively 

 shallow ? 



It is not our business here to discuss the various 

 speculative views which have from time to time 

 been put forward on the subject of Coral-forma- 

 tions. Let it suffice to say that Mr. Darwin's 

 theory of elevation and subsidence offers the only 

 consistent explanation of most of their known 

 phenomena which science is prepared to receive. 



If we suppose a Fringing reef, together with 

 the area which it surrounds, to sink at a rate not 

 more rapid than the upward growth of its consti- 

 tuent Corals, the reef itself will undergo little 

 apparent alteration, while a channel of water, 

 gradually increasing in width, will appear between 

 it and the more elevated regions of the slowly 

 submerging land. Thus a Barrier-reef is formed. 

 Depression still going on, the land encircled by 

 the reef is reduced to one or more projecting 

 peaks, as in those islands of the Pacific to which 

 allusion has been made. Further subsidence 

 causes these peaks to disappear beneath the 

 sea-level, and the Barrier-reef changes into an 

 Atoll. 



Fringing-reefs, therefore, show that the shores 

 which they skirt are stationary or rising, while 







