246 CORAL AND ATOLLS 



growth upon any bank that may happen to lie at or above 

 the Limiting Line of Sedimentation it follows that any eleva- 

 tion of the ocean floor, which rises to this plane, will furnish the 

 corals with a suitable basis. I do not doubt that many other 

 formations than the slowly formed sedimentation banks become 

 the basis of coral structures ; for land that has sunk beneath 

 the waves will also furnish the necessary building ground. 

 Land that has risen by earth movements to a station within 

 the wave-stirred zone, or loose volcanic structures that have 

 been denuded by the waves, may also become the sites of reef- 

 coral growth ; and doubtless in many cases they have done so. 

 In these cases the top of the platform may rise near enough 

 to the surface to be directly clothed with the reef corals, or the 

 intermediate building of calcareous algse, and deep-water corals, 

 may take place ; or it may not reach so high a plane, and the 

 processes of sedimentation may be called in to fill up the 

 necessary interval. 



It matters not what the base may be, so long as its plat- 

 form comes within the wave-stirred area. It may happen, 

 therefore, that the corals begin their growth on platforms 

 already having an upward or a downward earth movement, 

 and these movements will doubtless continue after the develop- 

 ment of the reef, with varying effects upon its structure. 



It is contended that, for the development of a coral reef, 

 some basis that reaches the wave-stirred limit is essential ; and 

 on such a basis a reef will form, when the other conditions, 

 such as the temperature of the sea, are favourable. In 

 some places, I do not doubt, a downward earth movement has 

 brought a base to the necessary depth for reef development : 

 the Great Barrier Reef of Australia is almost certainly an 

 example of this. In other places, an upward earth movement 

 has probably brought up some elevation sufficiently near to the 

 surface, and coral reefs have become developed on the basis 

 thus brought within their domain. The loose structure of sud- 

 denly erupted oceanic volcanic islands may doubtless be cut 

 down by waves and current, to some height at, or above, 

 the Limiting Line of Sedimentation ; but the cutting down of 



