CORAL REEFS AND ISLANDS 205 



but it is a very large assumption, for which there is little 

 or no justification. 



The Darwin-Dana hypothesis implies that a vast belt 

 of land in equatorial regions has been sinking down to the 

 extent of thousands of feet during more than a million 

 of years. If this has really taken place, it is one of the 

 greatest phenomena in the earth's history. 



Such was the position of affairs when the " Challenger " 

 sailed on her memorable exploring expedition, during which 

 the investigation of depths and bottom deposits over the 

 floor of the ocean enabled John Murray to construct his 

 theory of coral growth and atoll formation, which is perhaps 

 the best known after that of Darwin. Murray showed that 

 abundant platforms could be provided by the building up 

 of submarine volcanic elevations and banks by means of 

 calcareous deposits formed from the shells and other hard 

 parts of animals living on the bottom, and also of pelagic 

 organisms in the water above, such as form Globigerina ooze 

 and Pteropod ooze. He showed how the various agencies 

 at work all tend to wear down or to level up all elevations 

 rising from the floor of the ocean to about the lower limit 

 of wave-action, which is the correct depth at which to form 

 a suitable platform for reef -building corals to grow upon. 

 The coral colonies established on such a platform will then 

 naturally grow towards the surface and from the surface 

 outwards in all directions to form a small tableland or 

 plantation of coral. In such a plantation the conditions 

 of life will be more favourable round the edges, where the 

 breaking water brings abundant microscopic food and 

 oxygen, than in the centre where the water is more or less 

 stagnant and used-up. This leads to more active growth 

 on the periphery, and to starvation, death, and decay in the 

 centre, and thus a cup -shaped hollow is formed — a small 

 atoll (Fig. 11, Murray). 



This structure, once attained, remains and increases. 

 The outer rim of a coral reef is always the most actively 



