176 THE SEAS 



manner, Murray thought that a fringing reef could be 

 converted into a barrier reef. 



This theory has many points in its favour, notably in that 

 it does not demand any general subsidence of the land over 

 the tropics. It has been supported by many more recent 

 investigators of coral reefs, though often with qualifying 

 statements and additions. The most important criticisms 

 are concerned with the formation of the lagoon, for many 

 scientists think that these are at present being filled up with 

 sediment and not, as Murray's theory demands, gradually 

 increasing by further erosion and solution, and that, in any 

 case, they were originally formed by the scouring action of 

 the sea and by the action of boring animals and plants, 

 rather than by the dissolution on which Murray laid so much 

 stress. It is also very probable that the ring shape of 

 atolls is due to the action of the prevailing winds. All 

 atolls lie within the trade wind areas where the wind 

 blows from a constant quarter for the greater part of 

 the year. There is actually an atoll ring off Florida which 

 must have been moulded in this manner, it has not been 

 built by living corals but by deposits of various kinds formed 

 by submerged reefs of very mixed origin. 



Clearly a definite proof of the origin of atolls might be 

 hoped for if borings were made through the coral rock down 

 into the lower layers. If the substance of the atoll was 

 found to be of coral origin to great depths, then we should 

 have strong evidence in favour of Darwin's theory, the 

 coral limestone below about thirty fathoms being that 

 which had been carried down by the subsidence of the land ; 

 if, on the other hand, a surface layer of coral limestone 

 some thirty fathoms in thickness was found resting on a 

 layer of solidified bottom deposits, of volcanic fragments, 

 or similar material, then that would afford evidence that 

 Murray's \'iews are correct. With this end in view ex- 



