172 THE SEAS 



of which frequently descend to depths of hundreds and 

 thousands of fathoms respectively. It is the mystery 

 surrounding their mode of formation which has attracted the 

 attention of zoologists and geologists since reefs were first 

 accurately described, and has led to the development of a 

 number of theories which attempt to explain the mystery. 

 The most important of these are discussed below and, 

 though no one of these has met with universal acceptance, 

 yet all may contain elements of truth or apply to the forma- 

 tion of reefs in particular areas. 



Theories of Reef Formation 



The first, and most famous, theory concerning the origin 

 of barrier reefs and atolls was put forward by the great 

 Charles Darwin in 1842. He had observed, during his 

 voyage round the world in H.M.S. Beagle, that reef -building 

 corals can only exist in shallow water while, as a distin- 

 guished geologist, he also knew that the level of the earth 

 may vary from time to time. By putting these two facts 

 together he was able to elaborate a theory which, in its 

 simplicity, bears the stamp of genius. He imagined a 

 fringing reef developing, in the manner shown above, off 

 land which was slowly sinking as a result of a general 

 subsidence of the earth in that region. As the land sank, 

 the corals would grow upwards and keep pace, more or 

 less, with it, so that the outward edge of the reef about the 

 boulder zone would maintain its position near or above 

 the surface, but the shallow channel between it and the 

 land would become deeper and deeper as the land sank and 

 also broader as, bit by bit, the land was overrun by the sea. 

 Finally, a typical barrier reef would be formed, often many 

 miles from the new coastline, and with a channel up to fifty 

 fathoms in depth. Here the matter would rest should some 

 of the land remain above water — as in the case of Great 



