226 CORALS 



There is still another difficulty in tlie way of accepting 

 the original form of the subsidence theory. The lagoons of 

 the atolls and barrier reefs are not deep pits or troughs, but 

 usually extraordinarily fiat basins at a more or less uniform 

 depth of twenty fathoms. If these reefs had been formed 

 over long periods of time by gradual subsidence of a few 

 thousands of feet, the lagoons would have been of greater 

 depth and provided with sloping sides. 



To meet some of these difficulties Sir John Murray ^ 

 put forward an alternative theory which did not involve 

 the hypothesis of a long-continued subsidence of the land. 

 The discoveries made during the voyage of H. M.S. Challenger 

 concerning the constitution of the floor of the great oceans 

 and the nature of deposits on the sea-bottom, led him to the 

 conclusion that a continuous rain of calcareous organisms 

 from the surface-waters causes the formation of submarine 

 banks, which from time to time rise to the level at which 

 reef-building corals can thrive. \Mien the plantations thus 

 started reach the surface of the sea by upward growth they 

 gradually assume an atoll form by the death of the corals 

 in the centre and the outward growth, like a fairy ring, of 

 the corals on the edge, the lagoon being formed subsequently 

 bv solution of the dead coral by the sea-water which per- 

 colates through the mass. 



The barrier reefs are formed according to this theory by 

 the outward growth of fringing reefs on a basis formed 

 mainly by the talus of dead corals which are broken off 

 the growing edge bv storms, and the lagoon channels are 

 formed in the same way, by solution, as the lagoon of the 

 atolls. 



This theorv, of which only the briefest outline can be 

 given here, has been very unfairly termed the " still stand " 

 theory. It is true that it would account for the formation 

 of the characteristic coral reefs on a perfectly stationary 

 foundation ; but Sir John Murray was fully aware of the 

 probability of earth-movements both of elevation and sub- 

 sidence, and his theory w^ould hold good notwithstanding 

 slow movements of this kind in either direction. 



• Sir Jolm Murreiy, Proc. Roy. Soc. Ediu., \o\. x., 1S79-S0. 



