SUBMARINE PLATEAU. 273 



can live only in water of moderate depth, combined with the 

 assumption that the whole vast thickness of the reef was formed 

 by such shallow-water species — that such a reef could only have 

 been formed during a period of subsidence. Of course I freely 

 admit that the reef-building corals occur only down to a depth 

 of twenty fathoms at most. It is consequently impossible that, 

 during a period of upheaval, they should form a reef of more than 

 120 feet in perpendicular thickness. But we now know 

 that numerous animals live at much greater depths than had 

 hitherto been supposed. The great ' Challenger ' expedition and 

 other similar voyages have furnished proof that a very rich and 

 peculiar fauna is universally distributed throughout the ocean, 

 and we know, moreover, from Carpenter's investigations in the 

 Mediterranean, that merely local causes have there prevented 

 the development of an equally rich fauna. We may therefore 

 infer that the constructive activity of the greater number of 

 marine animals might very well supply materials for the forma- 

 tion of a solid submarine soil, even at much greater depths 

 than those at which the true reef-forming corals are found. 

 This probability is raised to certainty by the observations of 

 American naturalists in the West Indian seas. Pourtal^s 

 there discovered a plateau of solid rock many miles in ex- 

 tent, lying at an average depth of 150 fathoms, and consisting 

 of a compact conglomerate of fragments of shells and corals 

 in a calcareous mud with transported rolled pebbles. This 

 has been called the Pourtal6s Plateau, after its discoverer. 

 If this plateau were to be slowly elevated, at a rate somewhat 

 less rapid than that required for the deposition of the new layers 

 of solid rock, by degrees a sufficiently solid base would be raised 

 to the level requisite for the existence of reef-building corals, and 

 these might begin their work on the foundation prepared for 

 them by other animals. 



From all this I infer the process of reef-formation to be 

 somewhat as follows. In the first instance, during the rais- 

 ing of the sea-bottom, only the foundation for the coral building 

 would be formed, and the inclination of the plateau thus formed 

 would certainly be very inconsiderable. Here and there chan- 

 nels would already be formed by the action of deep-sea currents. 



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