Sea level 



Sections to show the Formation of an Atoll on a Submarine Mountain 

 Scale: — i cm. = ioo fathoms, or i cable. 



i showing the building up of mounds on the elevatic 



C. The highest mound has been built up to the bathymetrical limit of 

 the reef-building organisms, which at once raise a pinnacle on it to the 

 surface. 



D. By the further growth of the reef-building organisms, by detritus 

 from C and by other organic remains the pinnacle broadens and attains 

 the characteristic slope of a reef. 



E. The surface of the reef broadens by the fusion to it of buttresses, 

 formed by the direct upward growlh of the reef organisms on its shallow 



slopes. At the same time the whole extends outwards on its own talus, and 

 a pool begins to form m the centre by solution. (The sections outside the 

 reef down to 400 fathoms are from Sydney Island, Phoenix Group, on the 

 right and Canton Island, Phoenix Group, on the left.) 



F. By the further broadening of the reef and together with this the 

 solution of its central part an atoll is formed with a lagoon proportional in 

 depth to the size and depth of its passages. The floor of tlic lagoon is flat, 

 rising at its sides by a low cliff (F'). (The sections off the right and left of 

 the reef are those run by the Penguin lu the east and west of Funafuti 

 Atoll.) 



fN.B. Sections taken from the .Admiralty Charts.) 



