414 



Formation of Barrier Beefs and Atolls. [Feb, 7, 



important part in reducing elevated limestone islands to the level of 

 the sea, by riddling them with caverns and by forming extensive sinks, 

 often taken to be elevated lagoons. 



Closed atolls can hardly be said to exist ; Niau in the Paumotus is 

 the nearest approach to one, yet its shallow lagoon is fed by the sea 

 through its porous ring. Sea water may pass freely into a lagoon at 

 low tide over extensive shallow reef flats where there are no boat 

 passages. The land area of an atoll is relatively small compared to 

 that of the half submerged reef flats. This is specially the case in 

 the Marshall Islands and the Maldives where land areas are reduced to 

 a minimum. 



The Maldivian plateau with its thousands of small atolls, rings, or 

 lagoon reefs, rising from a depth varying from 20 to 30, fathoms, is 

 overwhelming testimony that atolls may rise from a plateau of suitable 

 depth, wherever and however it may have been formed and whatever 

 may be its geological structure. On the Yucatan plateau similar con- 

 ditions exist regarding the formation of atolls, only on a most limited 

 scale. 



The great coral reef regions are within the limits of the trades and 

 monsoons and areas of elevation, with the exception of the Ellice 

 and Marshall Islands and some of the Line islands. The extent of 

 the elevation is shown by the terraces of the elevated islands of the 

 Paumotus, Fiji, Tonga, Laclrones, Gilbert, and West Indies, or by the 

 lines of cliff" caverns indicating levels of marine erosion. 



In the regions I have examined the modern reef rock is of very 

 moderate thickness, within the limits of depth at which reef builders 

 begin to grow, and within which the land rims of atolls or of Barrier 

 Reefs are affected by mechanical causes. This does not affect the 

 existence of solitary deep-sea corals, of extensive growths of Oculina 

 or Lophohelia at great depths, or in any way challenge the formation 

 of thick beds of coralliferous limestone during periods of subsidence. 



The Marquesas, Galapagos, and a few islands in the Society and 

 West Indies have no corals, although they are within the limits of 

 coral areas. Their absence is due to the steepness of their shores 

 and to the absence or crumbling nature of their submarine platforms. 

 Coral reefs also cannot grow off' the steep cliff faces of elevated, 

 coralliferous limestone islands. 



Corals take their fullest development on the sea faces of reefs; 

 they grow sparingly in lagoons where coralline algae grow most 

 luxuriantly. Nullipores and corallines form an important part of 

 the reef-building material. 



