INTRODUCTION. xxi 



dition of exposure to oceanic currents is such that an immense body of water 

 is constantly flowing across the plateau during both the northeast and the 

 southwest monsoons. Where the plateaus are smaller or not so open to the 

 flow of currents as in such atolls as Addu, Karidu, Goifurfehendu, Gaha 

 Faro, Wataru, Makunudu, and others, Ave have only a single atoll developed ; 

 again in sufth plateaus as those upon which Kolumadulu, Haddummati, and 

 Suvadiva have developed, the conditions are oceanic, more similar to those 

 we find in the widely separated atolls of the Ellice, Gilbert, or Marshall 

 Islands. At the same time the lagoons of the southern atolls are far less 

 open to oceanic circulation than those of the northern plateaus, and thus we 

 find fewer banks or islands in the lagoons, and only here and there a trace 

 of those remarkable rings which are so characteristic a feature in Maldivian 

 coral-reef scenery. I have seen nothing so striking in my experience of 

 coral reefs as these rings with a light-colored rim standing out from the 

 deep blue water surrounding them, like ghosts of an atoll which had sunk, 

 and enclosing a lighter blue or emerald-colored lagoon indicative of its depth 

 below the surface. 



The conditions existing at the Maldives are repeated to a certain extent 

 on the Yucatan Plateau, where the Alacran reef, a regular atoll, rises from 

 it at a depth of about thirty fathoms. It is true that it is the only atoll 

 on this extensive plateau ; but there are also other irregularly shaped 

 patches of coral reefs. The absence of atolls may be traced to the fact 

 that the plateau is not within an area of such regular trades as are the 

 northeast and southwest monsoons in the region of the Maldives. The 

 central banks in the Tonga Archipelago, the Haapai and Nomuka groups, 

 with their irregularly shaped lagoon reefs and land-rim flats, remind us also 

 somewhat of Maldivian conditions. 



The moderate strength of the prevailing winds in the Maldives has 

 greatly influenced many of the chax'acteristic features of its atolls. The 

 effect of the southwest and of the northeast monsoons cannot be compared 

 to that of the trades in the Pacific. We have nothing in the Maldives corre- 

 sponding to the incessant breakers of the huge rollers which pound upon the 

 reef flats of the Paumotus and of the atolls and barrier reefs of the central 

 and western Pacific. The boulders thrown upon the reef flats are mere pyg- 



