President's Address. 21 



or when strong winds have blown the water off the flats. 

 Large heads of astrseans and Mceandrina occur here and 

 there towards the edge of the reef, which is occupied mainly 

 by clusters of Gorgonicc^. The destruction of the reefs by the 

 waves is very great, the sea being occasionally discoloured 

 with the chalky sediment to a distance of from 6 to 10 

 miles after a storm. Broken coral-heads and branches, dead 

 corallines, shells of mollusks, old serpulse tubes, stalks of 

 Gorgonice, and other organisms, are thrown up into lines that 

 consolidate into a low dyke, which in turn is pounded up 

 and removed by the breakers. A prodigious quantity of 

 calcareous sediment is thus produced, much of which is 

 swept into the interior of the reef, where it accumulates in 

 flats of sand and silt. It is only at the outer edge of the 

 reef, where the scour of the sea is greatest, that the corals 

 can flourish: elsewhere they are choked and buried under 

 the deposit of calcareous sediment. Some of this sediment 

 accumulates in steep submarine banks, like sand-dunes, 

 which shift to and fro as winds and currents vary ; though, 

 by the action of the carbonic acid of the sea- water, they are 

 apt to be cemented into solid slopes, some of which have an 

 angle of as much as 33°. So great is the destructive and 

 transporting influence of the sea, under the combined or 

 antagonistic working of tides, currents, and wind-waves, that 

 the whole mass of the reef, as well as the flats and shoals 

 inside, may be said to be in more or less active movement. 

 Hence none of the landmarks furnished by the islands are to 

 be relied upon for the location of buoys. 



A still more perfect example of an atoll formed under 

 similar conditions is that of Alacran, on the opposite coast of 

 Yucatan. Its eastern face is a great arc of about twenty 

 miles, where, exposed to the open sea and the easterly winds, 

 the corals flourish vigorously. On the eastern, or interior, 

 face of the western chord of the reef, however, the silt derived 

 from the pounding of the breakers to the eastward has 

 already killed the corals. The lagoon is occupied by detached 

 coral-heads, with lanes of clear water between them.^ 



To the east of the Tortugas, near the mainland of Florida, 



1 Bulletin Mus. Comp. Zool., v., No. 1. 



