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bulletin: museum of comparative zoology. 



There are eight of these islands, none of which exceeds a few acres in 

 area, and most of them are perfectly circular in outline. Five of the cir- 

 cular islands and one of quadrangular outline enclose interior lagoons. 

 Until one puts foot upon them they appear to stand eight or ten feet 

 above the sea. owing to the dense growth of mangrove trees which rise 

 to that height. Then it is seen that several of them show no land what- 

 ever above the water, but consist of circular patches of mangrove trees 

 growing out of the sea. Others visited by us consist of a low ring of 

 wind-blown shell and coral sand, nowhere rising over three feet above 

 the sea. A shallow living reef underlies the whole area of the sea where 

 these pseudo-atolls occur, and constitutes the platform from which they 

 rise. A typical island presents the following cross section, Figure 38. 



Figure 38. Cross Section of the Pseudo Atolls at Montego Bay. 

 (1. Interior Lagoon of brown colored brackish swamp water, 

 e. Land composed of coral sand and shell, covered by palms and cocoanut trees. 

 h. A ring of mangrove bushes growing from the water around the island. 

 s. The floor of the sea is a living coral reef, with many Algge, Echinoderms, Alollusks, 

 etc. The top of this reef is about two feet below the surface. 



We can offer only one hypothesis to explain these islets, and that is 

 as follows. As described by A. Agassiz ^ in the instance of the Florida 

 keys, ** young mangrove plants drift in immense quantities upon tho 

 submerged flats which reach nearly to the surface of the sea or be- 

 come awash." These grow up in clumps like some of the small islands 

 now seen. By wind and wave action the reef debris is accumulated 

 around tlio roots of these trees, gradually embedding them and ulti- 

 mately rising above the water as land. The circlet of land thus made 

 cuts off the older mangroves from the sea water. The older and origi- 

 nal mangroves, now sand clogged, become the interior of the islet, and 

 decay and die for want of access to salt water. Rain charged with 

 carbonic gas from the decaying vegetation then compacts and dissolves 

 tlie shell sand, and converts the central portion into lagoons around 

 wliich newer land continues to be made by wave and wind. Meanwhile 

 tho new growth flourishes adjacent to the salt water. 



1 Three Cruises of the Blake, Vol. I. pp. 52, 53. 



