104 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
There are eight of these islands, none of which exceeds a few acres in 
arca, 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 fect 
above the sea, owing to the dense growth of mangrove trees which riso 
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 psendo-atolls occur, and constitutes the platform from which they 
rise. A typical island presents the following cross section, Figure 38. 
Figure 98. Cross Section of the Pseudo Atolls at Montego Bay. 
d. Interior Lagoon of brown colored brackish swamp water. 
e. Land composed of coral sand and shell, covered by palms and cocoanut trees. 
b. 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 Algo, Echinoderms, Mollusks 
etc. The top of this reef is about two feet below the surface. 
We can offer only one hypothesis to explain these islots, and that is 
as follows. As described by A. Agassiz* in the instance of tho Florida 
keys, “ young mangrove plants drift in immense quantities upon tho 
submerged flats which reach noarly 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 débris is accumulated 
around the 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 
the shell sand, and converts the central portion into lagoons around 
which newer land continues to be made by wave and wind. Meanwhile 
the new growth flourishes adjacent to the salt water. 
1 Three Cruises of the Blake, Vol. I. pp. 52, 53, 
