STRUCTURE OF CORAL ISLANDS. 73 
shallow, and much obstructed by growing coral, the latter giving the 
water over it a clear light green colour. Platform, or outer coral shelf 
of the island, about two hundred and fifty feet wide; under water except 
at the lowest tides. Margin highest, and covered with Nullipore in- 
crustations, which give it a variety of delicate shades of colour, mostly 
reddish, or peach-blossom red, rose, scarlet. For thirty to fifty feet 
from the margin, very cavernous, and containing many Tridacne, 
lying half imbedded, with the variously tinted mantle expanded when 
the surface is covered with water. Rock of the platform either a 
compact white limestone or a solid conglomerate; dead over its sur- 
face, excepting a few Madrepore tufts or Astreas near the margin in 
pools. In this shelf there were long fissures, extending nearly parallel 
with the shore, a quarter to half an inch wide at top, and continuing 
sometimes a fourth of a mile or more. These fissures were commonly 
filled with coral sand. ‘The higher parts of the island either consist- 
ing of loose blocks of coral or covered with some soil; the soil mostly 
of comminuted coral and shells, with dark particles from vegetable de- 
composition intermingled. On the bottom exterior to the shore plat- 
form, observed the same corals growing as occurred in fragments 
upon the island; but the larger part of the bottom was without coral, 
or covered only with sand. 
Raraka, Paumotu Archipelago.—16° 10’ S., 145° W. 14 miles by 
8 miles, trending east and west. Shape nearly triangular. North side 
nearly continuously wooded: south angle and southwest reef bare. 
A large lagoon with an entrance for small vessels on the north side. 
A rapid current flows from the entrance, which it was difficult for 
a boat to pull against. Shore platform, as usual, about a hundred 
yards wide, with the edge rather higher than the surface back; the 
platform mostly bare of water at low tide. Several large masses of 
coral and coral rock, one to four hundred cubic feet, on the platform 
and upon the higher parts of the island, some of which stood five and 
six feet above high water mark. ‘They were cemented to the reef- 
rock below, and appeared like projecting parts of the reef. Layers 
of beach sand-rock on the lagoon shores, as well as on the seaward 
side, inclined at an angle of six or seven degrees: characters as 
already described. Growing coral in the entrance to the lagoon, 
within two feet of the surface, mostly a species of Millepora, (M. 
squarrosa.) Interior of the lagoon not examined for want of time. 
The water looked as blue as the ocean, and was much roughened 
by the winds. 
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