STRUCTURE OF CORAL ISLANDS. 201 
pore incrustations, which give it a variety of delicate shades of 
color, mostly reddish, of peach-blossom red, rose, scarlet. For 
thirty to fifty feet from the margin, very cavernous, and con 
taining many Tridacne, lying half imbedded, with the variously 
tinted mantle expanded when the surface is covered with wa- 
ter. Rock of the platform either a compact white limestone or 
a solid conglomerate; dead over its surface, 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 com- 
monly filled with coral sand. ‘The higher parts of the island 
either consisted of loose blocks of coral or were covered with 
some soil ; the soil mostly of comminuted coral and shells, with 
dark particles from vegetable decomposition intermingled. On 
the bottom, exterior to the shore platform, observed the same 
corals growing as occurred in fragments upon the island ; but 
the larger part of the bottom was without coral, or consisted 
only of sand. 
Raraka, Paumotu Archipelago.—16° 10’S., 145° W. 14 
miles by 8, trending east and west. Shape somewhat trian- 
_ gular. 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 project- 
