STRUCTURE OF CORAL ISLANDS. 59 
united action of winds and waves, and from opposite directions, which 
occasionally exceed half a mile.* 
Shore platform and emerged land.—The shore platform is from one 
to three hundred feet in width, and has the general features of a 
half-submerged outer reef. Its peculiarities arise solely from the 
accumulations which have changed the reef into an island. Much 
of it is commonly bare at low tide, though there are places where it is 
always covered with a few inches or a foot of water; and the elevated 
edge, the only part exposed, often seems like an embankment pre- 
venting the water from running off. The tides, as they rise, cover it 
with water throughout, and bear over it coral fragments and sand, 
comminuted shells and other animal remains, to add them to the 
beach. The heavier seas transport larger fragments; and at the foot 
of the beach there is often a deposit of blocks of coral or coral rock, a 
cubic foot or so in size, which low tide leaves standing commonly in 
a few inches of water.t 
Besides the deep channels cutting into the margin of the reef and 
giving it a broken outline, there are long fissures in some instances 
intersecting its surface. On Aratica, (Carlshoff,) and Ahn, (Peacock 
Island,) they extended along for a fourth to half a mile, generally 
running nearly parallel with the shore, and at top were from a fourth 
to half an inch wide. ‘These fissures are not essential features of the 
reef, and will come up for consideration on a future page of this 
volume, 
The beach usually slopes at an angle of 35 to 45 degrees, and con- 
sists of coral pebbles or sand, with some worn shells, and occasionally 
the exuvie of crabs and bones of fishes. Owing to its whiteness, and 
the contrast it affords to the massy verdure above, it is a remarkable 
feature in the distant view of these islands. It often seemed like an 
artificial wall or embankment, running parallel with the shores. On 
Clermont Tonnerre, the first of these islands visited by us, the natives 
* Beechey states that the rim is generally three to four hundred yards in width, and 
never exceeds half a mile.-— Voyage, Amer. ed., p. 160. 
{+ On moving these masses, which generally rest on a few points, and have an open 
space beneath, the waters at once become alive with fish, shrimps, and crabs, escaping 
from their disturbed shelter; and beneath appear various living flowers, the spiny echini 
and sluggish biche-la-mar, while swarms of shells, having a soldier crab for their tenant, 
walk off with unusual life and stateliness. Moreover, delicate corallines, ascidie and 
sponges tint with lively shades of red, green, and pink, the under surface of the block of 
coral which had formed the roof of the little grotto. 
