Corals and coral islands.

STR UCTURE OF CORAL REEFS.

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may be some hundreds of yards; over the bottom in these shal.
low waters are spread out the coral plantations, down to a
depth of 80 to 150 feet. Finally there is a rather abrupt de
scent to depths beyond the reach of an ordinary sounding-lead.
The great difference in the rapidity with which the water deepens depends chiefly on the varied character of submarine
slopes. Shallow waters may extend out for miles, especially
off the prominent points or angles; but it is more common to
meet with the opposite extreme-great depths within a few
hundred feet.
The outer reef or coral platform is generally a little the
highest at its seaward margin, owing partly to the growth of
ordinary corals and other species on this part, and also to the
accumulations which naturally would there be piled up by
the waves and become cemented. This part is therefore first
laid bare by the retreating tide; and though a tempting place
for a ramble, it is often a dangerous place on account of the
heavy breakers. There is not only greater height, but often
also a remarkably smooth surface to the reef-rock, looking as
if water-worn, and frequently a blotching of the rock with various shades of pink and purple. These colors and the smoothness, as observed by Chamisso, are due to incrusting Nullipores; and to the same calcareous sea-weeds, as Darwin first
observed, is often owing the increased height. The material
of the incrusting plant is more solid than ordinary coral, for it
is without a pore; and layer is added to layer until it has considerable thickness. It is thus an important protection to the
reef against the wash of the waters.
Darwin states that on Keeling Island, the Nullipore bed
has a thickness of two or three feet and a breadth of twenty
feet. Nullipores are abundant on the Paumotu reefs. Still,
they are not essential to the formation or protection of an