Corals and coral islands.

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CORALS AND CORAL ISLANDS.

low tide." "The northeastern part of the reef is called the,
Recife do Lixo, that is, Reef of the lixo, a shark-like ray which is
furnished with large crushing teeth and frequents the reef in
search of shell-fish."
The rock of'the submerged coral-heads is but a loose aggregation of corals in the position of growth, except probably,
in their lower portion, where the open spaces may be filled
with sand and fragments and all cemented together.
The deposits of sand or coral mud over the bottom of the
seas outside of barrier reefs are sometimes of great extent.
These sands are the fine detritus which the return flow of the
breaker bears seaward; and, in still deeper water, the deposits
should be mainly of the finest calcareous sand or mud-fit material for impalpable compact limestones. The waters outside
of the reef, especially when moved by heavy tidal currents or
storms, are often milky with the coral sand; and while the
coarser sand is dropped near the shores, the finer may be
carried for miles and distributed far out to sea. As Major
Hunt, in his observations on the Florida Reefs remarks, this
" white water " is one of the signs of proximity to a coral reef.
After storms, the white coral material subsides and the waters
become clear again.
Mr. Jukes, who made special examinations of the Australian reef region, and others in that vicinity, in H. M. S. Fly,
states that in the deeper waters outside of' the great barrier,
"and in all the neighboring East India seas, from Torres
Straits, north of' Australia, to the Straits of Malacca, wherever the bottom was brought up by the lead, it proved to
be a very fine-grained, impalpable, pale olive-green mud,
wholly soluble in dilute hydrochloric acid, and therefore essentially carbonate of lime. The substance, when dried, looked
much like chalk, excepting in its greener tinge. How far this