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Proceedings of the Royal Society 
breaking on tbe edge of the reef-flat, would naturally tend to 
collect at the foot of the first declivity in depths of 15 to 20 
fathoms; in such a situation living coral would he scarcely 
expected to thrive; but in the more level region beyond, as the 
sand and gravel thinned away, conditions more suitable for the 
growth of coral would be found, and this is the conclusion towards 
which my soundings pointed. There would thus appear to exist 
on the outer submarine slope of this barrier-reef, in depths 
of 15 to 20 fathoms, a belt of detritus dividing into two 
portions the zone in which the reef-building corals thrive. (I 
have marked the position of this belt in the section by a cross.) 
Had my soundings been confined to the upper of these two sub- 
zones, I should have been justified to a great extent, on reaching 
the belt of sand and gravel, in concluding that coral did not 
thrive in depths beyond 15 fathoms; but by subsequently 
extending such soundings seaward across this band of detritus into 
the lower or outer sub-zone, I should have exposed the fallacious 
character of such a conclusion. 
(c) Port Mary Reef Santa Anna ( vide section 1). — My soundings 
were taken off the outer edge of the reef enclosing the harbour. 
The seaward profile of the reef may be thus described. For a 
short distance from the edge of the reef-flat there is a gradual 
descent. This is followed by a rapid slope until a depth of about 
16 fathoms is reached, when within a distance of a couple of 
boats’ lengths, there is a drop of another 16 fathoms. From the 
foot of this declivity there is an easy descent for about 450 yards, 
with a fall of some 9 or 10 fathoms, which terminates in a precipi- 
tous slope where there is a drop of about 25 fathoms. The sub- 
marine slope beyond descends at an angle of 18° or 19° to con- 
siderable depths. Corals thrive down to a depth of 30 fathoms. 
Beyond this depth sand and gravel more frequently come up in the 
armings. It will be noticed that the lower limit of the coral zone 
apparently corresponds with the base of the first declivity (marked 
with a cross in the section), as in the cases of the two reefs pre- 
viously described, and that the deeper extension of the zone in this 
instance is evidently due to the deeper situation of the declivity in 
question. Sand and gravel cover the more level tract beyond ; but 
there were indications given by my soundings of the thinning 
