er in the inside, and the
deep blue of the sea beyond. That line of surf indicates the point at
which the waters of the ocean are breaking upon the coral reef which
surrounds the island. You see it sweep round the island upon all sides,
except where a river may chance to come down, and that always makes a
gap in the shore.

There are two or three points which I wish to bring clearly before your
notice about such a reef as this. In the first place, you perceive
it forms a kind of fringe round the island, and is therefore called a
"fringing reef." In the next place, if you go out in a boat, and take
soundings at the edge of the reef, you find that the depth of the water
is not more than from 20 to 25 fathoms--that is about 120 to 150 feet.
Outside that point you come to the natural sea bottom; but all inside
that depth is coral, built up from the bottom by the accumulation of the
skeletons of innumerable generations of coral polypes. So that you see
the coral forms a very considerable rampart round the island. What the
exact circumference may be I do not remember, but it cannot be less than
100 miles, and the outward height of this wall of coral rock nowhere
amounts to less than about 100 or 150 feet.

When the outward face of the reef is examined, you find that the upper
edge, which is exposed to the wash of the sea, and all the seaward face,
is covered with those living plant-like flowers which I have described
to you. They are the coral polypes which grow, flourish, and add to the
mass of calcareous matter which already forms the reef. But towards the
lower part of the reef, at a depth of about 120 feet, these creatures
are less active, and fewer of them at work; and at greater depths than
that you find no living coral polype at all; and it may be laid down
as a rule, derived from very extensive observation, that these
reef-building corals cannot live in a greater depth of water than about
120 to 150 feet. I beg you to recollect that fact, because it is one I
shall have to come back to by and by, and to show to what very curious
consequences that rule leads. Well then, coming back to the margin of
the reef, you find that part of it which lies just within the surf to be
coated by a very curious plant, a sort of seaweed, which contains in its
substance a very great deal of carbonate of lime, and looks almost like
rock; this is what is called the nulli pore. More towards the land,
we come to the shallow water upon the in