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 inside of the
reef, which has a particular name, derived from the Spanish or the
Portuguese--it is called a "lagoon," or lake. In this lagoon there is
comparatively little living coral; the bottom of it is formed of coral
mud. If we pounded this coral in water, it would be converted into
calcareous mud, and the waves during storms do for the coral skeletons
exactly what we might do for this coral in a mortar; the waves tear off
great fragments and crush them with prodigious force, until they are
ground into the merest powder, and that powder is washed into the
interior of the lagoon, and forms a muddy coating at the bottom. Beside
that there are a great many animals that prey upon the coral--fishes,