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.
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