CORAL AND CORAL REEFS 403 



of the ocean are breaking upon the coral reef which sur- 

 rounds 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 car- 

 bonate 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 



