THE GREAT BARRIER REEF. 817 



coral that form reefs, all those found in the hollows 

 or on the sheltered slopes of reefs, where alone they 

 can be examined. Whether it be universally true, 

 for all polyps depositing large masses of calcareous 

 matter, will perhaps admit of a doubt. 



These individual coral reefs are grouped together 

 in various ways, forming " atolls," "barriers," and 

 " fringing reefs," as described by Mr. Darwin. The 

 Great Barrier Reef of Australia is itself com- 

 posed of each of these different kinds of groups, 

 which I shall class as 1st — " The linear reefs," form- 

 ing the outer edge, or actual Barrier. 2nd — "De- 

 tached reefs," lvin^ outside that Barrier. 3rd.— 

 " Inner reefs," or those which lie between the 

 Barrier and the shore. The 1 st, or linear reefs, are 

 generally long and narrow, running along a line 

 more or less parallel with the shore, and divided 

 from each other by narrow passages. They vary from 

 200 yards to a mile in width, and from half a mile 

 to ten or fifteen miles in length. They have com- 

 monly unfathomable, or at least unfathomed, water 

 on the outside, and a depth of from ten to twenty 

 fathoms on the inside of them. The 2nd or " De- 

 tached reefs," are not common near the Barrier, 

 and occur only in one neighbourhood. They rise 

 from deep water all round, have more or less of a 

 circular form with lagoons inside them, and are 

 regular " atolls " on a small scale. The 3rd or 

 " Inner reefs," are very numerous. They are scat- 

 tered over the space between the Barrier and the 



