structure; of coral reefs. 135 



coral banks which are continuous with the higher portions, and 

 all together make up the coral reef-ground of an island. 



A wide difference in the extent of reef-grounds, follows 

 from the above-mentioned facts. On some coasts there are only 

 scattered groups of corals, or rising knolls, or mere points of 

 emerged coral rock ; but again, as for example, west of the two 

 large Feejee Islands, there may be three thousand square miles 

 of continuous reef-ground, occupied with coral patches and in- 

 termediate channels or seas. The enclosing barrier off Vanua 

 Levu alone is more than one hundred miles long. The Ex- 

 ploring Isles, in the eastern part of the Feejee group, have a 

 barrier eighty miles in circuit. New Caledonia has a reef 

 along its whole western shores, a distance of two hundred and 

 fifty miles, and it extends one hundred and fifty miles further 

 north, adding this much to the length of the island. The 

 great Australian barrier forms a broken line, twelve hundred 

 and fifty miles in length, lying off the coast from the Northern 

 Cape to the tropical circle. 



The seas outside of the lines of coral reef* are often unfath- 

 omable within a short distance of the line of breakers. 



In the further description of reef-grounds, or reef-forma- 

 tions, there are several distinct subjects for consideration, as 

 is obvious from the preceding remarks. These are — 



1. Outer reefs, or reefs formed from the growth of corals 

 exposed to the open seas. Of this character are all proper 

 barrier reefs, and such fringing reefs as are unprotected by a 

 barrier. 



2. Inner reefs, or reefs formed in quiet water between a 

 barrier and the shores of an island. 



3. Channels, or seas within barriers, which may receive de- 

 tritus either from the reefs, or from the shores, or from both of 

 these sources combined. 



