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 farther 
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. 
In the Louisiade Archipelago, Plate VII., the area within 
the great reef, one hundred and twenty-five miles long, is 
five sixths water, with depths of ten to two hundred feet ; 
and the westernmost island is an atoll. 
In the further description of reef-grounds, we note: 
1. Outer reefs, or reets 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 
