STRUCTURE OF CORAL REEFS. 9 



A wide difference in the extent of reefs would be inferred from 

 these facts. There is the mere point of coral rock ; and 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 intermediate channels or seas. The en- 

 closing barrier off Vanua Levu alone is more than one hundred 

 miles long. The Exploring Isles, in the eastern part of the 

 Feejee group, have a barrier eighty miles in circuit. New Cale- 

 donia, as often cited, 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, a thousand miles in length, lying off the coast from the 

 Northern Cape to the tropical circle ; and the channel within is 

 in some parts sixty miles from the coast, with a depth of thirty 

 to sixty fathoms. 



The seas outside of the lines of coral reef are often unfathom- 

 able within a short distance of the line of breakers. 



b. Structure of Reef Formations. 



In the description of reef grounds or reef- format ions 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 ex- 

 posed 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 bar- 

 rier and the shores of an island. 



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

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

 sources combined. 



4. Beaches and beach formations, produced by coral accumula- 

 tions on the shores through the action of the sea and winds. 



The outer and inner reefs, channels, and beaches, act each their 

 part in producing the coral formations in progress about islands. 



Outer reefs. — The outer reefs or flats of coral rock receive the 

 waves along their margin ; and the outline exposed to this action 

 is very much cut up with deep channels which give passage to 

 the advancing waters, and to the currents that flow back in prep- 

 aration for the next breaker. This margin, which we have said 

 rises but little above low-tide level, usually slopes beneath the 

 water at an angle of forty to seventy degrees to a depth of three 

 to eight fathoms; thence the waters deepen very gradually for 

 one to five hundred yards out, and from this there is finally an 

 abrupt descent, generally by an angle of at least forty degrees to 

 depths beyond the reach of a sounding lead. There is a great 

 difference in the rapidity with which the water deepens, as might 



2 



