On Coral Reefs and Islands. 361 
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 
or 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 Leyu 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-formations there are 
several distinct subjects for consideration, as is obvious from the 
preceding remarks. ‘These are— 
_ L. 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 onter and inner reefs, channels, and beaches, act each their 
part in producing the coral formations in progress about islands. | 
aration for the next breaker. ‘This marg 
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 fron this there 1s finally an’ — 
abrupt descent, generally by an angle of at least forty degrees to 
depths beyond the reach of a sounding legd.: There*is a great 
oie difference in the rapidity with which the water deepens, as might 
Bi ‘Ss aul . 46 : 
ND Senses, Vol. XI, No. 83—May, 1851. 
