36 CORAL FORMATIONS. 
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 unfathomable 
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 
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 nithin barriers, which may receive detritus 
either from the reefs, or the shores, or from both of these sources 
combined. 
4. Beaches and beach formations, produced by coral accumulations 
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 preparation 
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 be inferred from the varied 
character of submarine slopes; in some cases the shallow waters may 
extend for two or three miles beyond the reef, but it is far more 
common to meet with the opposite extreme—unfathomable depths 
within a few hundred feet. 
The growing corals are mostly confined to the shallow waters and 
to the sloping margin of the reef, up which they extend to within 
