g reef, and when an island or mountain-top
rose within it, for such a reef precedes the atoll in the order of
formation. Now in those islands in the Pacific, which are large enough
to feed small rivers, there is generally an opening or channel in the
surrounding coral reef at the point where the stream of fresh water
enters the sea. The depth of these channels rarely exceeds twenty-five
feet; and they may be attributed, says Captain Beechey, to the aversion
of the lithophytes to fresh water, and to the probable absence of the
mineral matter of which they construct their habitations.[1131]

Mr. Darwin, however, has shown, that mud at the bottom of river-courses
is far more influential than the freshness of the water in preventing
the growth of the polypi, for the walls which inclose the openings are
perpendicular, and do not slant off gradually, as would be the case, if
the nature of the element presented the only obstacle to the increase of
the coral-building animals.

When a breach has thus been made in the reef, it will be prevented from
closing up by the efflux of the sea at low tides; for it is sufficient
that a reef should rise a few feet above low-water mark to cause the
waters to collect in the lagoon at high tide, and when the sea falls, to
rush out at one or more points where the reef happens to be lowest or
weakest. This event is strictly analogous to that witnessed in our
estuaries, where a body of salt water accumulated during the flow issues
with great velocity at the ebb of the tide, and scours out or keeps open
a deep passage through the bar, which is almost always formed at the
mouth of a river. At first there are probably many openings, but the
growth of the coral tends to obstruct all those which do not serve as
the principal channels of discharge; so that their number is gradually
reduced to a few, and often finally to one. The fact observed
universally, that the principal opening fronts a considerable valley in
the encircled island, between the shores of which and the outer reef
there is often deep water, scarcely leaves any doubt as to the real
origin of the channel in all those countless atolls where the nucleus of
land has vanished.

_Size of atolls and barrier reefs._--In regard to the dimensions of
atolls, it was stated that some of the smallest observed by Beechey in
the Pacific were only a mile in diameter. If their external slope under
water equals upon an average an angle of 45 degrees, th