nant Smith, of the Blossom, that the trees came down to the beach
towards the centre of the island; a break at first sight resembling the
openings which usually lead into lagoons; but the trees stand on a steep
slope, and no hollow of an ancient lagoon was perceived.

Beechey also remarks, that the surface of Henderson's Island is flat,
and that in Queen Charlotte's Island, one of the same group, but under
water, there was no lagoon, the coral having grown up everywhere to one
level. The probable cause of this obliteration of the central basin or
lagoon will be considered in the sequel.

That the bed of the Pacific and Indian oceans, where atolls are
frequent, must have been sinking for ages, might be inferred, says Mr.
Darwin, from simply reflecting on two facts; first, that the efficient
coral-building zoophytes do not flourish in the ocean at a greater depth
than 120 feet; and, secondly, that there are spaces occupying areas of
many hundred thousand square miles, where all the islands consist of
coral, and yet none of which rise to a greater height than may be
accounted for by the action of the winds and waves on broken and
triturated coral. Were we to take for granted that the floor of the
ocean had remained stationary from the time when the coral began to
grow, we should be compelled to assume that an incredible number of
submarine mountains of vast height (for the ocean is always deep, and
often unfathomable between the different atolls) had all come to within
120 feet of the surface, and yet no one mountain had risen above water.
But no sooner do we admit the theory of subsidence, than this great,
difficulty vanishes. However varied may have been the altitude of
different islands, or the separate peaks of particular mountain-chains,
all may have been reduced to one uniform level by the gradual
submergence of the loftiest points, and the additions made to the
calcareous cappings of the less elevated summits as they subsided to
great depths.

_Openings into the lagoons._--In the general description of atolls and
encircling reefs, it was mentioned that there is almost always a deep
narrow passage opening into the lagoon, or into the still water between
the reef and the shore, which is kept open by the efflux of the sea as
the tide goes down.

The origin of this channel must, according to the theory of subsidence
before explained, be traced back to causes which were in action during
the existence of the encirclin