ntire corals, piled one upon another, for a
thickness of several thousand feet, he unquestionably mistakes
altogether the nature of the accumulations now in progress. In the first
place, the strata at present forming very extensively over the bottom of
the ocean, within such barrier reefs as those of Australia and New
Caledonia, are known to consist chiefly of horizontal layers of
calcareous sediment, while here and there an intermixture must occur of
the detritus of granitic and other rocks brought down by rivers from the
adjoining lands, or washed from sea-cliffs by the waves and currents.
Secondly, in regard to atolls, the stone-making polypifers grow most
luxuriantly on the outer edge of the island, to a thickness of a few
feet only. Beyond this margin broken pieces of coral and calcareous sand
are strewed by the breakers over a steep seaward slope, and as the
subsidence continues the next coating of live coral does not grow
vertically over the first layer, but on a narrow annular space within
it, the reef, as was before stated (p. 761), constantly contracting its
dimensions as it sinks. Thirdly, within the lagoon the accumulation of
calcareous matter is chiefly sedimentary, a kind of chalky mud derived
from the decay of the softer corallines, with a mixture of calcareous
sand swept by the winds and waves from the surrounding circular reef.
Here and there, but only in partial clumps, are found living corals,
which grow in the middle of the lagoon, and mixed with fine mud and
sand, a great variety of shells, and fragments of testacea and
echinoderms.

We owe to Lieutenant Nelson the discovery that in the Bermudas the
calcareous mud resulting from the decomposition of the softer corallines
is absolutely undistinguishable when dried from the ordinary white chalk
of Europe,[1133] and this mud is carried to great distances by currents,
and spread far and wide over the floor of the ocean. We also have
opportunities of seeing in upraised atolls, such as Elizabeth Island,
Tonga, and Hapai, which rise above the level of the sea to heights
varying from ten to eighty feet, that the rocks of which they consist do
not differ in structure or in the state of preservation of their
included zoophytes and shells from some of the oldest limestones known
to the geologist. Captain Beechey remarks that the dead coral in
Elizabeth Island is more or less porous and honeycombed at the surface,
and hardening into a compact rock which has the